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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：AVFrameFFMPEG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：
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）：
/* *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. */给我老师的人工智能教程打call！http://blog.youkuaiyun.com/jiangjunshow
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