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Decoding

Modules

 Frame parsing
 

Macros

#define AV_INPUT_BUFFER_PADDING_SIZE   32
 Required number of additionally allocated bytes at the end of the input bitstream for decoding. More...
 

Enumerations

enum  AVDiscard {
  AVDISCARD_NONE =-16, AVDISCARD_DEFAULT = 0, AVDISCARD_NONREF = 8, AVDISCARD_BIDIR = 16,
  AVDISCARD_NONINTRA = 24, AVDISCARD_NONKEY = 32, AVDISCARD_ALL = 48
}
 

Functions

AVCodecavcodec_find_decoder (enum AVCodecID id)
 Find a registered decoder with a matching codec ID. More...
 
AVCodecavcodec_find_decoder_by_name (const char *name)
 Find a registered decoder with the specified name. More...
 
int avcodec_default_get_buffer2 (AVCodecContext *s, AVFrame *frame, int flags)
 The default callback for AVCodecContext.get_buffer2(). More...
 
void avcodec_align_dimensions (AVCodecContext *s, int *width, int *height)
 Modify width and height values so that they will result in a memory buffer that is acceptable for the codec if you do not use any horizontal padding. More...
 
void avcodec_align_dimensions2 (AVCodecContext *s, int *width, int *height, int linesize_align[AV_NUM_DATA_POINTERS])
 Modify width and height values so that they will result in a memory buffer that is acceptable for the codec if you also ensure that all line sizes are a multiple of the respective linesize_align[i]. More...
 
int avcodec_enum_to_chroma_pos (int *xpos, int *ypos, enum AVChromaLocation pos)
 Converts AVChromaLocation to swscale x/y chroma position. More...
 
enum AVChromaLocation avcodec_chroma_pos_to_enum (int xpos, int ypos)
 Converts swscale x/y chroma position to AVChromaLocation. More...
 
attribute_deprecated int avcodec_decode_audio4 (AVCodecContext *avctx, AVFrame *frame, int *got_frame_ptr, const AVPacket *avpkt)
 Decode the audio frame of size avpkt->size from avpkt->data into frame. More...
 
attribute_deprecated int avcodec_decode_video2 (AVCodecContext *avctx, AVFrame *picture, int *got_picture_ptr, const AVPacket *avpkt)
 Decode the video frame of size avpkt->size from avpkt->data into picture. More...
 
int avcodec_decode_subtitle2 (AVCodecContext *avctx, AVSubtitle *sub, int *got_sub_ptr, AVPacket *avpkt)
 Decode a subtitle message. More...
 
int avcodec_send_packet (AVCodecContext *avctx, const AVPacket *avpkt)
 Supply raw packet data as input to a decoder. More...
 
int avcodec_receive_frame (AVCodecContext *avctx, AVFrame *frame)
 Return decoded output data from a decoder. More...
 
int avcodec_send_frame (AVCodecContext *avctx, const AVFrame *frame)
 Supply a raw video or audio frame to the encoder. More...
 
int avcodec_receive_packet (AVCodecContext *avctx, AVPacket *avpkt)
 Read encoded data from the encoder. More...
 
int avcodec_get_hw_frames_parameters (AVCodecContext *avctx, AVBufferRef *device_ref, enum AVPixelFormat hw_pix_fmt, AVBufferRef **out_frames_ref)
 Create and return a AVHWFramesContext with values adequate for hardware decoding. More...
 

Detailed Description

Macro Definition Documentation

#define AV_INPUT_BUFFER_PADDING_SIZE   32

Required number of additionally allocated bytes at the end of the input bitstream for decoding.

This is mainly needed because some optimized bitstream readers read 32 or 64 bit at once and could read over the end.
Note: If the first 23 bits of the additional bytes are not 0, then damaged MPEG bitstreams could cause overread and segfault.

Examples:
decode_audio.c, decode_video.c, and qsvdec.c.

Definition at line 769 of file avcodec.h.

Referenced by a64multi_encode_init(), aac_decode_frame(), aac_encode_init(), aac_sync(), adpcm_decode_frame(), adpcm_encode_init(), alac_encode_init(), allocate_buffers(), ape_tag_read_field(), apng_read_header(), apng_write_header(), append_extradata(), apply_color_indexing_transform(), asf_parse_packet(), asf_read_stream_properties(), atrac3_decode_init(), av_bitstream_filter_filter(), av_fast_padded_malloc(), av_fast_padded_mallocz(), av_grow_packet(), av_packet_from_data(), av_packet_merge_side_data(), av_packet_new_side_data(), av_packet_split_side_data(), av_parser_change(), av_parser_parse2(), av_shrink_packet(), avcodec_copy_context(), avcodec_encode_audio2(), avcodec_encode_video2(), avcodec_parameters_copy(), avcodec_parameters_from_context(), avcodec_parameters_to_context(), avi_read_header(), avio_close_dyn_buf(), avui_encode_init(), cbs_h2645_fragment_add_nals(), cbs_h2645_replace_ps(), cbs_h265_read_nal_unit(), cbs_mpeg2_read_unit(), cbs_mpeg2_split_fragment(), cdxl_decode_frame(), codec_reinit(), cook_decode_init(), copy_to_pbr(), create_new_buffer(), daala_header(), dca_parse_params(), decode_extradata_ps_mp4(), decode_frame(), decode_init(), decode_plane(), decode_plane10(), dshow_add_device(), dts_probe(), dv_decode_video_segment(), encode_frame(), encode_init(), extract_extradata(), extract_extradata_h2645(), extract_extradata_mpeg12(), extract_extradata_mpeg4(), extract_extradata_vc1(), extract_header(), FDBPrepare(), FDBRealloc(), ff_alloc_extradata(), ff_alloc_packet2(), ff_avc_write_annexb_extradata(), ff_cbs_write_extradata(), ff_combine_frame(), ff_flac_parse_picture(), ff_h2645_extract_rbsp(), ff_h264_parse_sprop_parameter_sets(), ff_id3v2_parse_apic(), ff_jpeg2000_init_component(), ff_mjpeg_find_marker(), ff_mov_read_stsd_entries(), ff_mpeg_update_thread_context(), ff_mpv_encode_picture(), ff_parse_mpeg2_descriptor(), ff_vaapi_encode_init(), ffat_decode(), ffat_init_decoder(), ffat_init_encoder(), ffm2_read_header(), ffm_read_header(), fixup_vorbis_headers(), flac_read_header(), flic_decode_frame_8BPP(), flush_packet(), flv_queue_extradata(), flv_write_packet(), g2m_init_buffers(), generate_fake_vps(), h264_mp4toannexb_init(), handle_packets(), hevc_extradata_to_annexb(), hevc_parse_sdp_line(), id3_has_changed_values(), iff_read_header(), imc_decode_frame(), init_fps(), jacosub_read_header(), jpg_decode_data(), latm_decode_audio_specific_config(), libkvazaar_init(), libopus_encode_init(), libvorbis_encode_init(), libx265_encode_init(), magy_encode_init(), main(), matroska_parse_tracks(), mov_read_atom_into_extradata(), mov_read_ddts(), mov_read_stsd(), mov_read_stsz(), mov_read_wave(), mov_realloc_extradata(), mov_rewrite_dvd_sub_extradata(), mov_text_encode_init(), mov_write_dvc1_structs(), mov_write_single_packet(), mp3_header_decompress(), mpc8_parse_seektable(), mpegts_push_data(), mxg_update_cache(), nal_unit_extract_rbsp(), new_pes_packet(), nvenc_setup_extradata(), ogg_new_buf(), ogg_new_stream(), ogg_read_page(), ogg_save(), ogm_header(), omx_encode_frame(), omx_encode_init(), opus_encode_init(), packet_alloc(), parse_fmtp_config(), parse_packed_headers(), parse_video_info(), parse_vtrk(), pcx_decode_frame(), qsv_retrieve_enc_params(), qt_rtp_parse_packet(), rdt_parse_b64buf(), read_apic(), read_header(), read_kuki_chunk(), read_packet(), read_sl_header(), recode_subtitle(), rm_read_audio_stream_info(), sao_edge_filter(), sao_filter_CTB(), set_extradata(), shn_probe(), shorten_decode_frame(), spdif_write_packet(), stream_seek(), stream_skip(), stream_write(), svc_encode_init(), svq3_decode_slice_header(), tak_read_header(), theora_header(), tiff_unpack_fax(), utvideo_encode_init(), vc1_decode_frame(), vc1_decode_init(), vfw_read_header(), wma_decode_superframe(), wmv2_encode_init(), wrapped_avframe_encode(), write_extradata(), wv_get_value_float(), and X264_init().

Enumeration Type Documentation

enum AVDiscard
Enumerator
AVDISCARD_NONE 

discard nothing

AVDISCARD_DEFAULT 

discard useless packets like 0 size packets in avi

AVDISCARD_NONREF 

discard all non reference

AVDISCARD_BIDIR 

discard all bidirectional frames

AVDISCARD_NONINTRA 

discard all non intra frames

AVDISCARD_NONKEY 

discard all frames except keyframes

AVDISCARD_ALL 

discard all

Definition at line 781 of file avcodec.h.

Function Documentation

AVCodec* avcodec_find_decoder ( enum AVCodecID  id)
AVCodec* avcodec_find_decoder_by_name ( const char *  name)

Find a registered decoder with the specified name.

Parameters
namename of the requested decoder
Returns
A decoder if one was found, NULL otherwise.
Examples:
qsvdec.c.

Definition at line 1254 of file utils.c.

Referenced by find_codec_or_die(), find_probe_decoder(), main(), show_help_codec(), and stream_component_open().

int avcodec_default_get_buffer2 ( AVCodecContext s,
AVFrame frame,
int  flags 
)

The default callback for AVCodecContext.get_buffer2().

It is made public so it can be called by custom get_buffer2() implementations for decoders without AV_CODEC_CAP_DR1 set.

Definition at line 1488 of file decode.c.

Referenced by alloc_frame_buffer(), ff_thread_release_buffer(), get_buffer(), init_context_defaults(), submit_packet(), and thread_get_buffer_internal().

void avcodec_align_dimensions ( AVCodecContext s,
int width,
int height 
)

Modify width and height values so that they will result in a memory buffer that is acceptable for the codec if you do not use any horizontal padding.

May only be used if a codec with AV_CODEC_CAP_DR1 has been opened.

Definition at line 421 of file utils.c.

void avcodec_align_dimensions2 ( AVCodecContext s,
int width,
int height,
int  linesize_align[AV_NUM_DATA_POINTERS] 
)

Modify width and height values so that they will result in a memory buffer that is acceptable for the codec if you also ensure that all line sizes are a multiple of the respective linesize_align[i].

May only be used if a codec with AV_CODEC_CAP_DR1 has been opened.

Definition at line 244 of file utils.c.

Referenced by avcodec_align_dimensions(), and update_frame_pool().

int avcodec_enum_to_chroma_pos ( int xpos,
int ypos,
enum AVChromaLocation  pos 
)

Converts AVChromaLocation to swscale x/y chroma position.

The positions represent the chroma (0,0) position in a coordinates system with luma (0,0) representing the origin and luma(1,1) representing 256,256

Parameters
xposhorizontal chroma sample position
yposvertical chroma sample position

Definition at line 436 of file utils.c.

Referenced by avcodec_chroma_pos_to_enum(), and mkv_write_video_color().

enum AVChromaLocation avcodec_chroma_pos_to_enum ( int  xpos,
int  ypos 
)

Converts swscale x/y chroma position to AVChromaLocation.

The positions represent the chroma (0,0) position in a coordinates system with luma (0,0) representing the origin and luma(1,1) representing 256,256

Parameters
xposhorizontal chroma sample position
yposvertical chroma sample position

Definition at line 448 of file utils.c.

Referenced by mkv_parse_video_color().

attribute_deprecated int avcodec_decode_audio4 ( AVCodecContext avctx,
AVFrame frame,
int got_frame_ptr,
const AVPacket avpkt 
)

Decode the audio frame of size avpkt->size from avpkt->data into frame.

Some decoders may support multiple frames in a single AVPacket. Such decoders would then just decode the first frame and the return value would be less than the packet size. In this case, avcodec_decode_audio4 has to be called again with an AVPacket containing the remaining data in order to decode the second frame, etc... Even if no frames are returned, the packet needs to be fed to the decoder with remaining data until it is completely consumed or an error occurs.

Some decoders (those marked with AV_CODEC_CAP_DELAY) have a delay between input and output. This means that for some packets they will not immediately produce decoded output and need to be flushed at the end of decoding to get all the decoded data. Flushing is done by calling this function with packets with avpkt->data set to NULL and avpkt->size set to 0 until it stops returning samples. It is safe to flush even those decoders that are not marked with AV_CODEC_CAP_DELAY, then no samples will be returned.

Warning
The input buffer, avpkt->data must be AV_INPUT_BUFFER_PADDING_SIZE larger than the actual read bytes because some optimized bitstream readers read 32 or 64 bits at once and could read over the end.
Note
The AVCodecContext MUST have been opened with avcodec_open2() before packets may be fed to the decoder.
Parameters
avctxthe codec context
[out]frameThe AVFrame in which to store decoded audio samples. The decoder will allocate a buffer for the decoded frame by calling the AVCodecContext.get_buffer2() callback. When AVCodecContext.refcounted_frames is set to 1, the frame is reference counted and the returned reference belongs to the caller. The caller must release the frame using av_frame_unref() when the frame is no longer needed. The caller may safely write to the frame if av_frame_is_writable() returns 1. When AVCodecContext.refcounted_frames is set to 0, the returned reference belongs to the decoder and is valid only until the next call to this function or until closing or flushing the decoder. The caller may not write to it.
[out]got_frame_ptrZero if no frame could be decoded, otherwise it is non-zero. Note that this field being set to zero does not mean that an error has occurred. For decoders with AV_CODEC_CAP_DELAY set, no given decode call is guaranteed to produce a frame.
[in]avpktThe input AVPacket containing the input buffer. At least avpkt->data and avpkt->size should be set. Some decoders might also require additional fields to be set.
Returns
A negative error code is returned if an error occurred during decoding, otherwise the number of bytes consumed from the input AVPacket is returned.
Deprecated:
Use avcodec_send_packet() and avcodec_receive_frame().
Examples:
demuxing_decoding.c, transcode_aac.c, and transcoding.c.

Definition at line 827 of file decode.c.

Referenced by decode_audio_frame(), decode_packet(), LLVMFuzzerTestOneInput(), main(), movie_push_frame(), run_test(), and wrap().

attribute_deprecated int avcodec_decode_video2 ( AVCodecContext avctx,
AVFrame picture,
int got_picture_ptr,
const AVPacket avpkt 
)

Decode the video frame of size avpkt->size from avpkt->data into picture.

Some decoders may support multiple frames in a single AVPacket, such decoders would then just decode the first frame.

Warning
The input buffer must be AV_INPUT_BUFFER_PADDING_SIZE larger than the actual read bytes because some optimized bitstream readers read 32 or 64 bits at once and could read over the end.
The end of the input buffer buf should be set to 0 to ensure that no overreading happens for damaged MPEG streams.
Note
Codecs which have the AV_CODEC_CAP_DELAY capability set have a delay between input and output, these need to be fed with avpkt->data=NULL, avpkt->size=0 at the end to return the remaining frames.
The AVCodecContext MUST have been opened with avcodec_open2() before packets may be fed to the decoder.
Parameters
avctxthe codec context
[out]pictureThe AVFrame in which the decoded video frame will be stored. Use av_frame_alloc() to get an AVFrame. The codec will allocate memory for the actual bitmap by calling the AVCodecContext.get_buffer2() callback. When AVCodecContext.refcounted_frames is set to 1, the frame is reference counted and the returned reference belongs to the caller. The caller must release the frame using av_frame_unref() when the frame is no longer needed. The caller may safely write to the frame if av_frame_is_writable() returns 1. When AVCodecContext.refcounted_frames is set to 0, the returned reference belongs to the decoder and is valid only until the next call to this function or until closing or flushing the decoder. The caller may not write to it.
[in]avpktThe input AVPacket containing the input buffer. You can create such packet with av_init_packet() and by then setting data and size, some decoders might in addition need other fields like flags&AV_PKT_FLAG_KEY. All decoders are designed to use the least fields possible.
[in,out]got_picture_ptrZero if no frame could be decompressed, otherwise, it is nonzero.
Returns
On error a negative value is returned, otherwise the number of bytes used or zero if no frame could be decompressed.
Deprecated:
Use avcodec_send_packet() and avcodec_receive_frame().
Examples:
demuxing_decoding.c, and transcoding.c.

Definition at line 820 of file decode.c.

Referenced by compute_crc_of_packets(), decode_frame(), decode_packet(), ff_load_image(), LLVMFuzzerTestOneInput(), main(), movie_push_frame(), smvjpeg_decode_frame(), try_decode_video_frame(), video_decode(), video_decode_example(), and wrap().

int avcodec_decode_subtitle2 ( AVCodecContext avctx,
AVSubtitle sub,
int got_sub_ptr,
AVPacket avpkt 
)

Decode a subtitle message.

Return a negative value on error, otherwise return the number of bytes used. If no subtitle could be decompressed, got_sub_ptr is zero. Otherwise, the subtitle is stored in *sub. Note that AV_CODEC_CAP_DR1 is not available for subtitle codecs. This is for simplicity, because the performance difference is expect to be negligible and reusing a get_buffer written for video codecs would probably perform badly due to a potentially very different allocation pattern.

Some decoders (those marked with AV_CODEC_CAP_DELAY) have a delay between input and output. This means that for some packets they will not immediately produce decoded output and need to be flushed at the end of decoding to get all the decoded data. Flushing is done by calling this function with packets with avpkt->data set to NULL and avpkt->size set to 0 until it stops returning subtitles. It is safe to flush even those decoders that are not marked with AV_CODEC_CAP_DELAY, then no subtitles will be returned.

Note
The AVCodecContext MUST have been opened with avcodec_open2() before packets may be fed to the decoder.
Parameters
avctxthe codec context
[out]subThe Preallocated AVSubtitle in which the decoded subtitle will be stored, must be freed with avsubtitle_free if *got_sub_ptr is set.
[in,out]got_sub_ptrZero if no subtitle could be decompressed, otherwise, it is nonzero.
[in]avpktThe input AVPacket containing the input buffer.

Definition at line 992 of file decode.c.

Referenced by decoder_decode_frame(), process_frame(), subtitle_handler(), transcode_subtitles(), try_decode_frame(), and wrap().

int avcodec_send_packet ( AVCodecContext avctx,
const AVPacket avpkt 
)

Supply raw packet data as input to a decoder.

Internally, this call will copy relevant AVCodecContext fields, which can influence decoding per-packet, and apply them when the packet is actually decoded. (For example AVCodecContext.skip_frame, which might direct the decoder to drop the frame contained by the packet sent with this function.)

Warning
The input buffer, avpkt->data must be AV_INPUT_BUFFER_PADDING_SIZE larger than the actual read bytes because some optimized bitstream readers read 32 or 64 bits at once and could read over the end.
Do not mix this API with the legacy API (like avcodec_decode_video2()) on the same AVCodecContext. It will return unexpected results now or in future libavcodec versions.
Note
The AVCodecContext MUST have been opened with avcodec_open2() before packets may be fed to the decoder.
Parameters
avctxcodec context
[in]avpktThe input AVPacket. Usually, this will be a single video frame, or several complete audio frames. Ownership of the packet remains with the caller, and the decoder will not write to the packet. The decoder may create a reference to the packet data (or copy it if the packet is not reference-counted). Unlike with older APIs, the packet is always fully consumed, and if it contains multiple frames (e.g. some audio codecs), will require you to call avcodec_receive_frame() multiple times afterwards before you can send a new packet. It can be NULL (or an AVPacket with data set to NULL and size set to 0); in this case, it is considered a flush packet, which signals the end of the stream. Sending the first flush packet will return success. Subsequent ones are unnecessary and will return AVERROR_EOF. If the decoder still has frames buffered, it will return them after sending a flush packet.
Returns
0 on success, otherwise negative error code: AVERROR(EAGAIN): input is not accepted in the current state - user must read output with avcodec_receive_frame() (once all output is read, the packet should be resent, and the call will not fail with EAGAIN). AVERROR_EOF: the decoder has been flushed, and no new packets can be sent to it (also returned if more than 1 flush packet is sent) AVERROR(EINVAL): codec not opened, it is an encoder, or requires flush AVERROR(ENOMEM): failed to add packet to internal queue, or similar other errors: legitimate decoding errors
Examples:
decode_audio.c, decode_video.c, filtering_audio.c, filtering_video.c, hw_decode.c, and qsvdec.c.

Definition at line 641 of file decode.c.

Referenced by compat_decode(), decode(), decode_packet(), decode_write(), decoder_decode_frame(), main(), process_frame(), tdsc_decode_jpeg_tile(), try_decode_frame(), and wrap().

int avcodec_receive_frame ( AVCodecContext avctx,
AVFrame frame 
)

Return decoded output data from a decoder.

Parameters
avctxcodec context
frameThis will be set to a reference-counted video or audio frame (depending on the decoder type) allocated by the decoder. Note that the function will always call av_frame_unref(frame) before doing anything else.
Returns
0: success, a frame was returned AVERROR(EAGAIN): output is not available in this state - user must try to send new input AVERROR_EOF: the decoder has been fully flushed, and there will be no more output frames AVERROR(EINVAL): codec not opened, or it is an encoder other negative values: legitimate decoding errors
Examples:
decode_audio.c, decode_video.c, filtering_audio.c, filtering_video.c, hw_decode.c, and qsvdec.c.

Definition at line 708 of file decode.c.

Referenced by compat_decode(), decode(), decode_packet(), decode_write(), decoder_decode_frame(), main(), process_frame(), tdsc_decode_jpeg_tile(), try_decode_frame(), and wrap().

int avcodec_send_frame ( AVCodecContext avctx,
const AVFrame frame 
)

Supply a raw video or audio frame to the encoder.

Use avcodec_receive_packet() to retrieve buffered output packets.

Parameters
avctxcodec context
[in]frameAVFrame containing the raw audio or video frame to be encoded. Ownership of the frame remains with the caller, and the encoder will not write to the frame. The encoder may create a reference to the frame data (or copy it if the frame is not reference-counted). It can be NULL, in which case it is considered a flush packet. This signals the end of the stream. If the encoder still has packets buffered, it will return them after this call. Once flushing mode has been entered, additional flush packets are ignored, and sending frames will return AVERROR_EOF.

For audio: If AV_CODEC_CAP_VARIABLE_FRAME_SIZE is set, then each frame can have any number of samples. If it is not set, frame->nb_samples must be equal to avctx->frame_size for all frames except the last. The final frame may be smaller than avctx->frame_size.

Returns
0 on success, otherwise negative error code: AVERROR(EAGAIN): input is not accepted in the current state - user must read output with avcodec_receive_packet() (once all output is read, the packet should be resent, and the call will not fail with EAGAIN). AVERROR_EOF: the encoder has been flushed, and no new frames can be sent to it AVERROR(EINVAL): codec not opened, refcounted_frames not set, it is a decoder, or requires flush AVERROR(ENOMEM): failed to add packet to internal queue, or similar other errors: legitimate decoding errors
Examples:
encode_audio.c, and encode_video.c.

Definition at line 396 of file encode.c.

Referenced by do_audio_out(), do_video_out(), encode(), encode_frame(), flush_encoders(), and wrap().

int avcodec_receive_packet ( AVCodecContext avctx,
AVPacket avpkt 
)

Read encoded data from the encoder.

Parameters
avctxcodec context
avpktThis will be set to a reference-counted packet allocated by the encoder. Note that the function will always call av_frame_unref(frame) before doing anything else.
Returns
0 on success, otherwise negative error code: AVERROR(EAGAIN): output is not available in the current state - user must try to send input AVERROR_EOF: the encoder has been fully flushed, and there will be no more output packets AVERROR(EINVAL): codec not opened, or it is an encoder other errors: legitimate decoding errors
Examples:
encode_audio.c, and encode_video.c.

Definition at line 426 of file encode.c.

Referenced by do_audio_out(), do_video_out(), encode(), encode_frame(), flush_encoders(), and wrap().

int avcodec_get_hw_frames_parameters ( AVCodecContext avctx,
AVBufferRef device_ref,
enum AVPixelFormat  hw_pix_fmt,
AVBufferRef **  out_frames_ref 
)

Create and return a AVHWFramesContext with values adequate for hardware decoding.

This is meant to get called from the get_format callback, and is a helper for preparing a AVHWFramesContext for AVCodecContext.hw_frames_ctx. This API is for decoding with certain hardware acceleration modes/APIs only.

The returned AVHWFramesContext is not initialized. The caller must do this with av_hwframe_ctx_init().

Calling this function is not a requirement, but makes it simpler to avoid codec or hardware API specific details when manually allocating frames.

Alternatively to this, an API user can set AVCodecContext.hw_device_ctx, which sets up AVCodecContext.hw_frames_ctx fully automatically, and makes it unnecessary to call this function or having to care about AVHWFramesContext initialization at all.

There are a number of requirements for calling this function:

  • It must be called from get_format with the same avctx parameter that was passed to get_format. Calling it outside of get_format is not allowed, and can trigger undefined behavior.
  • The function is not always supported (see description of return values). Even if this function returns successfully, hwaccel initialization could fail later. (The degree to which implementations check whether the stream is actually supported varies. Some do this check only after the user's get_format callback returns.)
  • The hw_pix_fmt must be one of the choices suggested by get_format. If the user decides to use a AVHWFramesContext prepared with this API function, the user must return the same hw_pix_fmt from get_format.
  • The device_ref passed to this function must support the given hw_pix_fmt.
  • After calling this API function, it is the user's responsibility to initialize the AVHWFramesContext (returned by the out_frames_ref parameter), and to set AVCodecContext.hw_frames_ctx to it. If done, this must be done before returning from get_format (this is implied by the normal AVCodecContext.hw_frames_ctx API rules).
  • The AVHWFramesContext parameters may change every time time get_format is called. Also, AVCodecContext.hw_frames_ctx is reset before get_format. So you are inherently required to go through this process again on every get_format call.
  • It is perfectly possible to call this function without actually using the resulting AVHWFramesContext. One use-case might be trying to reuse a previously initialized AVHWFramesContext, and calling this API function only to test whether the required frame parameters have changed.
  • Fields that use dynamically allocated values of any kind must not be set by the user unless setting them is explicitly allowed by the documentation. If the user sets AVHWFramesContext.free and AVHWFramesContext.user_opaque, the new free callback must call the potentially set previous free callback. This API call may set any dynamically allocated fields, including the free callback.

The function will set at least the following fields on AVHWFramesContext (potentially more, depending on hwaccel API):

  • All fields set by av_hwframe_ctx_alloc().
  • Set the format field to hw_pix_fmt.
  • Set the sw_format field to the most suited and most versatile format. (An implication is that this will prefer generic formats over opaque formats with arbitrary restrictions, if possible.)
  • Set the width/height fields to the coded frame size, rounded up to the API-specific minimum alignment.
  • Only if the hwaccel requires a pre-allocated pool: set the initial_pool_size field to the number of maximum reference surfaces possible with the codec, plus 1 surface for the user to work (meaning the user can safely reference at most 1 decoded surface at a time), plus additional buffering introduced by frame threading. If the hwaccel does not require pre-allocation, the field is left to 0, and the decoder will allocate new surfaces on demand during decoding.
  • Possibly AVHWFramesContext.hwctx fields, depending on the underlying hardware API.

Essentially, out_frames_ref returns the same as av_hwframe_ctx_alloc(), but with basic frame parameters set.

The function is stateless, and does not change the AVCodecContext or the device_ref AVHWDeviceContext.

Parameters
avctxThe context which is currently calling get_format, and which implicitly contains all state needed for filling the returned AVHWFramesContext properly.
device_refA reference to the AVHWDeviceContext describing the device which will be used by the hardware decoder.
hw_pix_fmtThe hwaccel format you are going to return from get_format.
out_frames_refOn success, set to a reference to an uninitialized AVHWFramesContext, created from the given device_ref. Fields will be set to values required for decoding. Not changed if an error is returned.
Returns
zero on success, a negative value on error. The following error codes have special semantics: AVERROR(ENOENT): the decoder does not support this functionality. Setup is always manual, or it is a decoder which does not support setting AVCodecContext.hw_frames_ctx at all, or it is a software format. AVERROR(EINVAL): it is known that hardware decoding is not supported for this configuration, or the device_ref is not supported for the hwaccel referenced by hw_pix_fmt.

Definition at line 1164 of file decode.c.

Referenced by ff_decode_get_hw_frames_ctx().