[置顶] FFmpeg总结（三）AV系列结构体之AVCodecContext

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描述：主要扩展API的结构体

New fields can be added to the end with minor version bumps. Removal, reordering and changes to existing fields require a major version bump. You can use AVOptions (av_opt* / av_set/get*()) to access these fields from user applications. The name string for AVOptions options matches the associated command line parameter name and can be found in libavcodec/options_table.h The AVOption/command line parameter names differ in some cases from the C structure field names for historic reasons or brevity. sizeof(AVCodecContext) must not be used outside libav*.

typedef struct AVCodecContext {
    /** * information on struct for av_log * - set by avcodec_alloc_context3 */
    const AVClass *av_class;
    int log_level_offset;

    enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
    const struct AVCodec  *codec;
#if FF_API_CODEC_NAME
    /** * @deprecated this field is not used for anything in libavcodec */
    attribute_deprecated
    char             codec_name[32];
#endif
    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;

#if FF_API_STREAM_CODEC_TAG
    /** * @deprecated this field is unused */
    attribute_deprecated
    unsigned int stream_codec_tag;
#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 user, may be overwritten by libavcodec * if this info is available in the stream */
    int64_t bit_rate;

    /** * number of bits the bitstream is allowed to diverge from the reference. * the reference can be CBR (for CBR pass1) or VBR (for pass2) * - encoding: Set by user; unused for constant quantizer encoding. * - decoding: unused */
    int bit_rate_tolerance;

    /** * Global quality for codecs which cannot change it per frame. * This should be proportional to MPEG-1/2/4 qscale. * - encoding: Set by user. * - decoding: unused */
    int global_quality;

    /** * - encoding: Set by user. * - decoding: unused */
    int compression_level;
#define FF_COMPRESSION_DEFAULT -1

    /** * AV_CODEC_FLAG_*. * - encoding: Set by user. * - decoding: Set by user. */
    int flags;

    /** * AV_CODEC_FLAG2_* * - encoding: Set by user. * - decoding: Set by user. */
    int flags2;

    /** * some codecs need / can use extradata like Huffman tables. * MJPEG: Huffman tables * rv10: additional flags * MPEG-4: global headers (they can be in the bitstream or here) * The allocated memory should be AV_INPUT_BUFFER_PADDING_SIZE bytes larger * than extradata_size to avoid problems if it is read with the bitstream reader. * The bytewise contents of extradata must not depend on the architecture or CPU endianness. * - encoding: Set/allocated/freed by libavcodec. * - decoding: Set/allocated/freed by user. */
    uint8_t *extradata;
    int extradata_size;

    /** * This is the fundamental unit of time (in seconds) in terms * of which frame timestamps are represented. For fixed-fps content, * timebase should be 1/framerate and timestamp increments should be * identically 1. * This often, but not always is the inverse of the frame rate or field rate * for video. 1/time_base is not the average frame rate if the frame rate is not * constant. * * Like containers, elementary streams also can store timestamps, 1/time_base * is the unit in which these timestamps are specified. * As example of such codec time base see ISO/IEC 14496-2:2001(E) * vop_time_increment_resolution and fixed_vop_rate * (fixed_vop_rate == 0 implies that it is different from the framerate) * * - encoding: MUST be set by user. * - decoding: the use of this field for decoding is deprecated. * Use framerate instead. */
    AVRational time_base;

    /** * For some codecs, the time base is closer to the field rate than the frame rate. * Most notably, H.264 and MPEG-2 specify time_base as half of frame duration * if no telecine is used ... * * Set to time_base ticks per frame. Default 1, e.g., H.264/MPEG-2 set it to 2. */
    int ticks_per_frame;

    /** * Codec delay. * * Encoding: Number of frames delay there will be from the encoder input to * the decoder output. (we assume the decoder matches the spec) * Decoding: Number of frames delay in addition to what a standard decoder * as specified in the spec would produce. * * Video: * Number of frames the decoded output will be delayed relative to the * encoded input. * * Audio: * For encoding, this field is unused (see initial_padding). * * For decoding, this is the number of samples the decoder needs to * output before the decoder's output is valid. When seeking, you should * start decoding this many samples prior to your desired seek point. * * - encoding: Set by libavcodec. * - decoding: Set by libavcodec. */
    int delay;


    /* video only */
    /** * picture width / height. * * @note Those fields may not match the values of the last * AVFrame output by avcodec_decode_video2 due frame * reordering. * * - encoding: MUST be set by user. * - decoding: May be set by the user before opening the decoder if known e.g. * from the container. Some decoders will require the dimensions * to be set by the caller. During decoding, the decoder may * overwrite those values as required while parsing the data. */
    int width, height;

    /** * Bitstream width / height, may be different from width/height e.g. when * the decoded frame is cropped before being output or lowres is enabled. * * @note Those field may not match the value of the last * AVFrame output by avcodec_receive_frame() due frame * reordering. * * - encoding: unused * - decoding: May be set by the user before opening the decoder if known * e.g. from the container. During decoding, the decoder may * overwrite those values as required while parsing the data. */
    int coded_width, coded_height;

#if FF_API_ASPECT_EXTENDED
#define FF_ASPECT_EXTENDED 15
#endif

    /** * the number of pictures in a group of pictures, or 0 for intra_only * - encoding: Set by user. * - decoding: unused */
    int gop_size;

    /** * Pixel format, see AV_PIX_FMT_xxx. * May be set by the demuxer if known from headers. * May be overridden by the decoder if it knows better. * * @note This field may not match the value of the last * AVFrame output by avcodec_receive_frame() due frame * reordering. * * - encoding: Set by user. * - decoding: Set by user if known, overridden by libavcodec while * parsing the data. */
    enum AVPixelFormat pix_fmt;

#if FF_API_MOTION_EST
    /** * This option does nothing * @deprecated use codec private options instead */
    attribute_deprecated int me_method;
#endif

    /** * 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. * @note The callback may be called again immediately if initialization for * the selected (hardware-accelerated) pixel format failed. * @warning Behavior is undefined if the callback returns a value not * in the fmt list of formats. * @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;

#if FF_API_RC_STRATEGY
    /** @deprecated use codec private option instead */
    attribute_deprecated int rc_strategy;
#define FF_RC_STRATEGY_XVID 1
#endif

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int b_frame_strategy;
#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;

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int mpeg_quant;
#endif

    /** * 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;

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
     int prediction_method;
#define FF_PRED_LEFT   0
#define FF_PRED_PLANE  1
#define FF_PRED_MEDIAN 2
#endif

    /** * 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;

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int pre_me;
#endif

    /** * 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;

#if FF_API_AFD
    /** * 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. * @deprecated Deprecated in favor of AVSideData */
    attribute_deprecated 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
#endif /* FF_API_AFD */

    /** * maximum motion estimation search range in subpel units * If 0 then no limit. * * - encoding: Set by user. * - decoding: unused */
    int me_range;

#if FF_API_QUANT_BIAS
    /** * @deprecated use encoder private option instead */
    attribute_deprecated int intra_quant_bias;
#define FF_DEFAULT_QUANT_BIAS 999999

    /** * @deprecated use encoder private option instead */
    attribute_deprecated int inter_quant_bias;
#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 (MPEG-2 field pics)
#define SLICE_FLAG_ALLOW_PLANE    0x0004 ///< allow draw_horiz_band() with 1 component at a time (SVQ1)

#if FF_API_XVMC
    /** * XVideo Motion Acceleration * - encoding: forbidden * - decoding: set by decoder * @deprecated XvMC doesn't need it anymore. */
    attribute_deprecated int xvmc_acceleration;
#endif /* FF_API_XVMC */

    /** * 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;

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int scenechange_threshold;

    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int noise_reduction;
#endif

#if FF_API_MPV_OPT
    /** * @deprecated this field is unused */
    attribute_deprecated
    int me_threshold;

    /** * @deprecated this field is unused */
    attribute_deprecated
    int mb_threshold;
#endif

    /** * precision of the intra DC coefficient - 8 * - encoding: Set by user. * - decoding: Set by libavcodec */
    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;

#if FF_API_MPV_OPT
    /** * @deprecated use encoder private options instead */
    attribute_deprecated
    float border_masking;
#endif

    /** * minimum MB Lagrange multiplier * - encoding: Set by user. * - decoding: unused */
    int mb_lmin;

    /** * maximum MB Lagrange multiplier * - encoding: Set by user. * - decoding: unused */
    int mb_lmax;

#if FF_API_PRIVATE_OPT
    /** * @deprecated use encoder private options instead */
    attribute_deprecated
    int me_penalty_compensation;
#endif

    /** * - encoding: Set by user. * - decoding: unused */
    int bidir_refine;

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int brd_scale;
#endif

    /** * 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;

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int chromaoffset;
#endif

#if FF_API_UNUSED_MEMBERS
    /** * Multiplied by qscale for each frame and added to scene_change_score. * - encoding: Set by user. * - decoding: unused */
    attribute_deprecated int scenechange_factor;
#endif

    /** * Note: Value depends upon the compare function used for fullpel ME. * - encoding: Set by user. * - decoding: unused */
    int mv0_threshold;

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int b_sensitivity;
#endif

    /** * 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. */
    /** * Number of samples per channel in an audio frame. * * - encoding: set by libavcodec in avcodec_open2(). Each submitted frame * except the last must contain exactly frame_size samples per channel. * May be 0 when the codec has AV_CODEC_CAP_VARIABLE_FRAME_SIZE set, then the * frame size is not restricted. * - decoding: may be set by some decoders to indicate constant frame size */
    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;

    /** * 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;

    /** * This callback is called at the beginning of each frame to get data * buffer(s) for it. There may be one contiguous buffer for all the data or * there may be a buffer per each data plane or anything in between. What * this means is, you may set however many entries in buf[] you feel necessary. * Each buffer must be reference-counted using the AVBuffer API (see description * of buf[] below). * * The following fields will be set in the frame before this callback is * called: * - format * - width, height (video only) * - sample_rate, channel_layout, nb_samples (audio only) * Their values may differ from the corresponding values in * AVCodecContext. This callback must use the frame values, not the codec * context values, to calculate the required buffer size. * * This callback must fill the following fields in the frame: * - data[] * - linesize[] * - extended_data: * * if the data is planar audio with more than 8 channels, then this * callback must allocate and fill extended_data to contain all pointers * to all data planes. data[] must hold as many pointers as it can. * extended_data must be allocated with av_malloc() and will be freed in * av_frame_unref(). * * otherwise extended_data must point to data * - buf[] must contain one or more pointers to AVBufferRef structures. Each of * the frame's data and extended_data pointers must be contained in these. That * is, one AVBufferRef for each allocated chunk of memory, not necessarily one * AVBufferRef per data[] entry. See: av_buffer_create(), av_buffer_alloc(), * and av_buffer_ref(). * - extended_buf and nb_extended_buf must be allocated with av_malloc() by * this callback and filled with the extra buffers if there are more * buffers than buf[] can hold. extended_buf will be freed in * av_frame_unref(). * * If AV_CODEC_CAP_DR1 is not set then get_buffer2() must call * avcodec_default_get_buffer2() instead of providing buffers allocated by * some other means. * * Each data plane must be aligned to the maximum required by the target * CPU. * * @see avcodec_default_get_buffer2() * * Video: * * If AV_GET_BUFFER_FLAG_REF is set in flags then the frame may be reused * (read and/or written to if it is writable) 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. * * Some decoders do not support linesizes changing between frames. * * If frame multithreading is used and thread_safe_callbacks is set, * this callback may be called from a different thread, but not from more * than one at once. Does not need to be reentrant. * * @see avcodec_align_dimensions2() * * Audio: * * Decoders request a buffer of a particular size by setting * AVFrame.nb_samples prior to calling get_buffer2(). The decoder may, * however, utilize only part of the buffer by setting AVFrame.nb_samples * to a smaller value in the output frame. * * As a convenience, av_samples_get_buffer_size() and * av_samples_fill_arrays() in libavutil may be used by custom get_buffer2() * 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_buffer2)(struct AVCodecContext *s, AVFrame *frame, int flags);

    /** * If non-zero, the decoded audio and video frames returned from * avcodec_decode_video2() and avcodec_decode_audio4() are reference-counted * and are valid indefinitely. The caller must free them with * av_frame_unref() when they are not needed anymore. * Otherwise, the decoded frames must not be freed by the caller and are * only valid until the next decode call. * * This is always automatically enabled if avcodec_receive_frame() is used. * * - encoding: unused * - decoding: set by the caller before avcodec_open2(). */
    int refcounted_frames;

    /* - 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;

#if FF_API_MPV_OPT
    /** * @deprecated use encoder private options instead */
    attribute_deprecated
    float rc_qsquish;

    attribute_deprecated
    float rc_qmod_amp;
    attribute_deprecated
    int rc_qmod_freq;
#endif

    /** * 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;

#if FF_API_MPV_OPT
    /** * @deprecated use encoder private options instead */
    attribute_deprecated
    const char *rc_eq;
#endif

    /** * maximum bitrate * - encoding: Set by user. * - decoding: Set by user, may be overwritten by libavcodec. */
    int64_t rc_max_rate;

    /** * minimum bitrate * - encoding: Set by user. * - decoding: unused */
    int64_t rc_min_rate;

#if FF_API_MPV_OPT
    /** * @deprecated use encoder private options instead */
    attribute_deprecated
    float rc_buffer_aggressivity;

    attribute_deprecated
    float rc_initial_cplx;
#endif

    /** * 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;

#if FF_API_CODER_TYPE
#define FF_CODER_TYPE_VLC       0
#define FF_CODER_TYPE_AC        1
#define FF_CODER_TYPE_RAW       2
#define FF_CODER_TYPE_RLE       3
#if FF_API_UNUSED_MEMBERS
#define FF_CODER_TYPE_DEFLATE   4
#endif /* FF_API_UNUSED_MEMBERS */
    /** * @deprecated use encoder private options instead */
    attribute_deprecated
    int coder_type;
#endif /* FF_API_CODER_TYPE */

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int context_model;
#endif

#if FF_API_MPV_OPT
    /** * @deprecated use encoder private options instead */
    attribute_deprecated
    int lmin;

    /** * @deprecated use encoder private options instead */
    attribute_deprecated
    int lmax;
#endif

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int frame_skip_threshold;

    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int frame_skip_factor;

    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int frame_skip_exp;

    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int frame_skip_cmp;
#endif /* FF_API_PRIVATE_OPT */

    /** * trellis RD quantization * - encoding: Set by user. * - decoding: unused */
    int trellis;

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int min_prediction_order;

    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int max_prediction_order;

    /** @deprecated use encoder private options instead */
    attribute_deprecated
    int64_t timecode_frame_start;
#endif

#if FF_API_RTP_CALLBACK
    /** * @deprecated unused */
    /* 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. */
    attribute_deprecated
    void (*rtp_callback)(struct AVCodecContext *avctx, void *data, int size, int mb_nb);
#endif

#if FF_API_PRIVATE_OPT
    /** @deprecated use encoder private options instead */
    attribute_deprecated
    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. */
#endif

#if FF_API_STAT_BITS
    /* statistics, used for 2-pass encoding */
    attribute_deprecated
    int mv_bits;
    attribute_deprecated
    int header_bits;
    attribute_deprecated
    int i_tex_bits;
    attribute_deprecated
    int p_tex_bits;
    attribute_deprecated
    int i_count;
    attribute_deprecated
    int p_count;
    attribute_deprecated
    int skip_count;
    attribute_deprecated
    int misc_bits;

    /** @deprecated this field is unused */
    attribute_deprecated
    int frame_bits;
#endif

    /** * 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
#if FF_API_OLD_MSMPEG4
#define FF_BUG_OLD_MSMPEG4      2
#endif
#define FF_BUG_XVID_ILACE       4
#define FF_BUG_UMP4             8
#define FF_BUG_NO_PADDING       16
#define FF_BUG_AMV              32
#if FF_API_AC_VLC
#define FF_BUG_AC_VLC           0  ///< Will be removed, libavcodec can now handle these non-compliant files by default.
#endif
#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 (MPEG-4, ...). * - 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
#define FF_EC_FAVOR_INTER 256

    /** * 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
#if FF_API_DEBUG_MV
/** * @deprecated this option does nothing */
#define FF_DEBUG_MV          32
#endif
#define FF_DEBUG_DCT_COEFF   0x00000040
#define FF_DEBUG_SKIP        0x00000080
#define FF_DEBUG_STARTCODE   0x00000100
#if FF_API_UNUSED_MEMBERS
#define FF_DEBUG_PTS         0x00000200
#endif /* FF_API_UNUSED_MEMBERS */
#define FF_DEBUG_ER          0x00000400
#define FF_DEBUG_MMCO        0x00000800
#define FF_DEBUG_BUGS        0x00001000
#if FF_API_DEBUG_MV
#define FF_DEBUG_VIS_QP      0x00002000 ///< only access through AVOptions from outside libavcodec
#define FF_DEBUG_VIS_MB_TYPE 0x00004000 ///< only access through AVOptions from outside libavcodec
#endif
#define FF_DEBUG_BUFFERS     0x00008000
#define FF_DEBUG_THREADS     0x00010000
#define FF_DEBUG_GREEN_MD    0x00800000
#define FF_DEBUG_NOMC        0x01000000

#if FF_API_DEBUG_MV
    /** * debug * Code outside libavcodec should access this field using AVOptions * - 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
#endif

    /** * Error recognition; may misdetect some more or less valid parts as errors. * - encoding: unused * - decoding: Set by user. */
    int err_recognition;

/** * Verify checksums embedded in the bitstream (could be of either encoded or * decoded data, depending on the codec) and print an error message on mismatch. * If AV_EF_EXPLODE is also set, a mismatching checksum will result in the * decoder returning an error. */
#define AV_EF_CRCCHECK  (1<<0)
#define AV_EF_BITSTREAM (1<<1)          ///< detect bitstream specification deviations
#define AV_EF_BUFFER    (1<<2)          ///< detect improper bitstream length
#define AV_EF_EXPLODE   (1<<3)          ///< abort decoding on minor error detection

#define AV_EF_IGNORE_ERR (1<<15)        ///< ignore errors and continue
#define AV_EF_CAREFUL    (1<<16)        ///< consider things that violate the spec, are fast to calculate and have not been seen in the wild as errors
#define AV_EF_COMPLIANT  (1<<17)        ///< consider all spec non compliances as errors
#define AV_EF_AGGRESSIVE (1<<18)        ///< consider things that a sane encoder should not do as an error


    /** * opaque 64-bit number (generally a PTS) that will be reordered and * output in AVFrame.reordered_opaque * - 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 & AV_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_ARM           7
#define FF_IDCT_ALTIVEC       8
#if FF_API_ARCH_SH4
#define FF_IDCT_SH4           9
#endif
#define FF_IDCT_SIMPLEARM     10
#if FF_API_UNUSED_MEMBERS
#define FF_IDCT_IPP           13
#endif /* FF_API_UNUSED_MEMBERS */
#define FF_IDCT_XVID          14
#if FF_API_IDCT_XVIDMMX
#define FF_IDCT_XVIDMMX       14
#endif /* FF_API_IDCT_XVIDMMX */
#define FF_IDCT_SIMPLEARMV5TE 16
#define FF_IDCT_SIMPLEARMV6   17
#if FF_API_ARCH_SPARC
#define FF_IDCT_SIMPLEVIS     18
#endif
#define FF_IDCT_FAAN          20
#define FF_IDCT_SIMPLENEON    22
#if FF_API_ARCH_ALPHA
#define FF_IDCT_SIMPLEALPHA   23
#endif
#define FF_IDCT_SIMPLEAUTO    128

    /** * 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;

#if FF_API_LOWRES
    /** * low resolution decoding, 1-> 1/2 size, 2->1/4 size * - encoding: unused * - decoding: Set by user. * Code outside libavcodec should access this field using: * av_codec_{get,set}_lowres(avctx) */
     int lowres;
#endif

#if FF_API_CODED_FRAME
    /** * the picture in the bitstream * - encoding: Set by libavcodec. * - decoding: unused * * @deprecated use the quality factor packet side data instead */
    attribute_deprecated AVFrame *coded_frame;
#endif

    /** * 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);

    /** * noise vs. sse weight for the nsse comparison 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_MPEG2_AAC_LOW 128
#define FF_PROFILE_MPEG2_AAC_HE  131

#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_DTS_EXPRESS 70

#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

#define FF_PROFILE_JPEG2000_CSTREAM_RESTRICTION_0   1
#define FF_PROFILE_JPEG2000_CSTREAM_RESTRICTION_1   2
#define FF_PROFILE_JPEG2000_CSTREAM_NO_RESTRICTION  32768
#define FF_PROFILE_JPEG2000_DCINEMA_2K              3
#define FF_PROFILE_JPEG2000_DCINEMA_4K              4

#define FF_PROFILE_VP9_0                            0
#define FF_PROFILE_VP9_1                            1
#define FF_PROFILE_VP9_2                            2
#define FF_PROFILE_VP9_3                            3

#define FF_PROFILE_HEVC_MAIN                        1
#define FF_PROFILE_HEVC_MAIN_10                     2
#define FF_PROFILE_HEVC_MAIN_STILL_PICTURE          3
#define FF_PROFILE_HEVC_REXT                        4

    /** * level * - encoding: Set by user. * - decoding: Set by libavcodec. */
     int level;
#define FF_LEVEL_UNKNOWN -99

    /** * Skip loop filtering for selected frames. * - encoding: unused * - decoding: Set by user. */
    enum AVDiscard skip_loop_filter;

    /** * Skip IDCT/dequantization for selected frames. * - encoding: unused * - decoding: Set by user. */
    enum AVDiscard skip_idct;

    /** * Skip decoding for selected frames. * - 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;

#if FF_API_ERROR_RATE
    /** * @deprecated use the 'error_rate' private AVOption of the mpegvideo * encoders */
    attribute_deprecated
    int error_rate;
#endif

#if FF_API_VBV_DELAY
    /** * 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. * @deprecated this value is now exported as a part of * AV_PKT_DATA_CPB_PROPERTIES packet side data */
    attribute_deprecated
    uint64_t vbv_delay;
#endif

#if FF_API_SIDEDATA_ONLY_PKT
    /** * Encoding only and set by default. Allow encoders to output packets * that do not contain any encoded data, only side data. * * Some encoders need to output such packets, e.g. to update some stream * parameters at the end of encoding. * * @deprecated this field disables the default behaviour and * it is kept only for compatibility. */
    attribute_deprecated
    int side_data_only_packets;
#endif

    /** * Audio only. The number of "priming" samples (padding) inserted by the * encoder at the beginning of the audio. I.e. this number of leading * decoded samples must be discarded by the caller to get the original audio * without leading padding. * * - decoding: unused * - encoding: Set by libavcodec. The timestamps on the output packets are * adjusted by the encoder so that they always refer to the * first sample of the data actually contained in the packet, * including any added padding. E.g. if the timebase is * 1/samplerate and the timestamp of the first input sample is * 0, the timestamp of the first output packet will be * -initial_padding. */
    int initial_padding;

    /** * - decoding: For codecs that store a framerate value in the compressed * bitstream, the decoder may export it here. { 0, 1} when * unknown. * - encoding: May be used to signal the framerate of CFR content to an * encoder. */
    AVRational framerate;

    /** * Nominal unaccelerated pixel format, see AV_PIX_FMT_xxx. * - encoding: unused. * - decoding: Set by libavcodec before calling get_format() */
    enum AVPixelFormat sw_pix_fmt;

    /** * Timebase in which pkt_dts/pts and AVPacket.dts/pts are. * Code outside libavcodec should access this field using: * av_codec_{get,set}_pkt_timebase(avctx) * - encoding unused. * - decoding set by user. */
    AVRational pkt_timebase;

    /** * AVCodecDescriptor * Code outside libavcodec should access this field using: * av_codec_{get,set}_codec_descriptor(avctx) * - encoding: unused. * - decoding: set by libavcodec. */
    const AVCodecDescriptor *codec_descriptor;

#if !FF_API_LOWRES
    /** * low resolution decoding, 1-> 1/2 size, 2->1/4 size * - encoding: unused * - decoding: Set by user. * Code outside libavcodec should access this field using: * av_codec_{get,set}_lowres(avctx) */
     int lowres;
#endif

    /** * 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

    /** * Character encoding of the input subtitles file. * - decoding: set by user * - encoding: unused */
    char *sub_charenc;

    /** * Subtitles character encoding mode. Formats or codecs might be adjusting * this setting (if they are doing the conversion themselves for instance). * - decoding: set by libavcodec * - encoding: unused */
    int sub_charenc_mode;
#define FF_SUB_CHARENC_MODE_DO_NOTHING  -1  ///< do nothing (demuxer outputs a stream supposed to be already in UTF-8, or the codec is bitmap for instance)
#define FF_SUB_CHARENC_MODE_AUTOMATIC    0  ///< libavcodec will select the mode itself
#define FF_SUB_CHARENC_MODE_PRE_DECODER  1  ///< the AVPacket data needs to be recoded to UTF-8 before being fed to the decoder, requires iconv

    /** * Skip processing alpha if supported by codec. * Note that if the format uses pre-multiplied alpha (common with VP6, * and recommended due to better video quality/compression) * the image will look as if alpha-blended onto a black background. * However for formats that do not use pre-multiplied alpha * there might be serious artefacts (though e.g. libswscale currently * assumes pre-multiplied alpha anyway). * Code outside libavcodec should access this field using AVOptions * * - decoding: set by user * - encoding: unused */
    int skip_alpha;

    /** * Number of samples to skip after a discontinuity * - decoding: unused * - encoding: set by libavcodec */
    int seek_preroll;

#if !FF_API_DEBUG_MV
    /** * debug motion vectors * Code outside libavcodec should access this field using AVOptions * - 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
#endif

    /** * custom intra quantization matrix * Code outside libavcodec should access this field using av_codec_g/set_chroma_intra_matrix() * - encoding: Set by user, can be NULL. * - decoding: unused. */
    uint16_t *chroma_intra_matrix;

    /** * dump format separator. * can be ", " or "\n " or anything else * Code outside libavcodec should access this field using AVOptions * (NO direct access). * - encoding: Set by user. * - decoding: Set by user. */
    uint8_t *dump_separator;

    /** * ',' separated list of allowed decoders. * If NULL then all are allowed * - encoding: unused * - decoding: set by user through AVOPtions (NO direct access) */
    char *codec_whitelist;

    /* * Properties of the stream that gets decoded * To be accessed through av_codec_get_properties() (NO direct access) * - encoding: unused * - decoding: set by libavcodec */
    unsigned properties;
#define FF_CODEC_PROPERTY_LOSSLESS        0x00000001
#define FF_CODEC_PROPERTY_CLOSED_CAPTIONS 0x00000002

    /** * Additional data associated with the entire coded stream. * * - decoding: unused * - encoding: may be set by libavcodec after avcodec_open2(). */
    AVPacketSideData *coded_side_data;
    int            nb_coded_side_data;

    /** * Encoding only. * * For hardware encoders configured to use a hwaccel pixel format, this * field should be set by the caller to a reference to the AVHWFramesContext * describing input frames. AVHWFramesContext.format must be equal to * AVCodecContext.pix_fmt. * * This field should be set before avcodec_open2() is called and is * afterwards owned and managed by libavcodec. */
    AVBufferRef *hw_frames_ctx;

    /** * Control the form of AVSubtitle.rects[N]->ass * - decoding: set by user * - encoding: unused */
    int sub_text_format;
#define FF_SUB_TEXT_FMT_ASS              0
#if FF_API_ASS_TIMING
#define FF_SUB_TEXT_FMT_ASS_WITH_TIMINGS 1
#endif

} AVCodecContext;

int width, height; 视频有关，画面宽高

int coded_width, coded_height; 码流宽高，不同于width/height，当解码出来帧在裁剪出来前输出时画面

int gop_size; (group of pictures) GOP画面组大小

int max_b_frames; B帧最大值

int has_b_frames; 解码器的帧重新排序缓冲区的大小，MPEG-2 1个IBP

int sample_rate; 音频采样率

int channels; 声道数目

enum AVSampleFormat sample_fmt; 音频采样格式，枚举类型

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 
}; 

int frame_size; 一个音频帧里每个声道采样数目

int frame_number; 帧数目，解码时是解码器总解码的帧数目，编码时是编码过的帧数目

int (*get_buffer2)(struct AVCodecContext *s, AVFrame *frame, int flags);这个回调被调用，当从buffer中得到每帧的数据时，每个buffer必须使用AVBuffer API进行引用技数

如下属性将被设置在callback回调之前

• format
• width, height(视频)
• sample_rate, channel_layout, nb_samples(音频)