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 4 is equivalent to --log-level debug.

Executable Options

--help, -h

Display help text


--version, -V

Display version details


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>

Logging level. Debug level enables per-frame QP, metric, and bitrate logging. If a CSV file is being generated, frame level makes the log be per-frame rather than per-encode. 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. frame
  5. debug
  6. full

Disable periodic progress reports from the CLI


--csv <filename>

Writes encoding results to a comma separated value log file. Creates the file if it doesnt already exist, else adds one line per run. if --log-level is frame or above, it writes one line per frame. Default none

When frame level logging is enabled, several frame performance statistics are listed:

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.

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.

--cu-stats, --no-cu-stats

Records statistics on how each CU was coded (split depths and other mode decisions) and reports those statistics at the end of the encode. Default disabled

--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 node 0
"-,+"     - allocate one pool, using all cores on node 1
"+,-,+"   - allocate two pools, using all cores on nodes 0 and 2
"+,-,+,-" - allocate two pools, using all cores on nodes 0 and 2
"-,*"     - allocate three pools, using all cores on nodes 1, 2 and 3
"8,8,8,8" - allocate four pools with up to 8 threads in each pool

The total number of threads will be determined by the number of threads assigned to all nodes. The worker threads will each be given affinity for their node, they will not be allowed to migrate between nodes, but they will be allowed to move between CPU cores within their node.

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.

Multiple thread pools will be allocated for any NUMA node with more than 64 logical CPU cores. But any given thread pool will always use at most one NUMA node.

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 thread is allocated per detected hardware thread (logical CPU cores) and one thread pool per NUMA node.

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.

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.



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


--input-depth <integer>

YUV only: Bit-depth of input file or stream

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



Enable high quality downscaling. 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 and internal bit depth is 8bits. Default disabled


--input-res <wxh>

YUV only: Source picture size [w x h]


--input-csp <integer|string>

YUV only: Source color space. Only i420, i422, and i444 are supported at this time. The internal color space is always the same as the source color space (libx265 does not support any color space conversions).

  1. i400
  2. i420 (default)
  3. i422
  4. i444
  5. nv12
  6. nv16
--fps <integer|float|numerator/denominator>

YUV only: Source frame rate

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

--interlaceMode <false|tff|bff>, --no-interlaceMode
  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


--frames, -f <integer>

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


--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.


--output-depth, -D 8|10

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, or libx265_main10 for a 10bit encoder, with the same API version as the linked libx265.


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).

API users must use 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.

Values: main, main10, mainstillpicture, main422-8, main422-10, main444-8, main444-10


--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, the option adds the intention to support the High tier of that level. If your specified level does not support a High tier, a warning is issued and this modifier flag is ignored. If --level-idc has been specified, but not –high-tier, then the encoder will attempt to encode at the specified level, main tier first, turning on high tier only if necessary and available at that level.

--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


--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.

Mode decision / Analysis

--rd <0..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
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
5 Adds RDO prediction decisions
6 Currently same as 5

Range of values: 0: 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 <64|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)


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.

--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 full CU size (2Nx2N) merge candidates first; if no residual is found the analysis is short circuited. 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

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 the resolution and slice type parameters do not match.

--analysis-mode <string|int>

Specify whether analysis information of each frame is output by encoder or input for reuse. By reading the analysis data writen by an earlier encode of the same sequence, substantial redundant work may be avoided.

The following data may be stored and reused: I frames - split decisions and luma intra directions of all CUs. P/B frames - motion vectors are dumped at each depth for all CUs.

Values: off(0), save(1): dump analysis data, load(2): read analysis data

--analysis-file <filename>

Specify a filename for analysis data (see --analysis-mode) If no filename is specified, x265_analysis.dat is used.

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.

--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.

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.

  1. dia
  2. hex (default)
  3. umh
  4. star
  5. 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

Spatial/intra options

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

Enable strong intra smoothing for 32x32 intra blocks. 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, preserving film grain for instance.

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 0.3

Range of values: 0 .. 2.0

--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 like film grain. Default: 1.0

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. Default 250

--min-keyint, -i <integer>

Minimum GOP size. Scenecuts closer together than this are coded as I or P, not IDR. Minimum keyint is clamped to be at least half of --keyint. If you wish to force regular keyframe intervals and disable adaptive I frame placement, you must use --no-scenecut.

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

--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

--lookahead-slices <0..16>

Use multiple worker threads to measure the estimated cost of each frame within the lookahead. 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. The effect on performance can be quite small. 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 encoder may internally lower the number of slices to ensure each slice codes at least 10 16x16 rows of lowres blocks. If slices are used in lookahead, they are logged in the list of tools as lslices.

Values: 0 - disabled (default). 1 is the same as 0. Max 16

--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

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)

--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

--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 (CRF)

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>

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
--aq-strength <float>

Adjust the strength of the adaptive quantization offsets. Setting --aq-strength to 0 disables AQ. Default 1.0.

Range of values: 0.0 to 3.0

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

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 provided this falls within the inclusive range [maxCUSize, minCUSize]. Experimental. 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 a slow and more detailed first pass encode in multi-pass rate control mode. Speed of the first pass encode is slightly lesser and quality midly improved when compared to the default settings in a multi-pass encode. Default disabled (turbo mode enabled)

When turbo first pass is not disabled, these options are set on the first pass to improve performance:

--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. Experimental.

--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). Default value is 0.6. Increasing it to 1 will effectively generate CQP

--qpstep <integer>

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

--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:

framenumber frametype QP

Frametype can be one of [I,i,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 a I frame that is not a keyframe (references are not broken).

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

--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.

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

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
--crop-rect <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 decoder may be directed to crop away this region before displaying the images via the --overscan option. Default undefined (not signaled)

--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 primitive to use when converting to RGB. Default undefined (not signaled)

  1. bt709
  2. undef
  3. reserved
  4. bt470m
  5. bt470bg
  6. smpte170m
  7. smpte240m
  8. film
  9. bt2020
--transfer <integer|string>

Specify transfer characteristics. Default undefined (not signaled)

  1. bt709
  2. undef
  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. smpte-st-2084
  17. smpte-st-428
--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
--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, white point X,Y and max,min luminance values. (HDR)

Example for P65D3 1000-nits:


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 and maximum frame average light level 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). (HDR)

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


--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

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.

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.


--recon-depth <integer>

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


--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”