Application Programming Interface


x265 is written primarily in C++ and x86 assembly language but the public facing programming interface is C for the widest possible portability. This C interface is wholly defined within x265.h in the source/ folder of our source tree. All of the functions and variables and enumerations meant to be used by the end-user are present in this header.

Where possible, x265 has tried to keep its public API as close as possible to x264’s public API. So those familiar with using x264 through its C interface will find x265 quite familiar.

This file is meant to be read in-order; the narrative follows linearly through the various sections

Build Considerations

The choice of Main or Main10 profile encodes is made at compile time; the internal pixel depth influences a great deal of variable sizes and thus 8 and 10bit pixels are handled as different build options (primarily to maintain the performance of the 8bit builds). libx265 exports a variable x265_max_bit_depth which indicates how the library was compiled (it will contain a value of 8 or 10). Further, x265_version_str is a pointer to a string indicating the version of x265 which was compiled, and x265_build_info_str is a pointer to a string identifying the compiler and build options.


x265_version_str is only updated when cmake runs. If you are making binaries for others to use, it is recommended to run cmake prior to make in your build scripts.

x265 will accept input pixels of any depth between 8 and 16 bits regardless of the depth of its internal pixels (8 or 10). It will shift and mask input pixels as required to reach the internal depth. If downshifting is being performed using our CLI application (to 8 bits), the --dither option may be enabled to reduce banding. This feature is not available through the C interface.


The primary object in x265 is the encoder object, and this is represented in the public API as an opaque typedef x265_encoder. Pointers of this type are passed to most encoder functions.

A single encoder generates a single output bitstream from a sequence of raw input pictures. Thus if you need multiple output bitstreams you must allocate multiple encoders. You may pass the same input pictures to multiple encoders, the encode function does not modify the input picture structures (the pictures are copied into the encoder as the first step of encode).

Encoder allocation is a reentrant function, so multiple encoders may be safely allocated in a single process. The encoder access functions are not reentrant for a single encoder, so the recommended use case is to allocate one client thread per encoder instance (one thread for all encoder instances is possible, but some encoder access functions are blocking and thus this would be less efficient).


There is one caveat to having multiple encoders within a single process. All of the encoders must use the same maximum CTU size because many global variables are configured based on this size. Encoder allocation will fail if a mis-matched CTU size is attempted. If no encoders are open, x265_cleanup() can be called to reset the configured CTU size so a new size can be used.

An encoder is allocated by calling x265_encoder_open():

/* x265_encoder_open:
 *      create a new encoder handler, all parameters from x265_param are copied */
x265_encoder* x265_encoder_open(x265_param *);

The returned pointer is then passed to all of the functions pertaining to this encode. A large amount of memory is allocated during this function call, but the encoder will continue to allocate memory as the first pictures are passed to the encoder; until its pool of picture structures is large enough to handle all of the pictures it must keep internally. The pool size is determined by the lookahead depth, the number of frame threads, and the maximum number of references.

As indicated in the comment, x265_param is copied internally so the user may release their copy after allocating the encoder. Changes made to their copy of the param structure have no affect on the encoder after it has been allocated.


The x265_param structure describes everything the encoder needs to know about the input pictures and the output bitstream and most everything in between.

The recommended way to handle these param structures is to allocate them from libx265 via:

/* x265_param_alloc:
 *  Allocates an x265_param instance. The returned param structure is not
 *  special in any way, but using this method together with x265_param_free()
 *  and x265_param_parse() to set values by name allows the application to treat
 *  x265_param as an opaque data struct for version safety */
x265_param *x265_param_alloc();

In this way, your application does not need to know the exact size of the param structure (the build of x265 could potentially be a bit newer than the copy of x265.h that your application compiled against).

Next you perform the initial rough cut configuration of the encoder by chosing a performance preset and optional tune factor x265_preset_names and x265_tune_names respectively hold the string names of the presets and tune factors (see presets for more detail on presets and tune factors):

/*      returns 0 on success, negative on failure (e.g. invalid preset/tune name). */
int x265_param_default_preset(x265_param *, const char *preset, const char *tune);

Now you may optionally specify a profile. x265_profile_names contains the string names this function accepts:

/*      (can be NULL, in which case the function will do nothing)
 *      returns 0 on success, negative on failure (e.g. invalid profile name). */
int x265_param_apply_profile(x265_param *, const char *profile);

Finally you configure any remaining options by name using repeated calls to:

/* x265_param_parse:
 *  set one parameter by name.
 *  returns 0 on success, or returns one of the following errors.
 *  note: BAD_VALUE occurs only if it can't even parse the value,
 *  numerical range is not checked until x265_encoder_open().
 *  value=NULL means "true" for boolean options, but is a BAD_VALUE for non-booleans. */
#define X265_PARAM_BAD_NAME  (-1)
#define X265_PARAM_BAD_VALUE (-2)
int x265_param_parse(x265_param *p, const char *name, const char *value);

See string options for the list of options (and their descriptions) which can be set by x265_param_parse().

After the encoder has been created, you may release the param structure:

/* x265_param_free:
 *  Use x265_param_free() to release storage for an x265_param instance
 *  allocated by x265_param_alloc() */
void x265_param_free(x265_param *);


Using these methods to allocate and release the param structures helps future-proof your code in many ways, but the x265 API is versioned in such a way that we prevent linkage against a build of x265 that does not match the version of the header you are compiling against (unless you use x265_api_query() to acquire the library’s interfaces). This is function of the X265_BUILD macro.

x265_encoder_parameters() may be used to get a copy of the param structure from the encoder after it has been opened, in order to see the changes made to the parameters for auto-detection and other reasons:

/* x265_encoder_parameters:
 *      copies the current internal set of parameters to the pointer provided
 *      by the caller.  useful when the calling application needs to know
 *      how x265_encoder_open has changed the parameters.
 *      note that the data accessible through pointers in the returned param struct
 *      (e.g. filenames) should not be modified by the calling application. */
void x265_encoder_parameters(x265_encoder *, x265_param *);

x265_encoder_reconfig() may be used to reconfigure encoder parameters mid-encode:

    /* x265_encoder_reconfig:
     *       used to modify encoder parameters.
     *      various parameters from x265_param are copied.
     *      this takes effect immediately, on whichever frame is encoded next;
     *      returns negative on parameter validation error, 0 on successful reconfigure
     *      and 1 when a reconfigure is already in progress.
     *      not all parameters can be changed; see the actual function for a
     *      detailed breakdown.  since not all parameters can be changed, moving
     *      from preset to preset may not always fully copy all relevant parameters,
     *      but should still work usably in practice. however, more so than for
     *      other presets, many of the speed shortcuts used in ultrafast cannot be
     *      switched out of; using reconfig to switch between ultrafast and other
     *      presets is not recommended without a more fine-grained breakdown of
     *      parameters to take this into account. */
int x265_encoder_reconfig(x265_encoder *, x265_param *);

x265_encoder_reconfig_zone() Used to reconfigure rate-contol settings of zones mid-encode:

/* x265_encoder_reconfig_zone:
 *      Properties of the zone will be copied to encoder's param and will be used only to
 *      influence rate-control decisions of the zone.
 *      returns 0 on successful copy and negative on failure.*/
int x265_encoder_reconfig(x265_encoder *, x265_param *);

x265_get_slicetype_poc_and_scenecut() may be used to fetch slice type, poc and scene cut information mid-encode:

/* x265_get_slicetype_poc_and_scenecut:
 *     get the slice type, poc and scene cut information for the current frame,
 *     returns negative on error, 0 on success.
 *     This API must be called after(poc >= lookaheadDepth + bframes + 2) condition check. */
 int x265_get_slicetype_poc_and_scenecut(x265_encoder *encoder, int *slicetype, int *poc, int* sceneCut);

x265_get_ref_frame_list() may be used to fetch forward and backward refrence list:

/* x265_get_ref_frame_list:
 *     returns negative on error, 0 when access unit were output.
 *     This API must be called after(poc >= lookaheadDepth + bframes + 2) condition check */
 int x265_get_ref_frame_list(x265_encoder *encoder, x265_picyuv**, x265_picyuv**, int, int, int*, int*);

x265_encoder_ctu_info may be used to provide additional CTU-specific information to the encoder:

/* x265_encoder_ctu_info:
 *    Copy CTU information such as ctu address and ctu partition structure of all
 *    CTUs in each frame. The function is invoked only if "--ctu-info" is enabled and
 *    the encoder will wait for this copy to complete if enabled.*/
int x265_encoder_ctu_info(x265_encoder *encoder, int poc, x265_ctu_info_t** ctu);

x265_set_analysis_data() may be used to recive analysis information from external application:

/* x265_set_analysis_data:
 *     set the analysis data. The incoming analysis_data structure is assumed to be AVC-sized blocks.
 *     returns negative on error, 0 access unit were output.*/
 int x265_set_analysis_data(x265_encoder *encoder, x265_analysis_data *analysis_data, int poc, uint32_t cuBytes);

x265_alloc_analysis_data() may be used to allocate memory for the x265_analysis_data:

/* x265_alloc_analysis_data:
 *     Allocate memory for the x265_analysis_data object's internal structures. */
 void x265_alloc_analysis_data(x265_param *param, x265_analysis_data* analysis);

x265_free_analysis_data() may be used to free memory for the x265_analysis_data:

/* x265_free_analysis_data:
 *    Free the allocated memory for x265_analysis_data object's internal structures. */
 void x265_free_analysis_data(x265_param *param, x265_analysis_data* analysis);


Raw pictures are passed to the encoder via the x265_picture structure. Just like the param structure we recommend you allocate this structure from the encoder to avoid potential size mismatches:

/* x265_picture_alloc:
 *  Allocates an x265_picture instance. The returned picture structure is not
 *  special in any way, but using this method together with x265_picture_free()
 *  and x265_picture_init() allows some version safety. New picture fields will
 *  always be added to the end of x265_picture */
x265_picture *x265_picture_alloc();

Regardless of whether you allocate your picture structure this way or whether you simply declare it on the stack, your next step is to initialize the structure via:

 * Initialize an x265_picture structure to default values. It sets the pixel
 * depth and color space to the encoder's internal values and sets the slice
 * type to auto - so the lookahead will determine slice type.
void x265_picture_init(x265_param *param, x265_picture *pic);

x265 does not perform any color space conversions, so the raw picture’s color space (chroma sampling) must match the color space specified in the param structure used to allocate the encoder. x265_picture_init initializes this field to the internal color space and it is best to leave it unmodified.

The picture bit depth is initialized to be the encoder’s internal bit depth but this value should be changed to the actual depth of the pixels being passed into the encoder. If the picture bit depth is more than 8, the encoder assumes two bytes are used to represent each sample (little-endian shorts).

The user is responsible for setting the plane pointers and plane strides (in units of bytes, not pixels). The presentation time stamp (pts) is optional, depending on whether you need accurate decode time stamps (dts) on output.

If you wish to override the lookahead or rate control for a given picture you may specify a slicetype other than X265_TYPE_AUTO, or a forceQP value other than 0.

x265 does not modify the picture structure provided as input, so you may reuse a single x265_picture for all pictures passed to a single encoder, or even all pictures passed to multiple encoders.

Structures allocated from the library should eventually be released:

/* x265_picture_free:
 *  Use x265_picture_free() to release storage for an x265_picture instance
 *  allocated by x265_picture_alloc() */
void x265_picture_free(x265_picture *);

Analysis Buffers

Analysis information can be saved and reused to between encodes of the same video sequence (generally for multiple bitrate encodes). The best results are attained by saving the analysis information of the highest bitrate encode and reuse it in lower bitrate encodes.

When saving or loading analysis data, buffers must be allocated for every picture passed into the encoder using:

/* x265_alloc_analysis_data:
 *  Allocate memory to hold analysis meta data, returns 1 on success else 0 */
int x265_alloc_analysis_data(x265_picture*);

Note that this is very different from the typical semantics of x265_picture, which can be reused many times. The analysis buffers must be re-allocated for every input picture.

Analysis buffers passed to the encoder are owned by the encoder until they pass the buffers back via an output x265_picture. The user is responsible for releasing the buffers when they are finished with them via:

/* x265_free_analysis_data:
 *  Use x265_free_analysis_data to release storage of members allocated by
 *  x265_alloc_analysis_data */
void x265_free_analysis_data(x265_picture*);

Encode Process

The output of the encoder is a series of NAL packets, which are always returned concatenated in consecutive memory. HEVC streams have SPS and PPS and VPS headers which describe how the following packets are to be decoded. If you specified --repeat-headers then those headers will be output with every keyframe. Otherwise you must explicitly query those headers using:

/* x265_encoder_headers:
 *      return the SPS and PPS that will be used for the whole stream.
 *      *pi_nal is the number of NAL units outputted in pp_nal.
 *      returns negative on error, total byte size of payload data on success
 *      the payloads of all output NALs are guaranteed to be sequential in memory. */
int x265_encoder_headers(x265_encoder *, x265_nal **pp_nal, uint32_t *pi_nal);

Now we get to the main encode loop. Raw input pictures are passed to the encoder in display order via:

/* x265_encoder_encode:
 *      encode one picture.
 *      *pi_nal is the number of NAL units outputted in pp_nal.
 *      returns negative on error, zero if no NAL units returned.
 *      the payloads of all output NALs are guaranteed to be sequential in memory. */
int x265_encoder_encode(x265_encoder *encoder, x265_nal **pp_nal, uint32_t *pi_nal, x265_picture *pic_in, x265_picture *pic_out);

These pictures are queued up until the lookahead is full, and then the frame encoders in turn are filled, and then finally you begin receiving a output NALs (corresponding to a single output picture) with each input picture you pass into the encoder.

Once the pipeline is completely full, x265_encoder_encode() will block until the next output picture is complete.


Optionally, if the pointer of a second x265_picture structure is provided, the encoder will fill it with data pertaining to the output picture corresponding to the output NALs, including the recontructed image, POC and decode timestamp. These pictures will be in encode (or decode) order. The encoder will also write corresponding frame encode statistics into x265_frame_stats.

When the last of the raw input pictures has been sent to the encoder, x265_encoder_encode() must still be called repeatedly with a pic_in argument of 0, indicating a pipeline flush, until the function returns a value less than or equal to 0 (indicating the output bitstream is complete).

At any time during this process, the application may query running statistics from the encoder:

/* x265_encoder_get_stats:
 *       returns encoder statistics */
void x265_encoder_get_stats(x265_encoder *encoder, x265_stats *, uint32_t statsSizeBytes);


At the end of the encode, the application will want to trigger logging of the final encode statistics, if --csv had been specified:

/* x265_encoder_log:
 *       write a line to the configured CSV file. If a CSV filename was not
 *       configured, or file open failed, this function will perform no write. */
void x265_encoder_log(x265_encoder *encoder, int argc, char **argv);

Finally, the encoder must be closed in order to free all of its resources. An encoder that has been flushed cannot be restarted and reused. Once x265_encoder_close() has been called, the encoder handle must be discarded:

/* x265_encoder_close:
 *      close an encoder handler */
void x265_encoder_close(x265_encoder *);

When the application has completed all encodes, it should call x265_cleanup() to free process global, particularly if a memory-leak detection tool is being used. x265_cleanup() also resets the saved CTU size so it will be possible to create a new encoder with a different CTU size:

/* x265_cleanup:
 *     release library static allocations, reset configured CTU size */
void x265_cleanup(void);

VMAF (Video Multi-Method Assessment Fusion)

If you set the ENABLE_LIBVMAF cmake option to ON, then x265 will report per frame and aggregate VMAF score for the given input and dump the scores in csv file. The user also need to specify the --recon in command line to get the VMAF scores.

/* x265_calculate_vmafScore:
  • returns VMAF score for the input video.
  • This api must be called only after encoding was done. */

double x265_calculate_vmafscore(x265_param*, x265_vmaf_data*);

/* x265_calculate_vmaf_framelevelscore:
  • returns VMAF score for each frame in a given input video. The frame level VMAF score does not include temporal scores. */

double x265_calculate_vmaf_framelevelscore(x265_vmaf_framedata*);


When setting ENABLE_LIBVMAF cmake option to ON, it is recommended to also set ENABLE_SHARED to OFF to prevent build problems. We only need the static library from these builds.

Binaries build with windows will not have VMAF support.

Multi-library Interface

If your application might want to make a runtime bit-depth selection, it will need to use one of these bit-depth introspection interfaces which returns an API structure containing the public function entry points and constants.

Instead of directly using all of the x265_ methods documented above, you query an x265_api structure from your libx265 and then use the function pointers of the same name (minus the x265_ prefix) within that structure. For instance x265_param_default() becomes api->param_default().


The first bit-depth instrospecton method is x265_api_get(). It designed for applications that might statically link with libx265, or will at least be tied to a particular SONAME or API version:

/* x265_api_get:
 *   Retrieve the programming interface for a linked x265 library.
 *   May return NULL if no library is available that supports the
 *   requested bit depth. If bitDepth is 0, the function is guarunteed
 *   to return a non-NULL x265_api pointer from the system default
 *   libx265 */
const x265_api* x265_api_get(int bitDepth);

Like x265_encoder_encode(), this function has the build number automatically appended to the function name via macros. This ties your application to a particular binary API version of libx265 (the one you compile against). If you attempt to link with a libx265 with a different API version number, the link will fail.

Obviously this has no meaningful effect on applications which statically link to libx265.


The second bit-depth introspection method is designed for applications which need more flexibility in API versioning. If you use x265_api_query() and dynamically link to libx265 at runtime (using dlopen() on POSIX or LoadLibrary() on Windows) your application is no longer directly tied to the API version that it was compiled against:

/* x265_api_query:
 *   Retrieve the programming interface for a linked x265 library, like
 *   x265_api_get(), except this function accepts X265_BUILD as the second
 *   argument rather than using the build number as part of the function name.
 *   Applications which dynamically link to libx265 can use this interface to
 *   query the library API and achieve a relative amount of version skew
 *   flexibility. The function may return NULL if the library determines that
 *   the apiVersion that your application was compiled against is not compatible
 *   with the library you have linked with.
 *   api_major_version will be incremented any time non-backward compatible
 *   changes are made to any public structures or functions. If
 *   api_major_version does not match X265_MAJOR_VERSION from the x265.h your
 *   application compiled against, your application must not use the returned
 *   x265_api pointer.
 *   Users of this API *must* also validate the sizes of any structures which
 *   are not treated as opaque in application code. For instance, if your
 *   application dereferences a x265_param pointer, then it must check that
 *   api->sizeof_param matches the sizeof(x265_param) that your application
 *   compiled with. */
const x265_api* x265_api_query(int bitDepth, int apiVersion, int* err);

A number of validations must be performed on the returned API structure in order to determine if it is safe for use by your application. If you do not perform these checks, your application is liable to crash:

if (api->api_major_version != X265_MAJOR_VERSION) /* do not use */
if (api->sizeof_param != sizeof(x265_param))      /* do not use */
if (api->sizeof_picture != sizeof(x265_picture))  /* do not use */
if (api->sizeof_stats != sizeof(x265_stats))      /* do not use */
if (api->sizeof_zone != sizeof(x265_zone))        /* do not use */

Note that if your application does not directly allocate or dereference one of these structures, if it treats the structure as opaque or does not use it at all, then it can skip the size check for that structure.

In particular, if your application uses api->param_alloc(), api->param_free(), api->param_parse(), etc and never directly accesses any x265_param fields, then it can skip the check on the sizeof(x265_parm) and thereby ignore changes to that structure (which account for a large percentage of X265_BUILD bumps).

Build Implications

By default libx265 will place all of its internal C++ classes and functions within an x265 namespace and export all of the C functions documented in this file. Obviously this prevents 8bit and 10bit builds of libx265 from being statically linked into a single binary, all of those symbols would collide.

However, if you set the EXPORT_C_API cmake option to OFF then libx265 will use a bit-depth specific namespace and prefix for its assembly functions (x265_8bit, x265_10bit or x265_12bit) and export no C functions.

In this way you can build one or more libx265 libraries without any exported C interface and link them into a libx265 build that does export a C interface. The build which exported the C functions becomes the default bit depth for the combined library, and the other bit depths are available via the bit-depth introspection methods.


When setting EXPORT_C_API cmake option to OFF, it is recommended to also set ENABLE_SHARED and ENABLE_CLI to OFF to prevent build problems. We only need the static library from these builds.

If an application requests a bit-depth that is not supported by the default library or any of the additionally linked libraries, the introspection method will fall-back to an attempt to dynamically bind a shared library with a name appropriate for the requested bit-depth:

8-bit:  libx265_main
10-bit: libx265_main10
12-bit: libx265_main12

If the profile-named library is not found, it will then try to bind a generic libx265 in the hopes that it is a multilib library with all bit depths.

Packaging and Distribution

We recommend that packagers distribute a single combined shared/static library build which includes all the bit depth libraries linked together. See the multilib scripts in our build/ subdirectories for examples of how to affect these combined library builds. It is the packager’s discretion which bit-depth exports the public C functions and thus becomes the default bit-depth for the combined library.


Windows packagers might want to build libx265 with WINXP_SUPPORT enabled. This makes the resulting binaries functional on XP and Vista. Without this flag, the minimum supported host O/S is Windows 7. Also note that binaries built with WINXP_SUPPORT will not have NUMA support and they will have slightly less performance.

STATIC_LINK_CRT is also recommended so end-users will not need to install any additional MSVC C runtime libraries.