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 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, 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.

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. 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 *);


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 *);

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.

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, or the log level indicated frame level
 *       logging, 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 resources like the thread pool; particularly if a memory-leak detection tool is being used:

 * Release library static allocations
void x265_cleanup(void);