From 8fe1597466a3e849f12696e0fa14e3403465020a Mon Sep 17 00:00:00 2001 From: wenchao1024 <87457873+wenchao1024@users.noreply.github.com> Date: Thu, 6 Jan 2022 14:45:33 +0800 Subject: [PATCH] =?UTF-8?q?Create=20FFmpeg=20=E7=BB=93=E6=9E=84=E4=BD=93?= =?UTF-8?q?=E5=AD=A6=E4=B9=A0(=E5=85=AD)=EF=BC=9A=20AVCodecContext=20?= =?UTF-8?q?=E5=88=86=E6=9E=90.md?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- ...体学习(六)： AVCodecContext 分析.md | 1806 +++++++++++++++++ 1 file changed, 1806 insertions(+) create mode 100644 FFmpeg 结构体学习(六)： AVCodecContext 分析.md diff --git a/FFmpeg 结构体学习(六)： AVCodecContext 分析.md b/FFmpeg 结构体学习(六)： AVCodecContext 分析.md new file mode 100644 index 0000000..ad2e00e --- /dev/null +++ b/FFmpeg 结构体学习(六)： AVCodecContext 分析.md @@ -0,0 +1,1806 @@ +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）。下面我们来分析一下该结构体里重要变量的含义和作用。 + +# 一、源码整理 + +首先我们先看一下结构体AVCodecContext的定义的结构体源码(位于libavcodec/avcodec.h)： + + + +``` +/** + * 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, 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, 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; +``` + + + +# 二、AVCodecContext 重点字段 + +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： + + + +``` +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： + + + +``` +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。 + + + +``` +#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 +``` +