这一节是最重要的一节，ffmpeg的解码，还是和以前一样，一定是先来一个上下文context，再一个具体的解码器类。
AVCodecContext和
下面是一个解码器定义，三部分组成，第一部分是设置参数，第二部分是基类，第三部分是具体实现。
如果你要实现自己的一个解码器，实现第三部分的那几个回调函数就可以了。
#define OFFSET(x) offsetof(H264Context, x)
#define VD AV_OPT_FLAG_VIDEO_PARAM | AV_OPT_FLAG_DECODING_PARAM
#define VDX VD | AV_OPT_FLAG_EXPORT
static const AVOption h264_options[] = {
{ "is_avc", "is avc", OFFSET(is_avc), AV_OPT_TYPE_BOOL, {.i64 = 0}, 0, 1, VDX },
{ "nal_length_size", "nal_length_size", OFFSET(nal_length_size), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 4, VDX },
{ "enable_er", "Enable error resilience on damaged frames (unsafe)", OFFSET(enable_er), AV_OPT_TYPE_BOOL, { .i64 = -1 }, -1, 1, VD },
{ "x264_build", "Assume this x264 version if no x264 version found in any SEI", OFFSET(x264_build), AV_OPT_TYPE_INT, {.i64 = -1}, -1, INT_MAX, VD },
{ NULL },
};
static const AVClass h264_class = {
.class_name = "H264 Decoder",
.item_name = av_default_item_name,
.option = h264_options,
.version = LIBAVUTIL_VERSION_INT,
};
const AVCodec ff_h264_decoder = {
.name = "h264",
.long_name = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
.type = AVMEDIA_TYPE_VIDEO,
.id = AV_CODEC_ID_H264,
.priv_data_size = sizeof(H264Context),
.init = h264_decode_init,
.close = h264_decode_end,
.decode = h264_decode_frame,
.capabilities = /*AV_CODEC_CAP_DRAW_HORIZ_BAND |*/ AV_CODEC_CAP_DR1 |
AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS |
AV_CODEC_CAP_FRAME_THREADS,
}
/**
* main external API structure.
* New fields can be added to the end with minor version bumps.
* Removal, reordering and changes to existing fields require a major
* version bump.
* You can use AVOptions (av_opt* / av_set/get*()) to access these fields from user
* applications.
* The name string for AVOptions options matches the associated command line
* parameter name and can be found in libavcodec/options_table.h
* The AVOption/command line parameter names differ in some cases from the C
* structure field names for historic reasons or brevity.
* sizeof(AVCodecContext) must not be used outside libav*.
*/
typedef struct AVCodecContext {
/**
* information on struct for av_log
* - set by avcodec_alloc_context3
*/
const AVClass *av_class;
int log_level_offset;
enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
const struct AVCodec *codec;
enum AVCodecID codec_id; /* see AV_CODEC_ID_xxx */
/**
* fourcc (LSB first, so "ABCD" -> ('D'<<24) + ('C'<<16) + ('B'<<8) + 'A').
* This is used to work around some encoder bugs.
* A demuxer should set this to what is stored in the field used to identify the codec.
* If there are multiple such fields in a container then the demuxer should choose the one
* which maximizes the information about the used codec.
* If the codec tag field in a container is larger than 32 bits then the demuxer should
* remap the longer ID to 32 bits with a table or other structure. Alternatively a new
* extra_codec_tag + size could be added but for this a clear advantage must be demonstrated
* first.
* - encoding: Set by user, if not then the default based on codec_id will be used.
* - decoding: Set by user, will be converted to uppercase by libavcodec during init.
*/
unsigned int codec_tag;
void *priv_data;
/**
* Private context used for internal data.
*
* Unlike priv_data, this is not codec-specific. It is used in general
* libavcodec functions.
*/
struct AVCodecInternal *internal;
/**
* Private data of the user, can be used to carry app specific stuff.
* - encoding: Set by user.
* - decoding: Set by user.
*/
void *opaque;
/**
* the average bitrate
* - encoding: Set by user; unused for constant quantizer encoding.
* - decoding: Set by user, may be overwritten by libavcodec
* if this info is available in the stream
*/
int64_t bit_rate;
/**
* number of bits the bitstream is allowed to diverge from the reference.
* the reference can be CBR (for CBR pass1) or VBR (for pass2)
* - encoding: Set by user; unused for constant quantizer encoding.
* - decoding: unused
*/
int bit_rate_tolerance;
/**
* Global quality for codecs which cannot change it per frame.
* This should be proportional to MPEG-1/2/4 qscale.
* - encoding: Set by user.
* - decoding: unused
*/
int global_quality;
/**
* - encoding: Set by user.
* - decoding: unused
*/
int compression_level;
#define FF_COMPRESSION_DEFAULT -1
/**
* AV_CODEC_FLAG_*.
* - encoding: Set by user.
* - decoding: Set by user.
*/
int flags;
/**
* AV_CODEC_FLAG2_*
* - encoding: Set by user.
* - decoding: Set by user.
*/
int flags2;
/**
* some codecs need / can use extradata like Huffman tables.
* MJPEG: Huffman tables
* rv10: additional flags
* MPEG-4: global headers (they can be in the bitstream or here)
* The allocated memory should be AV_INPUT_BUFFER_PADDING_SIZE bytes larger
* than extradata_size to avoid problems if it is read with the bitstream reader.
* The bytewise contents of extradata must not depend on the architecture or CPU endianness.
* Must be allocated with the av_malloc() family of functions.
* - encoding: Set/allocated/freed by libavcodec.
* - decoding: Set/allocated/freed by user.
*/
uint8_t *extradata;
int extradata_size;
/**
* This is the fundamental unit of time (in seconds) in terms
* of which frame timestamps are represented. For fixed-fps content,
* timebase should be 1/framerate and timestamp increments should be
* identically 1.
* This often, but not always is the inverse of the frame rate or field rate
* for video. 1/time_base is not the average frame rate if the frame rate is not
* constant.
*
* Like containers, elementary streams also can store timestamps, 1/time_base
* is the unit in which these timestamps are specified.
* As example of such codec time base see ISO/IEC 14496-2:2001(E)
* vop_time_increment_resolution and fixed_vop_rate
* (fixed_vop_rate == 0 implies that it is different from the framerate)
*
* - encoding: MUST be set by user.
* - decoding: the use of this field for decoding is deprecated.
* Use framerate instead.
*/
AVRational time_base;
/**
* For some codecs, the time base is closer to the field rate than the frame rate.
* Most notably, H.264 and MPEG-2 specify time_base as half of frame duration
* if no telecine is used ...
*
* Set to time_base ticks per frame. Default 1, e.g., H.264/MPEG-2 set it to 2.
*/
int ticks_per_frame;
/**
* Codec delay.
*
* Encoding: Number of frames delay there will be from the encoder input to
* the decoder output. (we assume the decoder matches the spec)
* Decoding: Number of frames delay in addition to what a standard decoder
* as specified in the spec would produce.
*
* Video:
* Number of frames the decoded output will be delayed relative to the
* encoded input.
*
* Audio:
* For encoding, this field is unused (see initial_padding).
*
* For decoding, this is the number of samples the decoder needs to
* output before the decoder's output is valid. When seeking, you should
* start decoding this many samples prior to your desired seek point.
*
* - encoding: Set by libavcodec.
* - decoding: Set by libavcodec.
*/
int delay;
/* video only */
/**
* picture width / height.
*
* @note Those fields may not match the values of the last
* AVFrame output by avcodec_receive_frame() due frame
* reordering.
*
* - encoding: MUST be set by user.
* - decoding: May be set by the user before opening the decoder if known e.g.
* from the container. Some decoders will require the dimensions
* to be set by the caller. During decoding, the decoder may
* overwrite those values as required while parsing the data.
*/
int width, height;
/**
* Bitstream width / height, may be different from width/height e.g. when
* the decoded frame is cropped before being output or lowres is enabled.
*
* @note Those field may not match the value of the last
* AVFrame output by avcodec_receive_frame() due frame
* reordering.
*
* - encoding: unused
* - decoding: May be set by the user before opening the decoder if known
* e.g. from the container. During decoding, the decoder may
* overwrite those values as required while parsing the data.
*/
int coded_width, coded_height;
/**
* the number of pictures in a group of pictures, or 0 for intra_only
* - encoding: Set by user.
* - decoding: unused
*/
int gop_size;
/**
* Pixel format, see AV_PIX_FMT_xxx.
* May be set by the demuxer if known from headers.
* May be overridden by the decoder if it knows better.
*
* @note This field may not match the value of the last
* AVFrame output by avcodec_receive_frame() due frame
* reordering.
*
* - encoding: Set by user.
* - decoding: Set by user if known, overridden by libavcodec while
* parsing the data.
*/
enum AVPixelFormat pix_fmt;
/**
* If non NULL, 'draw_horiz_band' is called by the libavcodec
* decoder to draw a horizontal band. It improves cache usage. Not
* all codecs can do that. You must check the codec capabilities
* beforehand.
* When multithreading is used, it may be called from multiple threads
* at the same time; threads might draw different parts of the same AVFrame,
* or multiple AVFrames, and there is no guarantee that slices will be drawn
* in order.
* The function is also used by hardware acceleration APIs.
* It is called at least once during frame decoding to pass
* the data needed for hardware render.
* In that mode instead of pixel data, AVFrame points to
* a structure specific to the acceleration API. The application
* reads the structure and can change some fields to indicate progress
* or mark state.
* - encoding: unused
* - decoding: Set by user.
* @param height the height of the slice
* @param y the y position of the slice
* @param type 1->top field, 2->bottom field, 3->frame
* @param offset offset into the AVFrame.data from which the slice should be read
*/
void (*draw_horiz_band)(struct AVCodecContext *s,
const AVFrame *src, int offset[AV_NUM_DATA_POINTERS],
int y, int type, int height);
/**
* Callback to negotiate the pixel format. Decoding only, may be set by the
* caller before avcodec_open2().
*
* Called by some decoders to select the pixel format that will be used for
* the output frames. This is mainly used to set up hardware acceleration,
* then the provided format list contains the corresponding hwaccel pixel
* formats alongside the "software" one. The software pixel format may also
* be retrieved from \ref sw_pix_fmt.
*
* This callback will be called when the coded frame properties (such as
* resolution, pixel format, etc.) change and more than one output format is
* supported for those new properties. If a hardware pixel format is chosen
* and initialization for it fails, the callback may be called again
* immediately.
*
* This callback may be called from different threads if the decoder is
* multi-threaded, but not from more than one thread simultaneously.
*
* @param fmt list of formats which may be used in the current
* configuration, terminated by AV_PIX_FMT_NONE.
* @warning Behavior is undefined if the callback returns a value other
* than one of the formats in fmt or AV_PIX_FMT_NONE.
* @return the chosen format or AV_PIX_FMT_NONE
*/
enum AVPixelFormat (*get_format)(struct AVCodecContext *s, const enum AVPixelFormat * fmt);
/**
* maximum number of B-frames between non-B-frames
* Note: The output will be delayed by max_b_frames+1 relative to the input.
* - encoding: Set by user.
* - decoding: unused
*/
int max_b_frames;
/**
* qscale factor between IP and B-frames
* If > 0 then the last P-frame quantizer will be used (q= lastp_q*factor+offset).
* If < 0 then normal ratecontrol will be done (q= -normal_q*factor+offset).
* - encoding: Set by user.
* - decoding: unused
*/
float b_quant_factor;
/**
* qscale offset between IP and B-frames
* - encoding: Set by user.
* - decoding: unused
*/
float b_quant_offset;
/**
* Size of the frame reordering buffer in the decoder.
* For MPEG-2 it is 1 IPB or 0 low delay IP.
* - encoding: Set by libavcodec.
* - decoding: Set by libavcodec.
*/
int has_b_frames;
/**
* qscale factor between P- and I-frames
* If > 0 then the last P-frame quantizer will be used (q = lastp_q * factor + offset).
* If < 0 then normal ratecontrol will be done (q= -normal_q*factor+offset).
* - encoding: Set by user.
* - decoding: unused
*/
float i_quant_factor;
/**
* qscale offset between P and I-frames
* - encoding: Set by user.
* - decoding: unused
*/
float i_quant_offset;
/**
* luminance masking (0-> disabled)
* - encoding: Set by user.
* - decoding: unused
*/
float lumi_masking;
/**
* temporary complexity masking (0-> disabled)
* - encoding: Set by user.
* - decoding: unused
*/
float temporal_cplx_masking;
/**
* spatial complexity masking (0-> disabled)
* - encoding: Set by user.
* - decoding: unused
*/
float spatial_cplx_masking;
/**
* p block masking (0-> disabled)
* - encoding: Set by user.
* - decoding: unused
*/
float p_masking;
/**
* darkness masking (0-> disabled)
* - encoding: Set by user.
* - decoding: unused
*/
float dark_masking;
/**
* slice count
* - encoding: Set by libavcodec.
* - decoding: Set by user (or 0).
*/
int slice_count;
/**
* slice offsets in the frame in bytes
* - encoding: Set/allocated by libavcodec.
* - decoding: Set/allocated by user (or NULL).
*/
int *slice_offset;
/**
* sample aspect ratio (0 if unknown)
* That is the width of a pixel divided by the height of the pixel.
* Numerator and denominator must be relatively prime and smaller than 256 for some video standards.
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
AVRational sample_aspect_ratio;
/**
* motion estimation comparison function
* - encoding: Set by user.
* - decoding: unused
*/
int me_cmp;
/**
* subpixel motion estimation comparison function
* - encoding: Set by user.
* - decoding: unused
*/
int me_sub_cmp;
/**
* macroblock comparison function (not supported yet)
* - encoding: Set by user.
* - decoding: unused
*/
int mb_cmp;
函数调用逻辑慢慢补充
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);
int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame);
int avcodec_send_frame(AVCodecContext *avctx, const AVFrame *frame);
int avcodec_receive_packet(AVCodecContext *avctx, AVPacket *avpkt);
int avcodec_get_hw_frames_parameters(AVCodecContext *avctx,
AVBufferRef *device_ref,
enum AVPixelFormat hw_pix_fmt,
AVBufferRef **out_frames_ref);