FFMPEG结构体分析：AVCodecContext（转）

注：写了一系列的结构体的分析的文章，在这里列一个列表：

FFMPEG结构体分析：AVFrame
FFMPEG结构体分析：AVFormatContext
FFMPEG结构体分析：AVCodecContext
FFMPEG结构体分析：AVIOContext
FFMPEG结构体分析：AVCodec
FFMPEG结构体分析：AVStream
FFMPEG结构体分析：AVPacket

FFMPEG有几个最重要的结构体，包含了解协议，解封装，解码操作，此前已经进行过分析：

FFMPEG中最关键的结构体之间的关系

在此不再详述，其中AVCodecContext是包含变量较多的结构体（感觉差不多是变量最多的结构体）。本文将会大概分析一下该结构体里每个变量的含义和作用。因为如果每个变量都分析的话，工作量太大，实在来不及。

首先看一下结构体的定义（位于avcodec.h）：

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/*
*雷霄骅
*leixiaohua1020@126.com
*中国传媒大学/数字电视技术
*/
/**
* 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.
* Please use AVOptions (av_opt* / av_set/get*()) to access these fields from user
* applications.
* 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;
char codec_name[32];
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;
/**
* fourcc from the AVI stream header (LSB first, so "ABCD" -> ('D'<<24) + ('C'<<16) + ('B'<<8) + 'A').
* This is used to work around some encoder bugs.
* - encoding: unused
* - decoding: Set by user, will be converted to uppercase by libavcodec during init.
*/
unsigned int stream_codec_tag;
#if FF_API_SUB_ID
/**
* @deprecated this field is unused
*/
attribute_deprecated int sub_id;
#endif
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 libavcodec. 0 or some bitrate if this info is available in the stream.
*/
int 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
/**
* CODEC_FLAG_*.
* - encoding: Set by user.
* - decoding: Set by user.
*/
int flags;
/**
* 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
* mpeg4: global headers (they can be in the bitstream or here)
* The allocated memory should be FF_INPUT_BUFFER_PADDING_SIZE bytes larger
* than extradata_size to avoid prolems if it is read with the bitstream reader.
* The bytewise contents of extradata must not depend on the architecture or CPU endianness.
* - 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.
* - encoding: MUST be set by user.
* - decoding: Set by libavcodec.
*/
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;
/**
* 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 is the number of "priming" samples added to the
* beginning of the stream. The decoded output will be delayed by this
* many samples relative to the input to the encoder. Note that this
* field is purely informational and does not directly affect the pts
* output by the encoder, which should always be based on the actual
* presentation time, including any delay.
* 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.
* - encoding: MUST be set by user.
* - decoding: Set by libavcodec.
* Note: For compatibility it is possible to set this instead of
* coded_width/height before decoding.
*/
int width, height;
/**
* Bitstream width / height, may be different from width/height if lowres enabled.
* - encoding: unused
* - decoding: Set by user before init if known. Codec should override / dynamically change if needed.
*/
int coded_width, coded_height;
#define FF_ASPECT_EXTENDED 15
/**
* 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.
* - encoding: Set by user.
* - decoding: Set by user if known, overridden by libavcodec if known
*/
enum AVPixelFormat pix_fmt;
/**
* Motion estimation algorithm used for video coding.
* 1 (zero), 2 (full), 3 (log), 4 (phods), 5 (epzs), 6 (x1), 7 (hex),
* 8 (umh), 9 (iter), 10 (tesa) [7, 8, 10 are x264 specific, 9 is snow specific]
* - encoding: MUST be set by user.
* - decoding: unused
*/
int me_method;
/**
* 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 pixelFormat
* @param fmt is the list of formats which are supported by the codec,
* it is terminated by -1 as 0 is a valid format, the formats are ordered by quality.
* The first is always the native one.
* @return the chosen format
* - encoding: unused
* - decoding: Set by user, if not set the native format will be chosen.
*/
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;
/** obsolete FIXME remove */
int rc_strategy;
#define FF_RC_STRATEGY_XVID 1
int b_frame_strategy;
#if FF_API_MPV_GLOBAL_OPTS
/**
* luma single coefficient elimination threshold
* - encoding: Set by user.
* - decoding: unused
*/
attribute_deprecated int luma_elim_threshold;
/**
* chroma single coeff elimination threshold
* - encoding: Set by user.
* - decoding: unused
*/
attribute_deprecated int chroma_elim_threshold;
#endif
/**
* 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;
/**
* 0-> h263 quant 1-> mpeg quant
* - encoding: Set by user.
* - decoding: unused
*/
int mpeg_quant;
/**
* 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;
/**
* prediction method (needed for huffyuv)
* - encoding: Set by user.
* - decoding: unused
*/
int prediction_method;
#define FF_PRED_LEFT 0
#define FF_PRED_PLANE 1
#define FF_PRED_MEDIAN 2
/**
* 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;
/**
* interlaced DCT comparison function
* - encoding: Set by user.
* - decoding: unused
*/
int ildct_cmp;
#define FF_CMP_SAD 0
#define FF_CMP_SSE 1
#define FF_CMP_SATD 2
#define FF_CMP_DCT 3
#define FF_CMP_PSNR 4
#define FF_CMP_BIT 5
#define FF_CMP_RD 6
#define FF_CMP_ZERO 7
#define FF_CMP_VSAD 8
#define FF_CMP_VSSE 9
#define FF_CMP_NSSE 10
#define FF_CMP_W53 11
#define FF_CMP_W97 12
#define FF_CMP_DCTMAX 13
#define FF_CMP_DCT264 14
#define FF_CMP_CHROMA 256
/**
* ME diamond size & shape
* - encoding: Set by user.
* - decoding: unused
*/
int dia_size;
/**
* amount of previous MV predictors (2a+1 x 2a+1 square)
* - encoding: Set by user.
* - decoding: unused
*/
int last_predictor_count;
/**
* prepass for motion estimation
* - encoding: Set by user.
* - decoding: unused
*/
int pre_me;
/**
* motion estimation prepass comparison function
* - encoding: Set by user.
* - decoding: unused
*/
int me_pre_cmp;
/**
* ME prepass diamond size & shape
* - encoding: Set by user.
* - decoding: unused
*/
int pre_dia_size;
/**
* subpel ME quality
* - encoding: Set by user.
* - decoding: unused
*/
int me_subpel_quality;
/**
* DTG active format information (additional aspect ratio
* information only used in DVB MPEG-2 transport streams)
* 0 if not set.
*
* - encoding: unused
* - decoding: Set by decoder.
*/
int dtg_active_format;
#define FF_DTG_AFD_SAME 8
#define FF_DTG_AFD_4_3 9
#define FF_DTG_AFD_16_9 10
#define FF_DTG_AFD_14_9 11
#define FF_DTG_AFD_4_3_SP_14_9 13
#define FF_DTG_AFD_16_9_SP_14_9 14
#define FF_DTG_AFD_SP_4_3 15
/**
* maximum motion estimation search range in subpel units
* If 0 then no limit.
*
* - encoding: Set by user.
* - decoding: unused
*/
int me_range;
/**
* intra quantizer bias
* - encoding: Set by user.
* - decoding: unused
*/
int intra_quant_bias;
#define FF_DEFAULT_QUANT_BIAS 999999
/**
* inter quantizer bias
* - encoding: Set by user.
* - decoding: unused
*/
int inter_quant_bias;
#if FF_API_COLOR_TABLE_ID
/**
* color table ID
* - encoding: unused
* - decoding: Which clrtable should be used for 8bit RGB images.
* Tables have to be stored somewhere. FIXME
*/
attribute_deprecated int color_table_id;
#endif
/**
* slice flags
* - encoding: unused
* - decoding: Set by user.
*/
int slice_flags;
#define SLICE_FLAG_CODED_ORDER 0x0001 ///< draw_horiz_band() is called in coded order instead of display
#define SLICE_FLAG_ALLOW_FIELD 0x0002 ///< allow draw_horiz_band() with field slices (MPEG2 field pics)
#define SLICE_FLAG_ALLOW_PLANE 0x0004 ///< allow draw_horiz_band() with 1 component at a time (SVQ1)
/**
* XVideo Motion Acceleration
* - encoding: forbidden
* - decoding: set by decoder
*/
int xvmc_acceleration;
/**
* macroblock decision mode
* - encoding: Set by user.
* - decoding: unused
*/
int mb_decision;
#define FF_MB_DECISION_SIMPLE 0 ///< uses mb_cmp
#define FF_MB_DECISION_BITS 1 ///< chooses the one which needs the fewest bits
#define FF_MB_DECISION_RD 2 ///< rate distortion
/**
* custom intra quantization matrix
* - encoding: Set by user, can be NULL.
* - decoding: Set by libavcodec.
*/
uint16_t *intra_matrix;
/**
* custom inter quantization matrix
* - encoding: Set by user, can be NULL.
* - decoding: Set by libavcodec.
*/
uint16_t *inter_matrix;
/**
* scene change detection threshold
* 0 is default, larger means fewer detected scene changes.
* - encoding: Set by user.
* - decoding: unused
*/
int scenechange_threshold;
/**
* noise reduction strength
* - encoding: Set by user.
* - decoding: unused
*/
int noise_reduction;
#if FF_API_INTER_THRESHOLD
/**
* @deprecated this field is unused
*/
attribute_deprecated int inter_threshold;
#endif
#if FF_API_MPV_GLOBAL_OPTS
/**
* @deprecated use mpegvideo private options instead
*/
attribute_deprecated int quantizer_noise_shaping;
#endif
/**
* Motion estimation threshold below which no motion estimation is
* performed, but instead the user specified motion vectors are used.
*
* - encoding: Set by user.
* - decoding: unused
*/
int me_threshold;
/**
* Macroblock threshold below which the user specified macroblock types will be used.
* - encoding: Set by user.
* - decoding: unused
*/
int mb_threshold;
/**
* precision of the intra DC coefficient - 8
* - encoding: Set by user.
* - decoding: unused
*/
int intra_dc_precision;
/**
* Number of macroblock rows at the top which are skipped.
* - encoding: unused
* - decoding: Set by user.
*/
int skip_top;
/**
* Number of macroblock rows at the bottom which are skipped.
* - encoding: unused
* - decoding: Set by user.
*/
int skip_bottom;
/**
* Border processing masking, raises the quantizer for mbs on the borders
* of the picture.
* - encoding: Set by user.
* - decoding: unused
*/
float border_masking;
/**
* minimum MB lagrange multipler
* - encoding: Set by user.
* - decoding: unused
*/
int mb_lmin;
/**
* maximum MB lagrange multipler
* - encoding: Set by user.
* - decoding: unused
*/
int mb_lmax;
/**
*
* - encoding: Set by user.
* - decoding: unused
*/
int me_penalty_compensation;
/**
*
* - encoding: Set by user.
* - decoding: unused
*/
int bidir_refine;
/**
*
* - encoding: Set by user.
* - decoding: unused
*/
int brd_scale;
/**
* minimum GOP size
* - encoding: Set by user.
* - decoding: unused
*/
int keyint_min;
/**
* number of reference frames
* - encoding: Set by user.
* - decoding: Set by lavc.
*/
int refs;
/**
* chroma qp offset from luma
* - encoding: Set by user.
* - decoding: unused
*/
int chromaoffset;
/**
* Multiplied by qscale for each frame and added to scene_change_score.
* - encoding: Set by user.
* - decoding: unused
*/
int scenechange_factor;
/**
*
* Note: Value depends upon the compare function used for fullpel ME.
* - encoding: Set by user.
* - decoding: unused
*/
int mv0_threshold;
/**
* Adjust sensitivity of b_frame_strategy 1.
* - encoding: Set by user.
* - decoding: unused
*/
int b_sensitivity;
/**
* Chromaticity coordinates of the source primaries.
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorPrimaries color_primaries;
/**
* Color Transfer Characteristic.
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorTransferCharacteristic color_trc;
/**
* YUV colorspace type.
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorSpace colorspace;
/**
* MPEG vs JPEG YUV range.
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVColorRange color_range;
/**
* This defines the location of chroma samples.
* - encoding: Set by user
* - decoding: Set by libavcodec
*/
enum AVChromaLocation chroma_sample_location;
/**
* Number of slices.
* Indicates number of picture subdivisions. Used for parallelized
* decoding.
* - encoding: Set by user
* - decoding: unused
*/
int slices;
/** Field order
* - encoding: set by libavcodec
* - decoding: Set by user.
*/
enum AVFieldOrder field_order;
/* audio only */
int sample_rate; ///< samples per second
int channels; ///< number of audio channels
/**
* audio sample format
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
enum AVSampleFormat sample_fmt; ///< sample format
/* The following data should not be initialized. */
/**
* Samples per packet, initialized when calling 'init'.
*/
int frame_size;
/**
* Frame counter, set by libavcodec.
*
* - decoding: total number of frames returned from the decoder so far.
* - encoding: total number of frames passed to the encoder so far.
*
* @note the counter is not incremented if encoding/decoding resulted in
* an error.
*/
int frame_number;
/**
* number of bytes per packet if constant and known or 0
* Used by some WAV based audio codecs.
*/
int block_align;
/**
* Audio cutoff bandwidth (0 means "automatic")
* - encoding: Set by user.
* - decoding: unused
*/
int cutoff;
#if FF_API_REQUEST_CHANNELS
/**
* Decoder should decode to this many channels if it can (0 for default)
* - encoding: unused
* - decoding: Set by user.
* @deprecated Deprecated in favor of request_channel_layout.
*/
int request_channels;
#endif
/**
* Audio channel layout.
* - encoding: set by user.
* - decoding: set by user, may be overwritten by libavcodec.
*/
uint64_t channel_layout;
/**
* Request decoder to use this channel layout if it can (0 for default)
* - encoding: unused
* - decoding: Set by user.
*/
uint64_t request_channel_layout;
/**
* Type of service that the audio stream conveys.
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
enum AVAudioServiceType audio_service_type;
/**
* desired sample format
* - encoding: Not used.
* - decoding: Set by user.
* Decoder will decode to this format if it can.
*/
enum AVSampleFormat request_sample_fmt;
/**
* Called at the beginning of each frame to get a buffer for it.
*
* The function will set AVFrame.data[], AVFrame.linesize[].
* AVFrame.extended_data[] must also be set, but it should be the same as
* AVFrame.data[] except for planar audio with more channels than can fit
* in AVFrame.data[]. In that case, AVFrame.data[] shall still contain as
* many data pointers as it can hold.
*
* if CODEC_CAP_DR1 is not set then get_buffer() must call
* avcodec_default_get_buffer() instead of providing buffers allocated by
* some other means.
*
* AVFrame.data[] should be 32- or 16-byte-aligned unless the CPU doesn't
* need it. avcodec_default_get_buffer() aligns the output buffer properly,
* but if get_buffer() is overridden then alignment considerations should
* be taken into account.
*
* @see avcodec_default_get_buffer()
*
* Video:
*
* If pic.reference is set then the frame will be read later by libavcodec.
* avcodec_align_dimensions2() should be used to find the required width and
* height, as they normally need to be rounded up to the next multiple of 16.
*
* If frame multithreading is used and thread_safe_callbacks is set,
* it may be called from a different thread, but not from more than one at
* once. Does not need to be reentrant.
*
* @see release_buffer(), reget_buffer()
* @see avcodec_align_dimensions2()
*
* Audio:
*
* Decoders request a buffer of a particular size by setting
* AVFrame.nb_samples prior to calling get_buffer(). The decoder may,
* however, utilize only part of the buffer by setting AVFrame.nb_samples
* to a smaller value in the output frame.
*
* Decoders cannot use the buffer after returning from
* avcodec_decode_audio4(), so they will not call release_buffer(), as it
* is assumed to be released immediately upon return.
*
* As a convenience, av_samples_get_buffer_size() and
* av_samples_fill_arrays() in libavutil may be used by custom get_buffer()
* functions to find the required data size and to fill data pointers and
* linesize. In AVFrame.linesize, only linesize[0] may be set for audio
* since all planes must be the same size.
*
* @see av_samples_get_buffer_size(), av_samples_fill_arrays()
*
* - encoding: unused
* - decoding: Set by libavcodec, user can override.
*/
int (*get_buffer)(struct AVCodecContext *c, AVFrame *pic);
/**
* Called to release buffers which were allocated with get_buffer.
* A released buffer can be reused in get_buffer().
* pic.data[*] must be set to NULL.
* May be called from a different thread if frame multithreading is used,
* but not by more than one thread at once, so does not need to be reentrant.
* - encoding: unused
* - decoding: Set by libavcodec, user can override.
*/
void (*release_buffer)(struct AVCodecContext *c, AVFrame *pic);
/**
* Called at the beginning of a frame to get cr buffer for it.
* Buffer type (size, hints) must be the same. libavcodec won't check it.
* libavcodec will pass previous buffer in pic, function should return
* same buffer or new buffer with old frame "painted" into it.
* If pic.data[0] == NULL must behave like get_buffer().
* if CODEC_CAP_DR1 is not set then reget_buffer() must call
* avcodec_default_reget_buffer() instead of providing buffers allocated by
* some other means.
* - encoding: unused
* - decoding: Set by libavcodec, user can override.
*/
int (*reget_buffer)(struct AVCodecContext *c, AVFrame *pic);
/* - encoding parameters */
float qcompress; ///< amount of qscale change between easy & hard scenes (0.0-1.0)
float qblur; ///< amount of qscale smoothing over time (0.0-1.0)
/**
* minimum quantizer
* - encoding: Set by user.
* - decoding: unused
*/
int qmin;
/**
* maximum quantizer
* - encoding: Set by user.
* - decoding: unused
*/
int qmax;
/**
* maximum quantizer difference between frames
* - encoding: Set by user.
* - decoding: unused
*/
int max_qdiff;
/**
* ratecontrol qmin qmax limiting method
* 0-> clipping, 1-> use a nice continuous function to limit qscale wthin qmin/qmax.
* - encoding: Set by user.
* - decoding: unused
*/
float rc_qsquish;
float rc_qmod_amp;
int rc_qmod_freq;
/**
* decoder bitstream buffer size
* - encoding: Set by user.
* - decoding: unused
*/
int rc_buffer_size;
/**
* ratecontrol override, see RcOverride
* - encoding: Allocated/set/freed by user.
* - decoding: unused
*/
int rc_override_count;
RcOverride *rc_override;
/**
* rate control equation
* - encoding: Set by user
* - decoding: unused
*/
const char *rc_eq;
/**
* maximum bitrate
* - encoding: Set by user.
* - decoding: unused
*/
int rc_max_rate;
/**
* minimum bitrate
* - encoding: Set by user.
* - decoding: unused
*/
int rc_min_rate;
float rc_buffer_aggressivity;
/**
* initial complexity for pass1 ratecontrol
* - encoding: Set by user.
* - decoding: unused
*/
float rc_initial_cplx;
/**
* Ratecontrol attempt to use, at maximum, <value> of what can be used without an underflow.
* - encoding: Set by user.
* - decoding: unused.
*/
float rc_max_available_vbv_use;
/**
* Ratecontrol attempt to use, at least, <value> times the amount needed to prevent a vbv overflow.
* - encoding: Set by user.
* - decoding: unused.
*/
float rc_min_vbv_overflow_use;
/**
* Number of bits which should be loaded into the rc buffer before decoding starts.
* - encoding: Set by user.
* - decoding: unused
*/
int rc_initial_buffer_occupancy;
#define FF_CODER_TYPE_VLC 0
#define FF_CODER_TYPE_AC 1
#define FF_CODER_TYPE_RAW 2
#define FF_CODER_TYPE_RLE 3
#define FF_CODER_TYPE_DEFLATE 4
/**
* coder type
* - encoding: Set by user.
* - decoding: unused
*/
int coder_type;
/**
* context model
* - encoding: Set by user.
* - decoding: unused
*/
int context_model;
/**
* minimum Lagrange multipler
* - encoding: Set by user.
* - decoding: unused
*/
int lmin;
/**
* maximum Lagrange multipler
* - encoding: Set by user.
* - decoding: unused
*/
int lmax;
/**
* frame skip threshold
* - encoding: Set by user.
* - decoding: unused
*/
int frame_skip_threshold;
/**
* frame skip factor
* - encoding: Set by user.
* - decoding: unused
*/
int frame_skip_factor;
/**
* frame skip exponent
* - encoding: Set by user.
* - decoding: unused
*/
int frame_skip_exp;
/**
* frame skip comparison function
* - encoding: Set by user.
* - decoding: unused
*/
int frame_skip_cmp;
/**
* trellis RD quantization
* - encoding: Set by user.
* - decoding: unused
*/
int trellis;
/**
* - encoding: Set by user.
* - decoding: unused
*/
int min_prediction_order;
/**
* - encoding: Set by user.
* - decoding: unused
*/
int max_prediction_order;
/**
* GOP timecode frame start number
* - encoding: Set by user, in non drop frame format
* - decoding: Set by libavcodec (timecode in the 25 bits format, -1 if unset)
*/
int64_t timecode_frame_start;
/* The RTP callback: This function is called */
/* every time the encoder has a packet to send. */
/* It depends on the encoder if the data starts */
/* with a Start Code (it should). H.263 does. */
/* mb_nb contains the number of macroblocks */
/* encoded in the RTP payload. */
void (*rtp_callback)(struct AVCodecContext *avctx, void *data, int size, int mb_nb);
int rtp_payload_size; /* The size of the RTP payload: the coder will */
/* do its best to deliver a chunk with size */
/* below rtp_payload_size, the chunk will start */
/* with a start code on some codecs like H.263. */
/* This doesn't take account of any particular */
/* headers inside the transmitted RTP payload. */
/* statistics, used for 2-pass encoding */
int mv_bits;
int header_bits;
int i_tex_bits;
int p_tex_bits;
int i_count;
int p_count;
int skip_count;
int misc_bits;
/**
* number of bits used for the previously encoded frame
* - encoding: Set by libavcodec.
* - decoding: unused
*/
int frame_bits;
/**
* pass1 encoding statistics output buffer
* - encoding: Set by libavcodec.
* - decoding: unused
*/
char *stats_out;
/**
* pass2 encoding statistics input buffer
* Concatenated stuff from stats_out of pass1 should be placed here.
* - encoding: Allocated/set/freed by user.
* - decoding: unused
*/
char *stats_in;
/**
* Work around bugs in encoders which sometimes cannot be detected automatically.
* - encoding: Set by user
* - decoding: Set by user
*/
int workaround_bugs;
#define FF_BUG_AUTODETECT 1 ///< autodetection
#define FF_BUG_OLD_MSMPEG4 2
#define FF_BUG_XVID_ILACE 4
#define FF_BUG_UMP4 8
#define FF_BUG_NO_PADDING 16
#define FF_BUG_AMV 32
#define FF_BUG_AC_VLC 0 ///< Will be removed, libavcodec can now handle these non-compliant files by default.
#define FF_BUG_QPEL_CHROMA 64
#define FF_BUG_STD_QPEL 128
#define FF_BUG_QPEL_CHROMA2 256
#define FF_BUG_DIRECT_BLOCKSIZE 512
#define FF_BUG_EDGE 1024
#define FF_BUG_HPEL_CHROMA 2048
#define FF_BUG_DC_CLIP 4096
#define FF_BUG_MS 8192 ///< Work around various bugs in Microsoft's broken decoders.
#define FF_BUG_TRUNCATED 16384
/**
* strictly follow the standard (MPEG4, ...).
* - encoding: Set by user.
* - decoding: Set by user.
* Setting this to STRICT or higher means the encoder and decoder will
* generally do stupid things, whereas setting it to unofficial or lower
* will mean the encoder might produce output that is not supported by all
* spec-compliant decoders. Decoders don't differentiate between normal,
* unofficial and experimental (that is, they always try to decode things
* when they can) unless they are explicitly asked to behave stupidly
* (=strictly conform to the specs)
*/
int strict_std_compliance;
#define FF_COMPLIANCE_VERY_STRICT 2 ///< Strictly conform to an older more strict version of the spec or reference software.
#define FF_COMPLIANCE_STRICT 1 ///< Strictly conform to all the things in the spec no matter what consequences.
#define FF_COMPLIANCE_NORMAL 0
#define FF_COMPLIANCE_UNOFFICIAL -1 ///< Allow unofficial extensions
#define FF_COMPLIANCE_EXPERIMENTAL -2 ///< Allow nonstandardized experimental things.
/**
* error concealment flags
* - encoding: unused
* - decoding: Set by user.
*/
int error_concealment;
#define FF_EC_GUESS_MVS 1
#define FF_EC_DEBLOCK 2
/**
* debug
* - encoding: Set by user.
* - decoding: Set by user.
*/
int debug;
#define FF_DEBUG_PICT_INFO 1
#define FF_DEBUG_RC 2
#define FF_DEBUG_BITSTREAM 4
#define FF_DEBUG_MB_TYPE 8
#define FF_DEBUG_QP 16
#define FF_DEBUG_MV 32
#define FF_DEBUG_DCT_COEFF 0x00000040
#define FF_DEBUG_SKIP 0x00000080
#define FF_DEBUG_STARTCODE 0x00000100
#define FF_DEBUG_PTS 0x00000200
#define FF_DEBUG_ER 0x00000400
#define FF_DEBUG_MMCO 0x00000800
#define FF_DEBUG_BUGS 0x00001000
#define FF_DEBUG_VIS_QP 0x00002000
#define FF_DEBUG_VIS_MB_TYPE 0x00004000
#define FF_DEBUG_BUFFERS 0x00008000
#define FF_DEBUG_THREADS 0x00010000
/**
* debug
* - encoding: Set by user.
* - decoding: Set by user.
*/
int debug_mv;
#define FF_DEBUG_VIS_MV_P_FOR 0x00000001 //visualize forward predicted MVs of P frames
#define FF_DEBUG_VIS_MV_B_FOR 0x00000002 //visualize forward predicted MVs of B frames
#define FF_DEBUG_VIS_MV_B_BACK 0x00000004 //visualize backward predicted MVs of B frames
/**
* Error recognition; may misdetect some more or less valid parts as errors.
* - encoding: unused
* - decoding: Set by user.
*/
int err_recognition;
#define AV_EF_CRCCHECK (1<<0)
#define AV_EF_BITSTREAM (1<<1)
#define AV_EF_BUFFER (1<<2)
#define AV_EF_EXPLODE (1<<3)
#define AV_EF_CAREFUL (1<<16)
#define AV_EF_COMPLIANT (1<<17)
#define AV_EF_AGGRESSIVE (1<<18)
/**
* opaque 64bit number (generally a PTS) that will be reordered and
* output in AVFrame.reordered_opaque
* @deprecated in favor of pkt_pts
* - encoding: unused
* - decoding: Set by user.
*/
int64_t reordered_opaque;
/**
* Hardware accelerator in use
* - encoding: unused.
* - decoding: Set by libavcodec
*/
struct AVHWAccel *hwaccel;
/**
* Hardware accelerator context.
* For some hardware accelerators, a global context needs to be
* provided by the user. In that case, this holds display-dependent
* data FFmpeg cannot instantiate itself. Please refer to the
* FFmpeg HW accelerator documentation to know how to fill this
* is. e.g. for VA API, this is a struct vaapi_context.
* - encoding: unused
* - decoding: Set by user
*/
void *hwaccel_context;
/**
* error
* - encoding: Set by libavcodec if flags&CODEC_FLAG_PSNR.
* - decoding: unused
*/
uint64_t error[AV_NUM_DATA_POINTERS];
/**
* DCT algorithm, see FF_DCT_* below
* - encoding: Set by user.
* - decoding: unused
*/
int dct_algo;
#define FF_DCT_AUTO 0
#define FF_DCT_FASTINT 1
#define FF_DCT_INT 2
#define FF_DCT_MMX 3
#define FF_DCT_ALTIVEC 5
#define FF_DCT_FAAN 6
/**
* IDCT algorithm, see FF_IDCT_* below.
* - encoding: Set by user.
* - decoding: Set by user.
*/
int idct_algo;
#define FF_IDCT_AUTO 0
#define FF_IDCT_INT 1
#define FF_IDCT_SIMPLE 2
#define FF_IDCT_SIMPLEMMX 3
#define FF_IDCT_LIBMPEG2MMX 4
#define FF_IDCT_MMI 5
#define FF_IDCT_ARM 7
#define FF_IDCT_ALTIVEC 8
#define FF_IDCT_SH4 9
#define FF_IDCT_SIMPLEARM 10
#define FF_IDCT_H264 11
#define FF_IDCT_VP3 12
#define FF_IDCT_IPP 13
#define FF_IDCT_XVIDMMX 14
#define FF_IDCT_CAVS 15
#define FF_IDCT_SIMPLEARMV5TE 16
#define FF_IDCT_SIMPLEARMV6 17
#define FF_IDCT_SIMPLEVIS 18
#define FF_IDCT_WMV2 19
#define FF_IDCT_FAAN 20
#define FF_IDCT_EA 21
#define FF_IDCT_SIMPLENEON 22
#define FF_IDCT_SIMPLEALPHA 23
#define FF_IDCT_BINK 24
#if FF_API_DSP_MASK
/**
* Unused.
* @deprecated use av_set_cpu_flags_mask() instead.
*/
attribute_deprecated unsigned dsp_mask;
#endif
/**
* bits per sample/pixel from the demuxer (needed for huffyuv).
* - encoding: Set by libavcodec.
* - decoding: Set by user.
*/
int bits_per_coded_sample;
/**
* Bits per sample/pixel of internal libavcodec pixel/sample format.
* - encoding: set by user.
* - decoding: set by libavcodec.
*/
int bits_per_raw_sample;
/**
* low resolution decoding, 1-> 1/2 size, 2->1/4 size
* - encoding: unused
* - decoding: Set by user.
*/
int lowres;
/**
* the picture in the bitstream
* - encoding: Set by libavcodec.
* - decoding: Set by libavcodec.
*/
AVFrame *coded_frame;
/**
* thread count
* is used to decide how many independent tasks should be passed to execute()
* - encoding: Set by user.
* - decoding: Set by user.
*/
int thread_count;
/**
* Which multithreading methods to use.
* Use of FF_THREAD_FRAME will increase decoding delay by one frame per thread,
* so clients which cannot provide future frames should not use it.
*
* - encoding: Set by user, otherwise the default is used.
* - decoding: Set by user, otherwise the default is used.
*/
int thread_type;
#define FF_THREAD_FRAME 1 ///< Decode more than one frame at once
#define FF_THREAD_SLICE 2 ///< Decode more than one part of a single frame at once
/**
* Which multithreading methods are in use by the codec.
* - encoding: Set by libavcodec.
* - decoding: Set by libavcodec.
*/
int active_thread_type;
/**
* Set by the client if its custom get_buffer() callback can be called
* synchronously from another thread, which allows faster multithreaded decoding.
* draw_horiz_band() will be called from other threads regardless of this setting.
* Ignored if the default get_buffer() is used.
* - encoding: Set by user.
* - decoding: Set by user.
*/
int thread_safe_callbacks;
/**
* The codec may call this to execute several independent things.
* It will return only after finishing all tasks.
* The user may replace this with some multithreaded implementation,
* the default implementation will execute the parts serially.
* @param count the number of things to execute
* - encoding: Set by libavcodec, user can override.
* - decoding: Set by libavcodec, user can override.
*/
int (*execute)(struct AVCodecContext *c, int (*func)(struct AVCodecContext *c2, void *arg), void *arg2, int *ret, int count, int size);
/**
* The codec may call this to execute several independent things.
* It will return only after finishing all tasks.
* The user may replace this with some multithreaded implementation,
* the default implementation will execute the parts serially.
* Also see avcodec_thread_init and e.g. the --enable-pthread configure option.
* @param c context passed also to func
* @param count the number of things to execute
* @param arg2 argument passed unchanged to func
* @param ret return values of executed functions, must have space for "count" values. May be NULL.
* @param func function that will be called count times, with jobnr from 0 to count-1.
* threadnr will be in the range 0 to c->thread_count-1 < MAX_THREADS and so that no
* two instances of func executing at the same time will have the same threadnr.
* @return always 0 currently, but code should handle a future improvement where when any call to func
* returns < 0 no further calls to func may be done and < 0 is returned.
* - encoding: Set by libavcodec, user can override.
* - decoding: Set by libavcodec, user can override.
*/
int (*execute2)(struct AVCodecContext *c, int (*func)(struct AVCodecContext *c2, void *arg, int jobnr, int threadnr), void *arg2, int *ret, int count);
/**
* thread opaque
* Can be used by execute() to store some per AVCodecContext stuff.
* - encoding: set by execute()
* - decoding: set by execute()
*/
void *thread_opaque;
/**
* noise vs. sse weight for the nsse comparsion function
* - encoding: Set by user.
* - decoding: unused
*/
int nsse_weight;
/**
* profile
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
int profile;
#define FF_PROFILE_UNKNOWN -99
#define FF_PROFILE_RESERVED -100
#define FF_PROFILE_AAC_MAIN 0
#define FF_PROFILE_AAC_LOW 1
#define FF_PROFILE_AAC_SSR 2
#define FF_PROFILE_AAC_LTP 3
#define FF_PROFILE_AAC_HE 4
#define FF_PROFILE_AAC_HE_V2 28
#define FF_PROFILE_AAC_LD 22
#define FF_PROFILE_AAC_ELD 38
#define FF_PROFILE_DTS 20
#define FF_PROFILE_DTS_ES 30
#define FF_PROFILE_DTS_96_24 40
#define FF_PROFILE_DTS_HD_HRA 50
#define FF_PROFILE_DTS_HD_MA 60
#define FF_PROFILE_MPEG2_422 0
#define FF_PROFILE_MPEG2_HIGH 1
#define FF_PROFILE_MPEG2_SS 2
#define FF_PROFILE_MPEG2_SNR_SCALABLE 3
#define FF_PROFILE_MPEG2_MAIN 4
#define FF_PROFILE_MPEG2_SIMPLE 5
#define FF_PROFILE_H264_CONSTRAINED (1<<9) // 8+1; constraint_set1_flag
#define FF_PROFILE_H264_INTRA (1<<11) // 8+3; constraint_set3_flag
#define FF_PROFILE_H264_BASELINE 66
#define FF_PROFILE_H264_CONSTRAINED_BASELINE (66|FF_PROFILE_H264_CONSTRAINED)
#define FF_PROFILE_H264_MAIN 77
#define FF_PROFILE_H264_EXTENDED 88
#define FF_PROFILE_H264_HIGH 100
#define FF_PROFILE_H264_HIGH_10 110
#define FF_PROFILE_H264_HIGH_10_INTRA (110|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_HIGH_422 122
#define FF_PROFILE_H264_HIGH_422_INTRA (122|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_HIGH_444 144
#define FF_PROFILE_H264_HIGH_444_PREDICTIVE 244
#define FF_PROFILE_H264_HIGH_444_INTRA (244|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_CAVLC_444 44
#define FF_PROFILE_VC1_SIMPLE 0
#define FF_PROFILE_VC1_MAIN 1
#define FF_PROFILE_VC1_COMPLEX 2
#define FF_PROFILE_VC1_ADVANCED 3
#define FF_PROFILE_MPEG4_SIMPLE 0
#define FF_PROFILE_MPEG4_SIMPLE_SCALABLE 1
#define FF_PROFILE_MPEG4_CORE 2
#define FF_PROFILE_MPEG4_MAIN 3
#define FF_PROFILE_MPEG4_N_BIT 4
#define FF_PROFILE_MPEG4_SCALABLE_TEXTURE 5
#define FF_PROFILE_MPEG4_SIMPLE_FACE_ANIMATION 6
#define FF_PROFILE_MPEG4_BASIC_ANIMATED_TEXTURE 7
#define FF_PROFILE_MPEG4_HYBRID 8
#define FF_PROFILE_MPEG4_ADVANCED_REAL_TIME 9
#define FF_PROFILE_MPEG4_CORE_SCALABLE 10
#define FF_PROFILE_MPEG4_ADVANCED_CODING 11
#define FF_PROFILE_MPEG4_ADVANCED_CORE 12
#define FF_PROFILE_MPEG4_ADVANCED_SCALABLE_TEXTURE 13
#define FF_PROFILE_MPEG4_SIMPLE_STUDIO 14
#define FF_PROFILE_MPEG4_ADVANCED_SIMPLE 15
/**
* level
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
int level;
#define FF_LEVEL_UNKNOWN -99
/**
*
* - encoding: unused
* - decoding: Set by user.
*/
enum AVDiscard skip_loop_filter;
/**
*
* - encoding: unused
* - decoding: Set by user.
*/
enum AVDiscard skip_idct;
/**
*
* - encoding: unused
* - decoding: Set by user.
*/
enum AVDiscard skip_frame;
/**
* Header containing style information for text subtitles.
* For SUBTITLE_ASS subtitle type, it should contain the whole ASS
* [Script Info] and [V4+ Styles] section, plus the [Events] line and
* the Format line following. It shouldn't include any Dialogue line.
* - encoding: Set/allocated/freed by user (before avcodec_open2())
* - decoding: Set/allocated/freed by libavcodec (by avcodec_open2())
*/
uint8_t *subtitle_header;
int subtitle_header_size;
/**
* Simulates errors in the bitstream to test error concealment.
* - encoding: Set by user.
* - decoding: unused
*/
int error_rate;
/**
* Current packet as passed into the decoder, to avoid having
* to pass the packet into every function. Currently only valid
* inside lavc and get/release_buffer callbacks.
* - decoding: set by avcodec_decode_*, read by get_buffer() for setting pkt_pts
* - encoding: unused
*/
AVPacket *pkt;
/**
* VBV delay coded in the last frame (in periods of a 27 MHz clock).
* Used for compliant TS muxing.
* - encoding: Set by libavcodec.
* - decoding: unused.
*/
uint64_t vbv_delay;
/**
* Timebase in which pkt_dts/pts and AVPacket.dts/pts are.
* Code outside libavcodec should access this field using:
* avcodec_set_pkt_timebase(avctx)
* - encoding unused.
* - decodimg set by user
*/
AVRational pkt_timebase;
/**
* AVCodecDescriptor
* Code outside libavcodec should access this field using:
* avcodec_get_codec_descriptior(avctx)
* - encoding: unused.
* - decoding: set by libavcodec.
*/
const AVCodecDescriptor *codec_descriptor;
/**
* Current statistics for PTS correction.
* - decoding: maintained and used by libavcodec, not intended to be used by user apps
* - encoding: unused
*/
int64_t pts_correction_num_faulty_pts; /// Number of incorrect PTS values so far
int64_t pts_correction_num_faulty_dts; /// Number of incorrect DTS values so far
int64_t pts_correction_last_pts; /// PTS of the last frame
int64_t pts_correction_last_dts; /// DTS of the last frame
} AVCodecContext;

光定义就真是够多的。下面挑一些关键的变量来看看（这里只考虑解码）。

enum AVMediaType codec_type：编解码器的类型（视频，音频...）

struct AVCodec *codec：采用的解码器AVCodec（H.264,MPEG2...）

int bit_rate：平均比特率

uint8_t *extradata; int extradata_size：针对特定编码器包含的附加信息（例如对于H.264解码器来说，存储SPS，PPS等）

AVRational time_base：根据该参数，可以把PTS转化为实际的时间（单位为秒s）

int width, height：如果是视频的话，代表宽和高

int refs：运动估计参考帧的个数（H.264的话会有多帧，MPEG2这类的一般就没有了）

int sample_rate：采样率（音频）

int channels：声道数（音频）

enum AVSampleFormat sample_fmt：采样格式

int profile：型（H.264里面就有，其他编码标准应该也有）

int level：级（和profile差不太多）

在这里需要注意：AVCodecContext中很多的参数是编码的时候使用的，而不是解码的时候使用的。

其实这些参数都比较容易理解。就不多费篇幅了。在这里看一下以下几个参数：

1.codec_type

编解码器类型有以下几种：

[cpp] view plaincopy

enum AVMediaType {
AVMEDIA_TYPE_UNKNOWN = -1, ///< Usually treated as AVMEDIA_TYPE_DATA
AVMEDIA_TYPE_VIDEO,
AVMEDIA_TYPE_AUDIO,
AVMEDIA_TYPE_DATA, ///< Opaque data information usually continuous
AVMEDIA_TYPE_SUBTITLE,
AVMEDIA_TYPE_ATTACHMENT, ///< Opaque data information usually sparse
AVMEDIA_TYPE_NB
};

2.sample_fmt

在FFMPEG中音频采样格式有以下几种：

[cpp] view plaincopy

enum AVSampleFormat {
AV_SAMPLE_FMT_NONE = -1,
AV_SAMPLE_FMT_U8, ///< unsigned 8 bits
AV_SAMPLE_FMT_S16, ///< signed 16 bits
AV_SAMPLE_FMT_S32, ///< signed 32 bits
AV_SAMPLE_FMT_FLT, ///< float
AV_SAMPLE_FMT_DBL, ///< double
AV_SAMPLE_FMT_U8P, ///< unsigned 8 bits, planar
AV_SAMPLE_FMT_S16P, ///< signed 16 bits, planar
AV_SAMPLE_FMT_S32P, ///< signed 32 bits, planar
AV_SAMPLE_FMT_FLTP, ///< float, planar
AV_SAMPLE_FMT_DBLP, ///< double, planar
AV_SAMPLE_FMT_NB ///< Number of sample formats. DO NOT USE if linking dynamically
};

3.profile

在FFMPEG中型有以下几种，可以看出AAC，MPEG2，H.264，VC-1，MPEG4都有型的概念。

[cpp] view plaincopy

#define FF_PROFILE_UNKNOWN -99
#define FF_PROFILE_RESERVED -100
#define FF_PROFILE_AAC_MAIN 0
#define FF_PROFILE_AAC_LOW 1
#define FF_PROFILE_AAC_SSR 2
#define FF_PROFILE_AAC_LTP 3
#define FF_PROFILE_AAC_HE 4
#define FF_PROFILE_AAC_HE_V2 28
#define FF_PROFILE_AAC_LD 22
#define FF_PROFILE_AAC_ELD 38
#define FF_PROFILE_DTS 20
#define FF_PROFILE_DTS_ES 30
#define FF_PROFILE_DTS_96_24 40
#define FF_PROFILE_DTS_HD_HRA 50
#define FF_PROFILE_DTS_HD_MA 60
#define FF_PROFILE_MPEG2_422 0
#define FF_PROFILE_MPEG2_HIGH 1
#define FF_PROFILE_MPEG2_SS 2
#define FF_PROFILE_MPEG2_SNR_SCALABLE 3
#define FF_PROFILE_MPEG2_MAIN 4
#define FF_PROFILE_MPEG2_SIMPLE 5
#define FF_PROFILE_H264_CONSTRAINED (1<<9) // 8+1; constraint_set1_flag
#define FF_PROFILE_H264_INTRA (1<<11) // 8+3; constraint_set3_flag
#define FF_PROFILE_H264_BASELINE 66
#define FF_PROFILE_H264_CONSTRAINED_BASELINE (66|FF_PROFILE_H264_CONSTRAINED)
#define FF_PROFILE_H264_MAIN 77
#define FF_PROFILE_H264_EXTENDED 88
#define FF_PROFILE_H264_HIGH 100
#define FF_PROFILE_H264_HIGH_10 110
#define FF_PROFILE_H264_HIGH_10_INTRA (110|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_HIGH_422 122
#define FF_PROFILE_H264_HIGH_422_INTRA (122|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_HIGH_444 144
#define FF_PROFILE_H264_HIGH_444_PREDICTIVE 244
#define FF_PROFILE_H264_HIGH_444_INTRA (244|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_CAVLC_444 44
#define FF_PROFILE_VC1_SIMPLE 0
#define FF_PROFILE_VC1_MAIN 1
#define FF_PROFILE_VC1_COMPLEX 2
#define FF_PROFILE_VC1_ADVANCED 3
#define FF_PROFILE_MPEG4_SIMPLE 0
#define FF_PROFILE_MPEG4_SIMPLE_SCALABLE 1
#define FF_PROFILE_MPEG4_CORE 2
#define FF_PROFILE_MPEG4_MAIN 3
#define FF_PROFILE_MPEG4_N_BIT 4
#define FF_PROFILE_MPEG4_SCALABLE_TEXTURE 5
#define FF_PROFILE_MPEG4_SIMPLE_FACE_ANIMATION 6
#define FF_PROFILE_MPEG4_BASIC_ANIMATED_TEXTURE 7
#define FF_PROFILE_MPEG4_HYBRID 8
#define FF_PROFILE_MPEG4_ADVANCED_REAL_TIME 9
#define FF_PROFILE_MPEG4_CORE_SCALABLE 10
#define FF_PROFILE_MPEG4_ADVANCED_CODING 11
#define FF_PROFILE_MPEG4_ADVANCED_CORE 12
#define FF_PROFILE_MPEG4_ADVANCED_SCALABLE_TEXTURE 13
#define FF_PROFILE_MPEG4_SIMPLE_STUDIO 14
#define FF_PROFILE_MPEG4_ADVANCED_SIMPLE 15

转载于:https://www.cnblogs.com/xihong2014/p/6707867.html

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