# Command Line Options¶

Note that unless an option is listed as CLI ONLY the option is also supported by x265_param_parse(). The CLI uses getopt to parse the command line options so the short or long versions may be used and the long options may be truncated to the shortest unambiguous abbreviation. Users of the API must pass x265_param_parse() the full option name.

Preset and tune have special implications. The API user must call x265_param_default_preset() with the preset and tune parameters they wish to use, prior to calling x265_param_parse() to set any additional fields. The CLI does this for the user implicitly, so all CLI options are applied after the user’s preset and tune choices, regardless of the order of the arguments on the command line.

If there is an extra command line argument (not an option or an option value) the CLI will treat it as the input filename. This effectively makes the --input specifier optional for the input file. If there are two extra arguments, the second is treated as the output bitstream filename, making --output also optional if the input filename was implied. This makes x265 in.y4m out.hevc a valid command line. If there are more than two extra arguments, the CLI will consider this an error and abort.

Generally, when an option expects a string value from a list of strings the user may specify the integer ordinal of the value they desire. ie: --log-level 3 is equivalent to --log-level debug.

## Executable Options¶

--help, -h

Display help text

CLI ONLY

--version, -V

Display version details

CLI ONLY

Command line executable return codes:

0. encode successful
1. unable to parse command line
2. unable to open encoder
3. unable to generate stream headers
4. encoder abort


## Logging/Statistic Options¶

--log-level <integer|string>

Controls the level of information displayed on the console. Debug level enables per-frame QP, metric, and bitrate logging. Full level enables hash and weight logging. -1 disables all logging, except certain fatal errors, and can be specified by the string “none”.

1. error
2. warning
3. info (default)
4. debug
5. full
--no-progress

Disable periodic progress reports from the CLI

CLI ONLY

--csv <filename>

Write encoding statistics to a Comma Separated Values log file. Creates the file if it doesn’t already exist. If --csv-log-level is 0, it adds one line per run. If --csv-log-level is greater than 0, it writes one line per frame. Default none

The following statistics are available when --csv-log-level is greater than or equal to 1:

Encode Order The frame order in which the encoder encodes.

Type Slice type of the frame.

POC Picture Order Count - The display order of the frames.

QP Quantization Parameter decided for the frame.

Bits Number of bits consumed by the frame.

Scenecut 1 if the frame is a scenecut, 0 otherwise.

RateFactor Applicable only when CRF is enabled. The rate factor depends on the CRF given by the user. This is used to determine the QP so as to target a certain quality.

BufferFill Bits available for the next frame. Includes bits carried over from the current frame.

BufferFillFinal Buffer bits available after removing the frame out of CPB.

Latency Latency in terms of number of frames between when the frame was given in and when the frame is given out.

PSNR Peak signal to noise ratio for Y, U and V planes.

SSIM A quality metric that denotes the structural similarity between frames.

Ref lists POC of references in lists 0 and 1 for the frame.

Several statistics about the encoded bitstream and encoder performance are available when --csv-log-level is greater than or equal to 2:

I/P cost ratio: The ratio between the cost when a frame is decided as an I frame to that when it is decided as a P frame as computed from the quarter-resolution frame in look-ahead. This, in combination with other parameters such as position of the frame in the GOP, is used to decide scene transitions.

Analysis statistics:

CU Statistics percentage of CU modes.

Distortion Average luma and chroma distortion. Calculated as SSE is done on fenc and recon(after quantization).

Psy Energy Average psy energy calculated as the sum of absolute difference between source and recon energy. Energy is measured by sa8d minus SAD.

Residual Energy Average residual energy. SSE is calculated on fenc and pred(before quantization).

Luma/Chroma Values minumum, maximum and average(averaged by area) luma and chroma values of source for each frame.

PU Statistics percentage of PU modes at each depth.

Performance statistics:

DecideWait ms number of milliseconds the frame encoder had to wait, since the previous frame was retrieved by the API thread, before a new frame has been given to it. This is the latency introduced by slicetype decisions (lookahead).

Row0Wait ms number of milliseconds since the frame encoder received a frame to encode before its first row of CTUs is allowed to begin compression. This is the latency introduced by reference frames making reconstructed and filtered rows available.

Wall time ms number of milliseconds between the first CTU being ready to be compressed and the entire frame being compressed and the output NALs being completed.

Ref Wait Wall ms number of milliseconds between the first reference row being available and the last reference row becoming available.

Total CTU time ms the total time (measured in milliseconds) spent by worker threads compressing and filtering CTUs for this frame.

Stall Time ms the number of milliseconds of the reported wall time that were spent with zero worker threads, aka all compression was completely stalled.

Total frame time Total time spent to encode the frame.

Avg WPP the average number of worker threads working on this frame, at any given time. This value is sampled at the completion of each CTU. This shows the effectiveness of Wavefront Parallel Processing.

Row Blocks the number of times a worker thread had to abandon the row of CTUs it was encoding because the row above it was not far enough ahead for the necessary reference data to be available. This is more of a problem for P frames where some blocks are much more expensive than others.

--csv-log-level <integer>

Controls the level of detail (and size) of –csv log files

1. summary (default)
2. frame level logging
3. frame level logging with performance statistics
--ssim, --no-ssim

Calculate and report Structural Similarity values. It is recommended to use --tune ssim if you are measuring ssim, else the results should not be used for comparison purposes. Default disabled

--psnr, --no-psnr

Calculate and report Peak Signal to Noise Ratio. It is recommended to use --tune psnr if you are measuring PSNR, else the results should not be used for comparison purposes. Default disabled

## Performance Options¶

--asm <integer:false:string>, --no-asm

x265 will use all detected CPU SIMD architectures by default. You can disable all assembly by using --no-asm or you can specify a comma separated list of SIMD architectures to use, matching these strings: MMX2, SSE, SSE2, SSE3, SSSE3, SSE4, SSE4.1, SSE4.2, AVX, XOP, FMA4, AVX2, FMA3

Some higher architectures imply lower ones being present, this is handled implicitly.

One may also directly supply the CPU capability bitmap as an integer.

Note that by specifying this option you are overriding x265’s CPU detection and it is possible to do this wrong. You can cause encoder crashes by specifying SIMD architectures which are not supported on your CPU.

Default: auto-detected SIMD architectures

--frame-threads, -F <integer>

Number of concurrently encoded frames. Using a single frame thread gives a slight improvement in compression, since the entire reference frames are always available for motion compensation, but it has severe performance implications. Default is an autodetected count based on the number of CPU cores and whether WPP is enabled or not.

Over-allocation of frame threads will not improve performance, it will generally just increase memory use.

Values: any value between 0 and 16. Default is 0, auto-detect

--pools <string>, --numa-pools <string>

Comma seperated list of threads per NUMA node. If “none”, then no worker pools are created and only frame parallelism is possible. If NULL or “” (default) x265 will use all available threads on each NUMA node:

'+'  is a special value indicating all cores detected on the node
'*'  is a special value indicating all cores detected on the node and all remaining nodes
'-'  is a special value indicating no cores on the node, same as '0'


example strings for a 4-node system:

""        - default, unspecified, all numa nodes are used for thread pools
"*"       - same as default
"none"    - no thread pools are created, only frame parallelism possible
"-"       - same as "none"
"10"      - allocate one pool, using up to 10 cores on all available nodes
"-,+"     - allocate one pool, using all cores on node 1
"+,-,+"   - allocate one pool, using only cores on nodes 0 and 2
"+,-,+,-" - allocate one pool, using only cores on nodes 0 and 2
"-,*"     - allocate one pool, using all cores on nodes 1, 2 and 3
"8,8,8,8" - allocate four pools with up to 8 threads in each pool
"8,+,+,+" - allocate two pools, the first with 8 threads on node 0, and the second with all cores on node 1,2,3


A thread pool dedicated to a given NUMA node is enabled only when the number of threads to be created on that NUMA node is explicitly mentioned in that corresponding position with the –pools option. Else, all threads are spawned from a single pool. The total number of threads will be determined by the number of threads assigned to the enabled NUMA nodes for that pool. The worker threads are be given affinity to all the enabled NUMA nodes for that pool and may migrate between them, unless explicitly specified as described above.

In the case that any threadpool has more than 64 threads, the threadpool may be broken down into multiple pools of 64 threads each; on 32-bit machines, this number is 32. All pools are given affinity to the NUMA nodes on which the original pool had affinity. For performance reasons, the last thread pool is spawned only if it has more than 32 threads for 64-bit machines, or 16 for 32-bit machines. If the total number of threads in the system doesn’t obey this constraint, we may spawn fewer threads than cores which has been emperically shown to be better for performance.

If the four pool features: --wpp, --pmode, --pme and --lookahead-slices are all disabled, then --pools is ignored and no thread pools are created.

If “none” is specified, then all four of the thread pool features are implicitly disabled.

Frame encoders are distributed between the available thread pools, and the encoder will never generate more thread pools than --frame-threads. The pools are used for WPP and for distributed analysis and motion search.

On Windows, the native APIs offer sufficient functionality to discover the NUMA topology and enforce the thread affinity that libx265 needs (so long as you have not chosen to target XP or Vista), but on POSIX systems it relies on libnuma for this functionality. If your target POSIX system is single socket, then building without libnuma is a perfectly reasonable option, as it will have no effect on the runtime behavior. On a multiple-socket system, a POSIX build of libx265 without libnuma will be less work efficient. See thread pools for more detail.

Default “”, one pool is created across all available NUMA nodes, with one thread allocated per detected hardware thread (logical CPU cores). In the case that the total number of threads is more than the maximum size that ATOMIC operations can handle (32 for 32-bit compiles, and 64 for 64-bit compiles), multiple thread pools may be spawned subject to the performance constraint described above.

Note that the string value will need to be escaped or quoted to protect against shell expansion on many platforms

--wpp, --no-wpp

Enable Wavefront Parallel Processing. The encoder may begin encoding a row as soon as the row above it is at least two CTUs ahead in the encode process. This gives a 3-5x gain in parallelism for about 1% overhead in compression efficiency.

This feature is implicitly disabled when no thread pool is present.

Default: Enabled

--pmode, --no-pmode

Parallel mode decision, or distributed mode analysis. When enabled the encoder will distribute the analysis work of each CU (merge, inter, intra) across multiple worker threads. Only recommended if x265 is not already saturating the CPU cores. In RD levels 3 and 4 it will be most effective if –rect is enabled. At RD levels 5 and 6 there is generally always enough work to distribute to warrant the overhead, assuming your CPUs are not already saturated.

–pmode will increase utilization without reducing compression efficiency. In fact, since the modes are all measured in parallel it makes certain early-outs impractical and thus you usually get slightly better compression when it is enabled (at the expense of not skipping improbable modes). This bypassing of early-outs can cause pmode to slow down encodes, especially at faster presets.

This feature is implicitly disabled when no thread pool is present.

Default disabled

--pme, --no-pme

Parallel motion estimation. When enabled the encoder will distribute motion estimation across multiple worker threads when more than two references require motion searches for a given CU. Only recommended if x265 is not already saturating CPU cores. --pmode is much more effective than this option, since the amount of work it distributes is substantially higher. With –pme it is not unusual for the overhead of distributing the work to outweigh the parallelism benefits.

This feature is implicitly disabled when no thread pool is present.

–pme will increase utilization on many core systems with no effect on the output bitstream.

Default disabled

--preset, -p <integer|string>

Sets parameters to preselected values, trading off compression efficiency against encoding speed. These parameters are applied before all other input parameters are applied, and so you can override any parameters that these values control. See presets for more detail.

1. ultrafast
2. superfast
3. veryfast
4. faster
5. fast
6. medium (default)
7. slow
8. slower
9. veryslow
10. placebo
--tune, -t <string>

Tune the settings for a particular type of source or situation. The changes will be applied after --preset but before all other parameters. Default none. See tunings for more detail.

Values: psnr, ssim, grain, zero-latency, fast-decode.

--slices <integer>

Encode each incoming frame as multiple parallel slices that may be decoded independently. Support available only for rectangular slices that cover the entire width of the image.

Recommended for improving encoder performance only if frame-parallelism and WPP are unable to maximize utilization on given hardware.

Default: 1 slice per frame. Experimental feature

--copy-pic, --no-copy-pic

Allow encoder to copy input x265 pictures to internal frame buffers. When disabled, x265 will not make an internal copy of the input picture and will work with the application’s buffers. While this allows for deeper integration, it is the responsbility of the application to (a) ensure that the allocated picture has extra space for padding that will be done by the library, and (b) the buffers aren’t recycled until the library has completed encoding this frame (which can be figured out by tracking NALs output by x265)

Default: enabled

## Input/Output File Options¶

These options all describe the input video sequence or, in the case of --dither, operations that are performed on the sequence prior to encode. All options dealing with files (names, formats, offsets or frame counts) are only applicable to the CLI application.

--input <filename>

Input filename, only raw YUV or Y4M supported. Use single dash for stdin. This option name will be implied for the first “extra” command line argument.

CLI ONLY

--y4m

Parse input stream as YUV4MPEG2 regardless of file extension, primarily intended for use with stdin (ie: --input - --y4m). This option is implied if the input filename has a “.y4m” extension

CLI ONLY

--input-depth <integer>

YUV only: Bit-depth of input file or stream

Values: any value between 8 and 16. Default is internal depth.

CLI ONLY

--frames <integer>

The number of frames intended to be encoded. It may be left unspecified, but when it is specified rate control can make use of this information. It is also used to determine if an encode is actually a stillpicture profile encode (single frame)

--dither

Enable high quality downscaling to the encoder’s internal bitdepth. Dithering is based on the diffusion of errors from one row of pixels to the next row of pixels in a picture. Only applicable when the input bit depth is larger than 8bits. Default disabled

CLI ONLY

--input-res <wxh>

YUV only: Source picture size [w x h]

CLI ONLY

--input-csp <integer|string>

Chroma Subsampling (YUV only): Only 4:0:0(monochrome), 4:2:0, 4:2:2, and 4:4:4 are supported at this time. The chroma subsampling format of your input must match your desired output chroma subsampling format (libx265 will not perform any chroma subsampling conversion), and it must be supported by the HEVC profile you have specified.

1. i400 (4:0:0 monochrome) - Not supported by Main or Main10 profiles
2. i420 (4:2:0 default) - Supported by all HEVC profiles
3. i422 (4:2:2) - Not supported by Main, Main10 and Main12 profiles
4. i444 (4:4:4) - Supported by Main 4:4:4, Main 4:4:4 10, Main 4:4:4 12, Main 4:4:4 16 Intra profiles
5. nv12
6. nv16
--fps <integer|float|numerator/denominator>

YUV only: Source frame rate

Range of values: positive int or float, or num/denom

--interlace <false|tff|bff>, --no-interlace
1. progressive pictures (default)
2. top field first
3. bottom field first

HEVC encodes interlaced content as fields. Fields must be provided to the encoder in the correct temporal order. The source dimensions must be field dimensions and the FPS must be in units of fields per second. The decoder must re-combine the fields in their correct orientation for display.

--seek <integer>

Number of frames to skip at start of input file. Default 0

CLI ONLY

--frames, -f <integer>

Number of frames of input sequence to be encoded. Default 0 (all)

CLI ONLY

--output, -o <filename>

Bitstream output file name. If there are two extra CLI options, the first is implicitly the input filename and the second is the output filename, making the --output option optional.

The output file will always contain a raw HEVC bitstream, the CLI does not support any container file formats.

CLI ONLY

--output-depth, -D 8|10|12

Bitdepth of output HEVC bitstream, which is also the internal bit depth of the encoder. If the requested bit depth is not the bit depth of the linked libx265, it will attempt to bind libx265_main for an 8bit encoder, libx265_main10 for a 10bit encoder, or libx265_main12 for a 12bit encoder, with the same API version as the linked libx265.

If the output depth is not specified but --profile is specified, the output depth will be derived from the profile name.

CLI ONLY

--chunk-start <integer>

First frame of the chunk. Frames preceeding this in display order will be encoded, however, they will be discarded in the bitstream. This feature can be enabled only in closed GOP structures. Default 0 (disabled).

--chunk-end <integer>

Last frame of the chunk. Frames following this in display order will be used in taking lookahead decisions, but, they will not be encoded. This feature can be enabled only in closed GOP structures. Default 0 (disabled).

## Profile, Level, Tier¶

--profile, -P <string>

Enforce the requirements of the specified profile, ensuring the output stream will be decodable by a decoder which supports that profile. May abort the encode if the specified profile is impossible to be supported by the compile options chosen for the encoder (a high bit depth encoder will be unable to output bitstreams compliant with Main or MainStillPicture).

The following profiles are supported in x265.

8bit profiles:

* main, main-intra, mainstillpicture (or msp for short)
* main444-8, main444-intra, main444-stillpicture


See note below on signaling intra and stillpicture profiles.

10bit profiles:

* main10, main10-intra
* main422-10, main422-10-intra
* main444-10, main444-10-intra


12bit profiles:

* main12, main12-intra
* main422-12, main422-12-intra
* main444-12, main444-12-intra


CLI ONLY

API users must call x265_param_apply_profile() after configuring their param structure. Any changes made to the param structure after this call might make the encode non-compliant.

The CLI application will derive the output bit depth from the profile name if --output-depth is not specified.

--level-idc <integer|float>

Minimum decoder requirement level. Defaults to 0, which implies auto-detection by the encoder. If specified, the encoder will attempt to bring the encode specifications within that specified level. If the encoder is unable to reach the level it issues a warning and aborts the encode. If the requested requirement level is higher than the actual level, the actual requirement level is signaled.

Beware, specifying a decoder level will force the encoder to enable VBV for constant rate factor encodes, which may introduce non-determinism.

The value is specified as a float or as an integer with the level times 10, for example level 5.1 is specified as “5.1” or “51”, and level 5.0 is specified as “5.0” or “50”.

Annex A levels: 1, 2, 2.1, 3, 3.1, 4, 4.1, 5, 5.1, 5.2, 6, 6.1, 6.2, 8.5

--high-tier, --no-high-tier

If --level-idc has been specified, –high-tier allows the support of high tier at that level. The encoder will first attempt to encode at the specified level, main tier first, turning on high tier only if necessary and available at that level.If your requested level does not support a High tier, high tier will not be supported. If –no-high-tier has been specified, then the encoder will attempt to encode only at the main tier.

Default: enabled

--ref <1..16>

Max number of L0 references to be allowed. This number has a linear multiplier effect on the amount of work performed in motion search, but will generally have a beneficial affect on compression and distortion.

Note that x265 allows up to 16 L0 references but the HEVC specification only allows a maximum of 8 total reference frames. So if you have B frames enabled only 7 L0 refs are valid and if you have --b-pyramid enabled (which is enabled by default in all presets), then only 6 L0 refs are the maximum allowed by the HEVC specification. If x265 detects that the total reference count is greater than 8, it will issue a warning that the resulting stream is non-compliant and it signals the stream as profile NONE and level NONE and will abort the encode unless --allow-non-conformance it specified. Compliant HEVC decoders may refuse to decode such streams.

Default 3

--allow-non-conformance, --no-allow-non-conformance

Allow libx265 to generate a bitstream with profile and level NONE. By default it will abort any encode which does not meet strict level compliance. The two most likely causes for non-conformance are --ctu being too small, --ref being too high, or the bitrate or resolution being out of specification.

Default: disabled

--uhd-bd

Enable Ultra HD Blu-ray format support. If specified with incompatible encoding options, the encoder will attempt to modify/set the right encode specifications. If the encoder is unable to do so, this option will be turned OFF. Highly experimental.

Default: disabled

Note

--profile, --level-idc, and --high-tier are only intended for use when you are targeting a particular decoder (or decoders) with fixed resource limitations and must constrain the bitstream within those limits. Specifying a profile or level may lower the encode quality parameters to meet those requirements but it will never raise them. It may enable VBV constraints on a CRF encode.

Also note that x265 determines the decoder requirement profile and level in three steps. First, the user configures an x265_param structure with their suggested encoder options and then optionally calls x265_param_apply_profile() to enforce a specific profile (main, main10, etc). Second, an encoder is created from this x265_param instance and the --level-idc and --high-tier parameters are used to reduce bitrate or other features in order to enforce the target level. Finally, the encoder re-examines the final set of parameters and detects the actual minimum decoder requirement level and this is what is signaled in the bitstream headers. The detected decoder level will only use High tier if the user specified a High tier level.

The signaled profile will be determined by the encoder’s internal bitdepth and input color space. If --keyint is 0 or 1, then an intra variant of the profile will be signaled.

If --total-frames is 1, then a stillpicture variant will be signaled, but this parameter is not always set by applications, particularly not when the CLI uses stdin streaming or when libx265 is used by third-party applications.

## Mode decision / Analysis¶

--rd <1..6>

Level of RDO in mode decision. The higher the value, the more exhaustive the analysis and the more rate distortion optimization is used. The lower the value the faster the encode, the higher the value the smaller the bitstream (in general). Default 3

Note that this table aims for accuracy, but is not necessarily our final target behavior for each mode.

Level Description
0 sa8d mode and split decisions, intra w/ source pixels, currently not supported
1 recon generated (better intra), RDO merge/skip selection
2 RDO splits and merge/skip selection
3 RDO mode and split decisions, chroma residual used for sa8d
4 Currently same as 3
6 Currently same as 5

Range of values: 1: least .. 6: full RDO analysis

Options which affect the coding unit quad-tree, sometimes referred to as the prediction quad-tree.

--ctu, -s <64|32|16>

Maximum CU size (width and height). The larger the maximum CU size, the more efficiently x265 can encode flat areas of the picture, giving large reductions in bitrate. However this comes at a loss of parallelism with fewer rows of CUs that can be encoded in parallel, and less frame parallelism as well. Because of this the faster presets use a CU size of 32. Default: 64

--min-cu-size <32|16|8>

Minimum CU size (width and height). By using 16 or 32 the encoder will not analyze the cost of CUs below that minimum threshold, saving considerable amounts of compute with a predictable increase in bitrate. This setting has a large effect on performance on the faster presets.

Default: 8 (minimum 8x8 CU for HEVC, best compression efficiency)

Note

All encoders within a single process must use the same settings for the CU size range. --ctu and --min-cu-size must be consistent for all of them since the encoder configures several key global data structures based on this range.

--limit-refs <0|1|2|3>

When set to X265_REF_LIMIT_DEPTH (1) x265 will limit the references analyzed at the current depth based on the references used to code the 4 sub-blocks at the next depth. For example, a 16x16 CU will only use the references used to code its four 8x8 CUs.

When set to X265_REF_LIMIT_CU (2), the rectangular and asymmetrical partitions will only use references selected by the 2Nx2N motion search (including at the lowest depth which is otherwise unaffected by the depth limit).

When set to 3 (X265_REF_LIMIT_DEPTH && X265_REF_LIMIT_CU), the 2Nx2N motion search at each depth will only use references from the split CUs and the rect/amp motion searches at that depth will only use the reference(s) selected by 2Nx2N.

For all non-zero values of limit-refs, the current depth will evaluate intra mode (in inter slices), only if intra mode was chosen as the best mode for atleast one of the 4 sub-blocks.

You can often increase the number of references you are using (within your decoder level limits) if you enable one or both of these flags.

Default 3.

--limit-modes, --no-limit-modes

When enabled, limit-modes will limit modes analyzed for each CU using cost metrics from the 4 sub-CUs. When multiple inter modes like --rect and/or --amp are enabled, this feature will use motion cost heuristics from the 4 sub-CUs to bypass modes that are unlikely to be the best choice. This can significantly improve performance when rect and/or --amp are enabled at minimal compression efficiency loss.

--rect, --no-rect

Enable analysis of rectangular motion partitions Nx2N and 2NxN (50/50 splits, two directions). Default disabled

--amp, --no-amp

Enable analysis of asymmetric motion partitions (75/25 splits, four directions). At RD levels 0 through 4, AMP partitions are only considered at CU sizes 32x32 and below. At RD levels 5 and 6, it will only consider AMP partitions as merge candidates (no motion search) at 64x64, and as merge or inter candidates below 64x64.

The AMP partitions which are searched are derived from the current best inter partition. If Nx2N (vertical rectangular) is the best current prediction, then left and right asymmetrical splits will be evaluated. If 2NxN (horizontal rectangular) is the best current prediction, then top and bottom asymmetrical splits will be evaluated, If 2Nx2N is the best prediction, and the block is not a merge/skip, then all four AMP partitions are evaluated.

This setting has no effect if rectangular partitions are disabled. Default disabled

--early-skip, --no-early-skip

Measure 2Nx2N merge candidates first; if no residual is found, additional modes at that depth are not analysed. Default disabled

--rskip, --no-rskip

This option determines early exit from CU depth recursion. When a skip CU is found, additional heuristics (depending on rd-level) are used to decide whether to terminate recursion. In rdlevels 5 and 6, comparison with inter2Nx2N is used, while at rdlevels 4 and neighbour costs are used to skip recursion. Provides minimal quality degradation at good performance gains when enabled.

Default: enabled, disabled for --tune grain

--splitrd-skip, --no-splitrd-skip

Enable skipping split RD analysis when sum of split CU rdCost larger than one split CU rdCost for Intra CU. Default disabled.

--fast-intra, --no-fast-intra

Perform an initial scan of every fifth intra angular mode, then check modes +/- 2 distance from the best mode, then +/- 1 distance from the best mode, effectively performing a gradient descent. When enabled 10 modes in total are checked. When disabled all 33 angular modes are checked. Only applicable for --rd levels 4 and below (medium preset and faster).

--b-intra, --no-b-intra

Enables the evaluation of intra modes in B slices. Default disabled.

--cu-lossless, --no-cu-lossless

For each CU, evaluate lossless (transform and quant bypass) encode of the best non-lossless mode option as a potential rate distortion optimization. If the global option --lossless has been specified, all CUs will be encoded as lossless unconditionally regardless of whether this option was enabled. Default disabled.

Only effective at RD levels 3 and above, which perform RDO mode decisions.

--tskip-fast, --no-tskip-fast

Only evaluate transform skip for NxN intra predictions (4x4 blocks). Only applicable if transform skip is enabled. For chroma, only evaluate if luma used tskip. Inter block tskip analysis is unmodified. Default disabled

--rd-refine, --no-rd-refine

For each analysed CU, calculate R-D cost on the best partition mode for a range of QP values, to find the optimal rounding effect. Default disabled.

Only effective at RD levels 5 and 6

Analysis re-use options, to improve performance when encoding the same sequence multiple times (presumably at varying bitrates). The encoder will not reuse analysis if slice type parameters do not match.

--analysis-save <filename>

Encoder outputs analysis information of each frame. Analysis data from save mode is written to the file specified. Requires cutree, pmode to be off. Default disabled.

--analysis-load <filename>

Encoder reuses analysis information from the file specified. By reading the analysis data writen by an earlier encode of the same sequence, substantial redundant work may be avoided. Requires cutree, pmode to be off. Default disabled.

The amount of analysis data stored/reused is determined by --analysis-reuse-level.

--analysis-reuse-file <filename>

Specify a filename for multi-pass-opt-analysis and multi-pass-opt-distortion. If no filename is specified, x265_analysis.dat is used.

--analysis-reuse-level <1..10>

Amount of information stored/reused in --analysis-reuse-mode is distributed across levels. Higher the value, higher the information stored/reused, faster the encode. Default 5.

Note that –analysis-reuse-level must be paired with analysis-reuse-mode.

Level Description
2 to 4 Level 1 + intra/inter modes, ref’s
5 and 6 Level 2 + rect-amp
7 Level 5 + AVC size CU refinement
8 and 9 Level 5 + AVC size Full CU analysis-info
10 Level 5 + Full CU analysis-info
--refine-mv-type <string>

Reuse MV information received through API call. Currently receives information for AVC size and the accepted string input is “avc”. Default is disabled.

--scale-factor

Factor by which input video is scaled down for analysis save mode. This option should be coupled with analysis-reuse-mode option, –analysis-reuse-level 10. The ctu size of load can either be the same as that of save or double the size of save. Default 0.

--refine-intra <0..4>

Enables refinement of intra blocks in current encode.

Level 0 - Forces both mode and depth from the save encode.

Level 1 - Evaluates all intra modes at current depth(n) and at depth (n+1) when current block size is one greater than the min-cu-size. Forces modes for larger blocks.

Level 2 - In addition to the functionality of level 1, at all depths, force (a) only depth when angular mode is chosen by the save encode. (b) depth and mode when other intra modes are chosen by the save encode.

Level 3 - Perform analysis of intra modes for depth reused from first encode.

Level 4 - Does not reuse any analysis information - redo analysis for the intra block.

Default 0.

--refine-inter <0..3>

Enables refinement of inter blocks in current encode.

Level 0 - Forces both mode and depth from the save encode.

Level 1 - Evaluates all inter modes at current depth(n) and at depth (n+1) when current block size is one greater than the min-cu-size. Forces modes for larger blocks.

Level 2 - In addition to the functionality of level 1, restricts the modes evaluated when specific modes are decided as the best mode by the save encode.

2nx2n in save encode - disable re-evaluation of rect and amp.

skip in save encode - re-evaluates only skip, merge and 2nx2n modes.

Level 3 - Perform analysis of inter modes while reusing depths from the save encode.

Default 0.

--dynamic-refine, --no-dynamic-refine

Dynamically switches --refine-inter levels 0-3 based on the content and the encoder settings. It is recommended to use --refine-intra 4 with dynamic refinement. Default disabled.

--refine-mv

Enables refinement of motion vector for scaled video. Evaluates the best motion vector by searching the surrounding eight integer and subpel pixel positions.

Options which affect the transform unit quad-tree, sometimes referred to as the residual quad-tree (RQT).

--rdoq-level <0|1|2>, --no-rdoq-level

Specify the amount of rate-distortion analysis to use within quantization:

At level 0 rate-distortion cost is not considered in quant

At level 1 rate-distortion cost is used to find optimal rounding values for each level (and allows psy-rdoq to be effective). It trades-off the signaling cost of the coefficient vs its post-inverse quant distortion from the pre-quant coefficient. When --psy-rdoq is enabled, this formula is biased in favor of more energy in the residual (larger coefficient absolute levels)

At level 2 rate-distortion cost is used to make decimate decisions on each 4x4 coding group, including the cost of signaling the group within the group bitmap. If the total distortion of not signaling the entire coding group is less than the rate cost, the block is decimated. Next, it applies rate-distortion cost analysis to the last non-zero coefficient, which can result in many (or all) of the coding groups being decimated. Psy-rdoq is less effective at preserving energy when RDOQ is at level 2, since it only has influence over the level distortion costs.

--tu-intra-depth <1..4>

The transform unit (residual) quad-tree begins with the same depth as the coding unit quad-tree, but the encoder may decide to further split the transform unit tree if it improves compression efficiency. This setting limits the number of extra recursion depth which can be attempted for intra coded units. Default: 1, which means the residual quad-tree is always at the same depth as the coded unit quad-tree

Note that when the CU intra prediction is NxN (only possible with 8x8 CUs), a TU split is implied, and thus the residual quad-tree begins at 4x4 and cannot split any futhrer.

--tu-inter-depth <1..4>

The transform unit (residual) quad-tree begins with the same depth as the coding unit quad-tree, but the encoder may decide to further split the transform unit tree if it improves compression efficiency. This setting limits the number of extra recursion depth which can be attempted for inter coded units. Default: 1. which means the residual quad-tree is always at the same depth as the coded unit quad-tree unless the CU was coded with rectangular or AMP partitions, in which case a TU split is implied and thus the residual quad-tree begins one layer below the CU quad-tree.

--limit-tu <0..4>

Enables early exit from TU depth recursion, for inter coded blocks.

Level 1 - decides to recurse to next higher depth based on cost comparison of full size TU and split TU.

Level 2 - based on first split subTU’s depth, limits recursion of other split subTUs.

Level 3 - based on the average depth of the co-located and the neighbor CUs’ TU depth, limits recursion of the current CU.

Level 4 - uses the depth of the neighbouring/ co-located CUs TU depth to limit the 1st subTU depth. The 1st subTU depth is taken as the limiting depth for the other subTUs.

Enabling levels 3 or 4 may cause a mismatch in the output bitstreams between option:–analysis-save and option:–analysis-load as all neighbouring CUs TU depth may not be available in the option:–analysis-load run as only the best mode’s information is available to it.

Default: 0

--nr-intra <integer>, --nr-inter <integer>

Noise reduction - an adaptive deadzone applied after DCT (subtracting from DCT coefficients), before quantization. It does no pixel-level filtering, doesn’t cross DCT block boundaries, has no overlap, The higher the strength value parameter, the more aggressively it will reduce noise.

Enabling noise reduction will make outputs diverge between different numbers of frame threads. Outputs will be deterministic but the outputs of -F2 will no longer match the outputs of -F3, etc.

Values: any value in range of 0 to 2000. Default 0 (disabled).

--tskip, --no-tskip

Enable evaluation of transform skip (bypass DCT but still use quantization) coding for 4x4 TU coded blocks.

Only effective at RD levels 3 and above, which perform RDO mode decisions. Default disabled

--rdpenalty <0..2>

When set to 1, transform units of size 32x32 are given a 4x bit cost penalty compared to smaller transform units, in intra coded CUs in P or B slices.

When set to 2, transform units of size 32x32 are not even attempted, unless otherwise required by the maximum recursion depth. For this option to be effective with 32x32 intra CUs, --tu-intra-depth must be at least 2. For it to be effective with 64x64 intra CUs, --tu-intra-depth must be at least 3.

Note that in HEVC an intra transform unit (a block of the residual quad-tree) is also a prediction unit, meaning that the intra prediction signal is generated for each TU block, the residual subtracted and then coded. The coding unit simply provides the prediction modes that will be used when predicting all of the transform units within the CU. This means that when you prevent 32x32 intra transform units, you are preventing 32x32 intra predictions.

Default 0, disabled.

Values: 0:disabled 1:4x cost penalty 2:force splits

--max-tu-size <32|16|8|4>

Maximum TU size (width and height). The residual can be more efficiently compressed by the DCT transform when the max TU size is larger, but at the expense of more computation. Transform unit quad-tree begins at the same depth of the coded tree unit, but if the maximum TU size is smaller than the CU size then transform QT begins at the depth of the max-tu-size. Default: 32.

--dynamic-rd <0..4>

Increases the RD level at points where quality drops due to VBV rate control enforcement. The number of CUs for which the RD is reconfigured is determined based on the strength. Strength 1 gives the best FPS, strength 4 gives the best SSIM. Strength 0 switches this feature off. Default: 0.

Effective for RD levels 4 and below.

--ssim-rd, --no-ssim-rd

Enable/Disable SSIM RDO. SSIM is a better perceptual quality assessment method as compared to MSE. SSIM based RDO calculation is based on residual divisive normalization scheme. This normalization is consistent with the luminance and contrast masking effect of Human Visual System. It is used for mode selection during analysis of CTUs and can achieve significant gain in terms of objective quality metrics SSIM and PSNR. It only has effect on presets which use RDO-based mode decisions (--rd 3 and above).

## Temporal / motion search options¶

--max-merge <1..5>

Maximum number of neighbor (spatial and temporal) candidate blocks that the encoder may consider for merging motion predictions. If a merge candidate results in no residual, it is immediately selected as a “skip”. Otherwise the merge candidates are tested as part of motion estimation when searching for the least cost inter option. The max candidate number is encoded in the SPS and determines the bit cost of signaling merge CUs. Default 2

--me <integer|string>

Motion search method. Generally, the higher the number the harder the ME method will try to find an optimal match. Diamond search is the simplest. Hexagon search is a little better. Uneven Multi-Hexegon is an adaption of the search method used by x264 for slower presets. Star is a three step search adapted from the HM encoder: a star-pattern search followed by an optional radix scan followed by an optional star-search refinement. Full is an exhaustive search; an order of magnitude slower than all other searches but not much better than umh or star. SEA is similar to FULL search; a three step motion search adopted from x264: DC calculation followed by ADS calculation followed by SAD of the passed motion vector candidates, hence faster than Full search.

1. dia
2. hex (default)
3. umh
4. star
5. sea
6. full
--subme, -m <0..7>

Amount of subpel refinement to perform. The higher the number the more subpel iterations and steps are performed. Default 2

-m HPEL iters HPEL dirs QPEL iters QPEL dirs HPEL SATD
0 1 4 0 4 false
1 1 4 1 4 false
2 1 4 1 4 true
3 2 4 1 4 true
4 2 4 2 4 true
5 1 8 1 8 true
6 2 8 1 8 true
7 2 8 2 8 true

At –subme values larger than 2, chroma residual cost is included in all subpel refinement steps and chroma residual is included in all motion estimation decisions (selecting the best reference picture in each list, and chosing between merge, uni-directional motion and bi-directional motion). The ‘slow’ preset is the first preset to enable the use of chroma residual.

--merange <integer>

Motion search range. Default 57

The default is derived from the default CTU size (64) minus the luma interpolation half-length (4) minus maximum subpel distance (2) minus one extra pixel just in case the hex search method is used. If the search range were any larger than this, another CTU row of latency would be required for reference frames.

Range of values: an integer from 0 to 32768

--temporal-mvp, --no-temporal-mvp

Enable temporal motion vector predictors in P and B slices. This enables the use of the motion vector from the collocated block in the previous frame to be used as a predictor. Default is enabled

--weightp, -w, --no-weightp

Enable weighted prediction in P slices. This enables weighting analysis in the lookahead, which influences slice decisions, and enables weighting analysis in the main encoder which allows P reference samples to have a weight function applied to them prior to using them for motion compensation. In video which has lighting changes, it can give a large improvement in compression efficiency. Default is enabled

--weightb, --no-weightb

Enable weighted prediction in B slices. Default disabled

--analyze-src-pics, --no-analyze-src-pics

Enable motion estimation with source frame pixels, in this mode, motion estimation can be computed independently. Default disabled.

## Spatial/intra options¶

--strong-intra-smoothing, --no-strong-intra-smoothing

Enable strong intra smoothing for 32x32 intra blocks. This flag performs bi-linear interpolation of the corner reference samples for a strong smoothing effect. The purpose is to prevent blocking or banding artifacts in regions with few/zero AC coefficients. Default enabled

--constrained-intra, --no-constrained-intra

Constrained intra prediction. When generating intra predictions for blocks in inter slices, only intra-coded reference pixels are used. Inter-coded reference pixels are replaced with intra-coded neighbor pixels or default values. The general idea is to block the propagation of reference errors that may have resulted from lossy signals. Default disabled

## Psycho-visual options¶

Left to its own devices, the encoder will make mode decisions based on a simple rate distortion formula, trading distortion for bitrate. This is generally effective except for the manner in which this distortion is measured. It tends to favor blurred reconstructed blocks over blocks which have wrong motion. The human eye generally prefers the wrong motion over the blur and thus x265 offers psycho-visual adjustments to the rate distortion algorithm.

--psy-rd will add an extra cost to reconstructed blocks which do not match the visual energy of the source block. The higher the strength of --psy-rd the more strongly it will favor similar energy over blur and the more aggressively it will ignore rate distortion. If it is too high, it will introduce visal artifacts and increase bitrate enough for rate control to increase quantization globally, reducing overall quality. psy-rd will tend to reduce the use of blurred prediction modes, like DC and planar intra and bi-directional inter prediction.

--psy-rdoq will adjust the distortion cost used in rate-distortion optimized quantization (RDO quant), enabled by --rdoq-level 1 or 2, favoring the preservation of energy in the reconstructed image. --psy-rdoq prevents RDOQ from blurring all of the encoding options which psy-rd has to chose from. At low strength levels, psy-rdoq will influence the quantization level decisions, favoring higher AC energy in the reconstructed image. As psy-rdoq strength is increased, more non-zero coefficient levels are added and fewer coefficients are zeroed by RDOQ’s rate distortion analysis. High levels of psy-rdoq can double the bitrate which can have a drastic effect on rate control, forcing higher overall QP, and can cause ringing artifacts. psy-rdoq is less accurate than psy-rd, it is biasing towards energy in general while psy-rd biases towards the energy of the source image. But very large psy-rdoq values can sometimes be beneficial.

As a general rule, when both psycho-visual features are disabled, the encoder will tend to blur blocks in areas of difficult motion. Turning on small amounts of psy-rd and psy-rdoq will improve the perceived visual quality. Increasing psycho-visual strength further will improve quality and begin introducing artifacts and increase bitrate, which may force rate control to increase global QP. Finding the optimal psycho-visual parameters for a given video requires experimentation. Our recommended defaults (1.0 for both) are generally on the low end of the spectrum.

The lower the bitrate, the lower the optimal psycho-visual settings. If the bitrate is too low for the psycho-visual settings, you will begin to see temporal artifacts (motion judder). This is caused when the encoder is forced to code skip blocks (no residual) in areas of difficult motion because it is the best option psycho-visually (they have great amounts of energy and no residual cost). One can lower psy-rd settings when judder is happening, and allow the encoder to use some blur in these areas of high motion.

--psy-rd <float>

Influence rate distortion optimizated mode decision to preserve the energy of the source image in the encoded image at the expense of compression efficiency. It only has effect on presets which use RDO-based mode decisions (--rd 3 and above). 1.0 is a typical value. Default 2.0

Range of values: 0 .. 5.0

--psy-rdoq <float>

Influence rate distortion optimized quantization by favoring higher energy in the reconstructed image. This generally improves perceived visual quality at the cost of lower quality metric scores. It only has effect when --rdoq-level is 1 or 2. High values can be beneficial in preserving high-frequency detail. Default: 0.0 (1.0 for presets slow, slower, veryslow)

Range of values: 0 .. 50.0

## Slice decision options¶

--open-gop, --no-open-gop

Enable open GOP, allow I-slices to be non-IDR. Default enabled

--keyint, -I <integer>

Max intra period in frames. A special case of infinite-gop (single keyframe at the beginning of the stream) can be triggered with argument -1. Use 1 to force all-intra. When intra-refresh is enabled it specifies the interval between which refresh sweeps happen. Default 250

--min-keyint, -i <integer>

Minimum GOP size. Scenecuts beyond this interval are coded as IDR and start a new keyframe, while scenecuts closer together are coded as I or P. For fixed keyframe interval, set value to be equal to keyint.

Range of values: >=0 (0: auto)

--scenecut <integer>, --no-scenecut

How aggressively I-frames need to be inserted. The higher the threshold value, the more aggressive the I-frame placement. --scenecut 0 or --no-scenecut disables adaptive I frame placement. Default 40

--scenecut-bias <0..100.0>

This value represents the percentage difference between the inter cost and intra cost of a frame used in scenecut detection. For example, a value of 5 indicates, if the inter cost of a frame is greater than or equal to 95 percent of the intra cost of the frame, then detect this frame as scenecut. Values between 5 and 15 are recommended. Default 5.

--radl <integer>

Number of RADL pictures allowed infront of IDR. Requires fixed keyframe interval. Recommended value is 2-3. Default 0 (disabled).

**Range of values: Between 0 and –bframes

--ctu-info <0, 1, 2, 4, 6>

This value enables receiving CTU information asynchronously and determine reaction to the CTU information. Default 0. 1: force the partitions if CTU information is present. 2: functionality of (1) and reduce qp if CTU information has changed. 4: functionality of (1) and force Inter modes when CTU Information has changed, merge/skip otherwise. This option should be enabled only when planning to invoke the API function x265_encoder_ctu_info to copy ctu-info asynchronously. If enabled without calling the API function, the encoder will wait indefinitely.

--intra-refresh

Enables Periodic Intra Refresh(PIR) instead of keyframe insertion. PIR can replace keyframes by inserting a column of intra blocks in non-keyframes, that move across the video from one side to the other and thereby refresh the image but over a period of multiple frames instead of a single keyframe.

--rc-lookahead <integer>

Number of frames for slice-type decision lookahead (a key determining factor for encoder latency). The longer the lookahead buffer the more accurate scenecut decisions will be, and the more effective cuTree will be at improving adaptive quant. Having a lookahead larger than the max keyframe interval is not helpful. Default 20

Range of values: Between the maximum consecutive bframe count (--bframes) and 250

--gop-lookahead <integer>

Number of frames for GOP boundary decision lookahead. If a scenecut frame is found within this from the gop boundary set by –keyint, the GOP will be extented until such a point, otherwise the GOP will be terminated as set by –keyint. Default 0.

Range of values: Between 0 and (–rc-lookahead - mini-GOP length)

It is recommended to have –gop-lookahaed less than –min-keyint as scenecuts beyond –min-keyint are already being coded as keyframes.

--lookahead-slices <0..16>

Use multiple worker threads to measure the estimated cost of each frame within the lookahead. The frame is divided into the specified number of slices, and one-thread is launched per slice. When --b-adapt is 2, most frame cost estimates will be performed in batch mode (many cost estimates at the same time) and lookahead-slices is ignored for batched estimates; it may still be used for single cost estimations. The higher this parameter, the less accurate the frame costs will be (since context is lost across slice boundaries) which will result in less accurate B-frame and scene-cut decisions. The effect on performance can be significant especially on systems with many threads.

The encoder may internally lower the number of slices or disable slicing to ensure each slice codes at least 10 16x16 rows of lowres blocks to minimize the impact on quality. For example, for 720p and 1080p videos, the number of slices is capped to 4 and 6, respectively. For resolutions lesser than 720p, slicing is auto-disabled.

If slices are used in lookahead, they are logged in the list of tools as lslices

Values: 0 - disabled. 1 is the same as 0. Max 16. Default: 8 for ultrafast, superfast, faster, fast, medium

4 for slow, slower disabled for veryslow, slower
--lookahead-threads <integer>

Values: 0 - disabled(default). Max - Half of available hardware threads.

--b-adapt <integer>

Set the level of effort in determining B frame placement.

With b-adapt 0, the GOP structure is fixed based on the values of --keyint and --bframes.

With b-adapt 1 a light lookahead is used to choose B frame placement.

With b-adapt 2 (trellis) a viterbi B path selection is performed

Values: 0:none; 1:fast; 2:full(trellis) default

--bframes, -b <0..16>

Maximum number of consecutive b-frames. Use --bframes 0 to force all P/I low-latency encodes. Default 4. This parameter has a quadratic effect on the amount of memory allocated and the amount of work performed by the full trellis version of --b-adapt lookahead.

--bframe-bias <integer>

Bias towards B frames in slicetype decision. The higher the bias the more likely x265 is to use B frames. Can be any value between -90 and 100 and is clipped to that range. Default 0

--b-pyramid, --no-b-pyramid

Use B-frames as references, when possible. Default enabled

--force-flush <integer>

Force the encoder to flush frames. Default is 0.

Values: 0 - flush the encoder only when all the input pictures are over. 1 - flush all the frames even when the input is not over.

slicetype decision may change with this option.

2 - flush the slicetype decided frames only.

## Quality, rate control and rate distortion options¶

--bitrate <integer>

Enables single-pass ABR rate control. Specify the target bitrate in kbps. Default is 0 (CRF)

Range of values: An integer greater than 0

--crf <0..51.0>

Quality-controlled variable bitrate. CRF is the default rate control method; it does not try to reach any particular bitrate target, instead it tries to achieve a given uniform quality and the size of the bitstream is determined by the complexity of the source video. The higher the rate factor the higher the quantization and the lower the quality. Default rate factor is 28.0.

--crf-max <0..51.0>

Specify an upper limit to the rate factor which may be assigned to any given frame (ensuring a max QP). This is dangerous when CRF is used in combination with VBV as it may result in buffer underruns. Default disabled

--crf-min <0..51.0>

Specify an lower limit to the rate factor which may be assigned to any given frame (ensuring a min compression factor).

--vbv-bufsize <integer>

Specify the size of the VBV buffer (kbits). Enables VBV in ABR mode. In CRF mode, --vbv-maxrate must also be specified. Default 0 (vbv disabled)

--vbv-maxrate <integer>

Maximum local bitrate (kbits/sec). Will be used only if vbv-bufsize is also non-zero. Both vbv-bufsize and vbv-maxrate are required to enable VBV in CRF mode. Default 0 (disabled)

Note that when VBV is enabled (with a valid --vbv-bufsize), VBV emergency denoising is turned on. This will turn on aggressive denoising at the frame level when frame QP > QP_MAX_SPEC (51), drastically reducing bitrate and allowing ratecontrol to assign lower QPs for the following frames. The visual effect is blurring, but removes significant blocking/displacement artifacts.

--vbv-init <float>

Initial buffer occupancy. The portion of the decode buffer which must be full before the decoder will begin decoding. Determines absolute maximum frame size. May be specified as a fractional value between 0 and 1, or in kbits. In other words these two option pairs are equivalent:

--vbv-bufsize 1000 --vbv-init 900
--vbv-bufsize 1000 --vbv-init 0.9


Default 0.9

Range of values: fractional: 0 - 1.0, or kbits: 2 .. bufsize

--vbv-end <float>

Final buffer emptiness. The portion of the decode buffer that must be available after all the specified frames have been inserted into the decode buffer. Specified as a fractional value between 0 and 1, or in kbits. Default 0 (disabled)

This enables basic support for chunk-parallel encoding where each segment can specify the starting and ending state of the VBV buffer so that VBV compliance can be maintained when chunks are independently encoded and stitched together.

--vbv-end-fr-adj <float>

Frame from which qp has to be adjusted to achieve final decode buffer emptiness. Specified as a fraction of the total frames. Fractions > 0 are supported only when the total number of frames is known. Default 0.

--qp, -q <integer>

Specify base quantization parameter for Constant QP rate control. Using this option enables Constant QP rate control. The specified QP is assigned to P slices. I and B slices are given QPs relative to P slices using param->rc.ipFactor and param->rc.pbFactor unless QP 0 is specified, in which case QP 0 is used for all slice types. Note that QP 0 does not cause lossless encoding, it only disables quantization. Default disabled.

Range of values: an integer from 0 to 51

--lossless, --no-lossless

Enables true lossless coding by bypassing scaling, transform, quantization and in-loop filter processes. This is used for ultra-high bitrates with zero loss of quality. Reconstructed output pictures are bit-exact to the input pictures. Lossless encodes implicitly have no rate control, all rate control options are ignored. Slower presets will generally achieve better compression efficiency (and generate smaller bitstreams). Default disabled.

--aq-mode <0|1|2|3>

Adaptive Quantization operating mode. Raise or lower per-block quantization based on complexity analysis of the source image. The more complex the block, the more quantization is used. This offsets the tendency of the encoder to spend too many bits on complex areas and not enough in flat areas.

1. disabled
2. AQ enabled (default)
3. AQ enabled with auto-variance

3. AQ enabled with auto-variance and bias to dark scenes. This is recommended for 8-bit encodes or low-bitrate 10-bit encodes, to prevent color banding/blocking.

--aq-strength <float>

Adjust the strength of the adaptive quantization offsets. Setting --aq-strength to 0 disables AQ. At aq-modes 2 and 3, high aq-strengths will lead to high QP offsets resulting in a large difference in achieved bitrates.

Default 1.0. Range of values: 0.0 to 3.0

--aq-motion, --no-aq-motion

Adjust the AQ offsets based on the relative motion of each block with respect to the motion of the frame. The more the relative motion of the block, the more quantization is used. Default disabled. Experimental Feature

--qg-size <64|32|16|8>

Enable adaptive quantization for sub-CTUs. This parameter specifies the minimum CU size at which QP can be adjusted, ie. Quantization Group size. Allowed range of values are 64, 32, 16, 8 provided this falls within the inclusive range [maxCUSize, minCUSize]. Default: same as maxCUSize

--cutree, --no-cutree

Enable the use of lookahead’s lowres motion vector fields to determine the amount of reuse of each block to tune adaptive quantization factors. CU blocks which are heavily reused as motion reference for later frames are given a lower QP (more bits) while CU blocks which are quickly changed and are not referenced are given less bits. This tends to improve detail in the backgrounds of video with less detail in areas of high motion. Default enabled

--pass <integer>

Enable multi-pass rate control mode. Input is encoded multiple times, storing the encoded information of each pass in a stats file from which the consecutive pass tunes the qp of each frame to improve the quality of the output. Default disabled

1. First pass, creates stats file
2. Last pass, does not overwrite stats file
3. Nth pass, overwrites stats file

Range of values: 1 to 3

--stats <filename>

Specify file name of of the multi-pass stats file. If unspecified the encoder will use x265_2pass.log

--slow-firstpass, --no-slow-firstpass

Enable first pass encode with the exact settings specified. The quality in subsequent multi-pass encodes is better (compared to first pass) when the settings match across each pass. Default enabled.

When slow first pass is disabled, a turbo encode with the following go-fast options is used to improve performance:

--multi-pass-opt-analysis, --no-multi-pass-opt-analysis

Enable/Disable multipass analysis refinement along with multipass ratecontrol. Based on the information stored in pass 1, in subsequent passes analysis data is refined and also redundant steps are skipped. In pass 1 analysis information like motion vector, depth, reference and prediction modes of the final best CTU partition is stored for each CTU. Multipass analysis refinement cannot be enabled when ‘analysis-save/analysis-load’ option is enabled and both will be disabled when enabled together. This feature requires ‘pmode/pme’ to be disabled and hence pmode/pme will be disabled when enabled at the same time.

Default: disabled.

--multi-pass-opt-distortion, --no-multi-pass-opt-distortion

Enable/Disable multipass refinement of qp based on distortion data along with multipass ratecontrol. In pass 1 distortion of best CTU partition is stored. CTUs with high distortion get lower(negative)qp offsets and vice-versa for low distortion CTUs in pass 2. This helps to improve the subjective quality. Multipass refinement of qp cannot be enabled when ‘analysis-save/analysis-load’ option is enabled and both will be disabled when enabled together. ‘multi-pass-opt-distortion’ requires ‘pmode/pme’ to be disabled and hence pmode/pme will be disabled when enabled along with it.

Default: disabled.

--strict-cbr, --no-strict-cbr

Enables stricter conditions to control bitrate deviance from the target bitrate in ABR mode. Bit rate adherence is prioritised over quality. Rate tolerance is reduced to 50%. Default disabled.

This option is for use-cases which require the final average bitrate to be within very strict limits of the target; preventing overshoots, while keeping the bit rate within 5% of the target setting, especially in short segment encodes. Typically, the encoder stays conservative, waiting until there is enough feedback in terms of encoded frames to control QP. strict-cbr allows the encoder to be more aggressive in hitting the target bitrate even for short segment videos.

--cbqpoffs <integer>

Offset of Cb chroma QP from the luma QP selected by rate control. This is a general way to spend more or less bits on the chroma channel. Default 0

Range of values: -12 to 12

--crqpoffs <integer>

Offset of Cr chroma QP from the luma QP selected by rate control. This is a general way to spend more or less bits on the chroma channel. Default 0

Range of values: -12 to 12

--ipratio <float>

QP ratio factor between I and P slices. This ratio is used in all of the rate control modes. Some --tune options may change the default value. It is not typically manually specified. Default 1.4

--pbratio <float>

QP ratio factor between P and B slices. This ratio is used in all of the rate control modes. Some --tune options may change the default value. It is not typically manually specified. Default 1.3

--qcomp <float>

qComp sets the quantizer curve compression factor. It weights the frame quantizer based on the complexity of residual (measured by lookahead). It’s value must be between 0.5 and 1.0. Default value is 0.6. Increasing it to 1.0 will effectively generate CQP.

--qpstep <integer>

The maximum single adjustment in QP allowed to rate control. Default 4

--qpmin <integer>

sets a hard lower limit on QP allowed to ratecontrol. Default 0

--qpmax <integer>

sets a hard upper limit on QP allowed to ratecontrol. Default 69

--rc-grain, --no-rc-grain

Enables a specialised ratecontrol algorithm for film grain content. This parameter strictly minimises QP fluctuations within and across frames and removes pulsing of grain. Default disabled. Enabled when :option:’–tune’ grain is applied. It is highly recommended that this option is used through the tune grain feature where a combination of param options are used to improve visual quality.

--const-vbv, --no-const-vbv

Enables VBV algorithm to be consistent across runs. Default disabled. Enabled when :option:’–tune’ grain is applied.

--qblur <float>

Temporally blur quants. Default 0.5

--cplxblur <float>

temporally blur complexity. default 20

--zones <zone0>/<zone1>/...

Tweak the bitrate of regions of the video. Each zone takes the form:

<start frame>,<end frame>,<option> where <option> is either q=<integer> (force QP) or b=<float> (bitrate multiplier).

If zones overlap, whichever comes later in the list takes precedence. Default none

## Quantization Options¶

Note that rate-distortion optimized quantization (RDOQ) is enabled implicitly at --rd 4, 5, and 6 and disabled implicitly at all other levels.

--signhide, --no-signhide

Hide sign bit of one coeff per TU (rdo). The last sign is implied. This requires analyzing all the coefficients to determine if a sign must be toggled, and then to determine which one can be toggled with the least amount of distortion. Default enabled

--qpfile <filename>

Specify a text file which contains frametypes and QPs for some or all frames. The format of each line is:

Frametype can be one of [I,i,K,P,B,b]. B is a referenced B frame, b is an unreferenced B frame. I is a keyframe (random access point) while i is an I frame that is not a keyframe (references are not broken). K implies I if closed_gop option is enabled, and i otherwise.

Specifying QP (integer) is optional, and if specified they are clamped within the encoder to qpmin/qpmax.

--scaling-list <filename>

Quantization scaling lists. HEVC supports 6 quantization scaling lists to be defined; one each for Y, Cb, Cr for intra prediction and one each for inter prediction.

x265 does not use scaling lists by default, but this can also be made explicit by --scaling-list off.

HEVC specifies a default set of scaling lists which may be enabled without requiring them to be signaled in the SPS. Those scaling lists can be enabled via --scaling-list default.

All other strings indicate a filename containing custom scaling lists in the HM format. The encode will abort if the file is not parsed correctly. Custom lists must be signaled in the SPS. A sample scaling list file is available in the downloads page

--lambda-file <filename>

Specify a text file containing values for x265_lambda_tab and x265_lambda2_tab. Each table requires MAX_MAX_QP+1 (70) float values.

The text file syntax is simple. Comma is considered to be white-space. All white-space is ignored. Lines must be less than 2k bytes in length. Content following hash (#) characters are ignored. The values read from the file are logged at --log-level debug.

Note that the lambda tables are process-global and so the new values affect all encoders running in the same process.

Lambda values affect encoder mode decisions, the lower the lambda the more bits it will try to spend on signaling information (motion vectors and splits) and less on residual. This feature is intended for experimentation.

--max-ausize-factor <float>

It controls the maximum AU size defined in specification. It represents the percentage of maximum AU size used. Default is 1. Range is 0.5 to 1.

## Loop filters¶

--deblock=<int>:<int>, --no-deblock

Toggle deblocking loop filter, optionally specify deblocking strength offsets.

<int>:<int> - parsed as tC offset and Beta offset <int>,<int> - parsed as tC offset and Beta offset <int> - both tC and Beta offsets assigned the same value

If unspecified, the offsets default to 0. The offsets must be in a range of -6 (lowest strength) to 6 (highest strength).

To disable the deblocking filter entirely, use –no-deblock or –deblock=false. Default enabled, with both offsets defaulting to 0

If deblocking is disabled, or the offsets are non-zero, these changes from the default configuration are signaled in the PPS.

--sao, --no-sao

Toggle Sample Adaptive Offset loop filter, default enabled

--sao-non-deblock, --no-sao-non-deblock

Specify how to handle depencency between SAO and deblocking filter. When enabled, non-deblocked pixels are used for SAO analysis. When disabled, SAO analysis skips the right/bottom boundary areas. Default disabled

--limit-sao, --no-limit-sao

Limit SAO filter computation by early terminating SAO process based on inter prediction mode, CTU spatial-domain correlations, and relations between luma and chroma. Default disabled

## VUI (Video Usability Information) options¶

x265 emits a VUI with only the timing info by default. If the SAR is specified (or read from a Y4M header) it is also included. All other VUI fields must be manually specified.

--sar <integer|w:h>

Sample Aspect Ratio, the ratio of width to height of an individual sample (pixel). The user may supply the width and height explicitly or specify an integer from the predefined list of aspect ratios defined in the HEVC specification. Default undefined (not signaled)

1. 1:1 (square)
2. 12:11
3. 10:11
4. 16:11
5. 40:33
6. 24:11
7. 20:11
8. 32:11
9. 80:33
10. 18:11
11. 15:11
12. 64:33
13. 160:99
14. 4:3
15. 3:2
16. 2:1
--display-window <left,top,right,bottom>

Define the (overscan) region of the image that does not contain information because it was added to achieve certain resolution or aspect ratio (the areas are typically black bars). The decoder may be directed to crop away this region before displaying the images via the --overscan option. Default undefined (not signaled).

Note that this has nothing to do with padding added internally by the encoder to ensure the pictures size is a multiple of the minimum coding unit (4x4). That padding is signaled in a separate “conformance window” and is not user-configurable.

--overscan <show|crop>

Specify whether it is appropriate for the decoder to display or crop the overscan area. Default unspecified (not signaled)

--videoformat <integer|string>

Specify the source format of the original analog video prior to digitizing and encoding. Default undefined (not signaled)

1. component
2. pal
3. ntsc
4. secam
5. mac
6. undefined
--range <full|limited>

Specify output range of black level and range of luma and chroma signals. Default undefined (not signaled)

--colorprim <integer|string>

Specify color primaries to use when converting to RGB. Default undefined (not signaled)

1. bt709
2. unknown
3. reserved
4. bt470m
5. bt470bg
6. smpte170m
7. smpte240m
8. film
9. bt2020
10. smpte428
11. smpte431
12. smpte432
--transfer <integer|string>

Specify transfer characteristics. Default undefined (not signaled)

1. bt709
2. unknown
3. reserved
4. bt470m
5. bt470bg
6. smpte170m
7. smpte240m
8. linear
9. log100
10. log316
11. iec61966-2-4
12. bt1361e
13. iec61966-2-1
14. bt2020-10
15. bt2020-12
16. smpte2084
17. smpte428
18. arib-std-b67
--colormatrix <integer|string>

Specify color matrix setting i.e set the matrix coefficients used in deriving the luma and chroma. Default undefined (not signaled)

1. GBR
2. bt709
3. undef
4. reserved
5. fcc
6. bt470bg
7. smpte170m
8. smpte240m
9. YCgCo
10. bt2020nc
11. bt2020c
12. smpte2085
13. chroma-derived-nc
14. chroma-derived-c
15. ictcp
--chromaloc <0..5>

Specify chroma sample location for 4:2:0 inputs. Consult the HEVC specification for a description of these values. Default undefined (not signaled)

--master-display <string>

SMPTE ST 2086 mastering display color volume SEI info, specified as a string which is parsed when the stream header SEI are emitted. The string format is “G(%hu,%hu)B(%hu,%hu)R(%hu,%hu)WP(%hu,%hu)L(%u,%u)” where %hu are unsigned 16bit integers and %u are unsigned 32bit integers. The SEI includes X,Y display primaries for RGB channels and white point (WP) in units of 0.00002 and max,min luminance (L) values in units of 0.0001 candela per meter square. Applicable for HDR content.

Example for a P3D65 1000-nits monitor, where G(x=0.265, y=0.690), B(x=0.150, y=0.060), R(x=0.680, y=0.320), WP(x=0.3127, y=0.3290), L(max=1000, min=0.0001):

G(13250,34500)B(7500,3000)R(34000,16000)WP(15635,16450)L(10000000,1)

Note that this string value will need to be escaped or quoted to protect against shell expansion on many platforms. No default.

--max-cll <string>

Maximum content light level (MaxCLL) and maximum frame average light level (MaxFALL) as required by the Consumer Electronics Association 861.3 specification.

Specified as a string which is parsed when the stream header SEI are emitted. The string format is “%hu,%hu” where %hu are unsigned 16bit integers. The first value is the max content light level (or 0 if no maximum is indicated), the second value is the maximum picture average light level (or 0). Applicable for HDR content.

Example for MaxCLL=1000 candela per square meter, MaxFALL=400 candela per square meter:

–max-cll “1000,400”

Note that this string value will need to be escaped or quoted to protect against shell expansion on many platforms. No default.

--hdr, --no-hdr

Force signalling of HDR parameters in SEI packets. Enabled automatically when --master-display or --max-cll is specified. Useful when there is a desire to signal 0 values for max-cll and max-fall. Default disabled.

--hdr-opt, --no-hdr-opt

Add luma and chroma offsets for HDR/WCG content. Input video should be 10 bit 4:2:0. Applicable for HDR content. It is recommended that AQ-mode be enabled along with this feature. Default disabled.

--dhdr10-info <filename>

Inserts tone mapping information as an SEI message. It takes as input, the path to the JSON file containing the Creative Intent Metadata to be encoded as Dynamic Tone Mapping into the bitstream.

--dhdr10-opt, --no-dhdr10-opt

Limits the frames for which tone mapping information is inserted as SEI message. Inserts SEI only for IDR frames and for frames where tone mapping information has changed.

--min-luma <integer>

Minimum luma value allowed for input pictures. Any values below min-luma are clipped. No default.

--max-luma <integer>

Maximum luma value allowed for input pictures. Any values above max-luma are clipped. No default.

--nalu-file <filename>

Text file containing userSEI in POC order : <POC><space><PREFIX><space><NAL UNIT TYPE>/<SEI TYPE><space><SEI Payload> Parse the input file specified and inserts SEI messages into the bitstream. Currently, we support only PREFIX SEI messages. This is an “application-only” feature.

--atc-sei <integer>

Emit the alternative transfer characteristics SEI message where the integer is the preferred transfer characteristics. Required for HLG (Hybrid Log Gamma) signalling. Not signalled by default.

--pic-struct <integer>

Set the picture structure and emits it in the picture timing SEI message. Values in the range 0..12. See D.3.3 of the HEVC spec. for a detailed explanation. Required for HLG (Hybrid Log Gamma) signalling. Not signalled by default.

## Bitstream options¶

--annexb, --no-annexb

If enabled, x265 will produce Annex B bitstream format, which places start codes before NAL. If disabled, x265 will produce file format, which places length before NAL. x265 CLI will choose the right option based on output format. Default enabled

API ONLY

--repeat-headers, --no-repeat-headers

If enabled, x265 will emit VPS, SPS, and PPS headers with every keyframe. This is intended for use when you do not have a container to keep the stream headers for you and you want keyframes to be random access points. Default disabled

--aud, --no-aud

Emit an access unit delimiter NAL at the start of each slice access unit. If --repeat-headers is not enabled (indicating the user will be writing headers manually at the start of the stream) the very first AUD will be skipped since it cannot be placed at the start of the access unit, where it belongs. Default disabled

--hrd, --no-hrd

Enable the signalling of HRD parameters to the decoder. The HRD parameters are carried by the Buffering Period SEI messages and Picture Timing SEI messages providing timing information to the decoder. Default disabled

--info, --no-info

Emit an informational SEI with the stream headers which describes the encoder version, build info, and encode parameters. This is very helpful for debugging purposes but encoding version numbers and build info could make your bitstreams diverge and interfere with regression testing. Default enabled

--hash <integer>

Emit decoded picture hash SEI, so the decoder may validate the reconstructed pictures and detect data loss. Also useful as a debug feature to validate the encoder state. Default None

1. MD5
2. CRC
3. Checksum
--temporal-layers,--no-temporal-layers

Enable a temporal sub layer. All referenced I/P/B frames are in the base layer and all unreferenced B frames are placed in a temporal enhancement layer. A decoder may chose to drop the enhancement layer and only decode and display the base layer slices.

If used with a fixed GOP (--b-adapt 0) and --bframes 3 then the two layers evenly split the frame rate, with a cadence of PbBbP. You probably also want --no-scenecut and a keyframe interval that is a multiple of 4.

--log2-max-poc-lsb <integer>

Maximum of the picture order count. Default 8

--vui-timing-info, --no-vui-timing-info

Emit VUI timing info in bitstream. Default enabled.

--vui-hrd-info, --no-vui-hrd-info

Emit VUI HRD info in bitstream. Default enabled when --hrd is enabled.

--opt-qp-pps, --no-opt-qp-pps

Optimize QP in PPS (instead of default value of 26) based on the QP values observed in last GOP. Default disabled.

--opt-ref-list-length-pps, --no-opt-ref-list-length-pps

Optimize L0 and L1 ref list length in PPS (instead of default value of 0) based on the lengths observed in the last GOP. Default disabled.

--multi-pass-opt-rps, --no-multi-pass-opt-rps

Enable storing commonly used RPS in SPS in multi pass mode. Default disabled.

--opt-cu-delta-qp, --no-opt-cu-delta-qp

Optimize CU level QPs by pulling up lower QPs to value close to meanQP thereby minimizing fluctuations in deltaQP signalling. Default disabled.

Only effective at RD levels 5 and 6

--idr-recovery-sei, --no-idr-recovery-sei
Emit RecoveryPoint info as sei in bitstream for each IDR frame. Default disabled.
--single-sei, --no-single-sei
Emit SEI messages in a single NAL unit instead of multiple NALs. Default disabled.
When HRD SEI is enabled the HM decoder will throw a warning.

## DCT Approximations¶

--lowpass-dct

If enabled, x265 will use low-pass subband dct approximation instead of the standard dct for 16x16 and 32x32 blocks. This approximation is less computational intensive but it generates truncated coefficient matrixes for the transformed block. Empirical analysis shows marginal loss in compression and performance gains up to 10%, paticularly at moderate bit-rates.

This approximation should be considered for platforms with performance and time constrains.

Default disabled. Experimental feature

## Debugging options¶

--recon, -r <filename>

Output file containing reconstructed images in display order. If the file extension is “.y4m” the file will contain a YUV4MPEG2 stream header and frame headers. Otherwise it will be a raw YUV file in the encoder’s internal bit depth.

CLI ONLY

--recon-depth <integer>

Bit-depth of output file. This value defaults to the internal bit depth and currently cannot to be modified.

CLI ONLY

--recon-y4m-exec <string>

If you have an application which can play a Y4MPEG stream received on stdin, the x265 CLI can feed it reconstructed pictures in display order. The pictures will have no timing info, obviously, so the picture timing will be determined primarily by encoding elapsed time and latencies, but it can be useful to preview the pictures being output by the encoder to validate input settings and rate control parameters.

Example command for ffplay (assuming it is in your PATH):

–recon-y4m-exec “ffplay -i pipe:0 -autoexit”

CLI ONLY