FFMPEG结构体分析：AVCodecContext

注：写了一系列的结构体的分析的文章，在这里列一个列表：

FFMPEG结构体分析：AVFrame

FFMPEG结构体分析：AVFormatContext

FFMPEG结构体分析：AVCodecContext

FFMPEG结构体分析：AVIOContext

FFMPEG结构体分析：AVCodec

FFMPEG结构体分析：AVStream
FFMPEG结构体分析：AVPacket

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

FFMPEG中最关键的结构体之间的关系

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

首先看一下结构体的定义（位于avcodec.h）：

 /*
  *雷霄骅
  *[email protected]
  *中国传媒大学/数字电视技术
  */
 /**
  * main external API structure.
  * New fields can be added to the end with minor version bumps.
  * Removal, reordering and changes to existing fields require a major
  * version bump.
  * Please use AVOptions (av_opt* / av_set/get*()) to access these fields from user
  * applications.
  * sizeof(AVCodecContext) must not be used outside libav*.
  */
 typedef struct AVCodecContext {
     /**
      * information on struct for av_log
      * - set by avcodec_alloc_context3
      */
     const AVClass *av_class;
     int log_level_offset;

     enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
     const struct AVCodec  *codec;
     char             codec_name[32];
     enum AVCodecID     codec_id; /* see AV_CODEC_ID_xxx */

     /**
      * fourcc (LSB first, so "ABCD" -> ('D'<<24) + ('C'<<16) + ('B'<<8) + 'A').
      * This is used to work around some encoder bugs.
      * A demuxer should set this to what is stored in the field used to identify the codec.
      * If there are multiple such fields in a container then the demuxer should choose the one
      * which maximizes the information about the used codec.
      * If the codec tag field in a container is larger than 32 bits then the demuxer should
      * remap the longer ID to 32 bits with a table or other structure. Alternatively a new
      * extra_codec_tag + size could be added but for this a clear advantage must be demonstrated
      * first.
      * - encoding: Set by user, if not then the default based on codec_id will be used.
      * - decoding: Set by user, will be converted to uppercase by libavcodec during init.
      */
     unsigned int codec_tag;

     /**
      * fourcc from the AVI stream header (LSB first, so "ABCD" -> ('D'<<24) + ('C'<<16) + ('B'<<8) + 'A').
      * This is used to work around some encoder bugs.
      * - encoding: unused
      * - decoding: Set by user, will be converted to uppercase by libavcodec during init.
      */
     unsigned int stream_codec_tag;

 #if FF_API_SUB_ID
     /**
      * @deprecated this field is unused
      */
     attribute_deprecated int sub_id;
 #endif

     void *priv_data;

     /**
      * Private context used for internal data.
      *
      * Unlike priv_data, this is not codec-specific. It is used in general
      * libavcodec functions.
      */
     struct AVCodecInternal *internal;

     /**
      * Private data of the user, can be used to carry app specific stuff.
      * - encoding: Set by user.
      * - decoding: Set by user.
      */
     void *opaque;

     /**
      * the average bitrate
      * - encoding: Set by user; unused for constant quantizer encoding.
      * - decoding: Set by libavcodec. 0 or some bitrate if this info is available in the stream.
      */
     int bit_rate;

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

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

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

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

     /**
      * CODEC_FLAG2_*
      * - encoding: Set by user.
      * - decoding: Set by user.
      */
     int flags2;

     /**
      * some codecs need / can use extradata like Huffman tables.
      * mjpeg: Huffman tables
      * rv10: additional flags
      * mpeg4: global headers (they can be in the bitstream or here)
      * The allocated memory should be FF_INPUT_BUFFER_PADDING_SIZE bytes larger
      * than extradata_size to avoid prolems if it is read with the bitstream reader.
      * The bytewise contents of extradata must not depend on the architecture or CPU endianness.
      * - encoding: Set/allocated/freed by libavcodec.
      * - decoding: Set/allocated/freed by user.
      */
     uint8_t *extradata;
     int extradata_size;

     /**
      * This is the fundamental unit of time (in seconds) in terms
      * of which frame timestamps are represented. For fixed-fps content,
      * timebase should be 1/framerate and timestamp increments should be
      * identically 1.
      * - encoding: MUST be set by user.
      * - decoding: Set by libavcodec.
      */
     AVRational time_base;

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

     /**
      * Encoding: Number of frames delay there will be from the encoder input to
      *           the decoder output. (we assume the decoder matches the spec)
      * Decoding: Number of frames delay in addition to what a standard decoder
      *           as specified in the spec would produce.
      *
      * Video:
      *   Number of frames the decoded output will be delayed relative to the
      *   encoded input.
      *
      * Audio:
      *   For encoding, this is the number of "priming" samples added to the
      *   beginning of the stream. The decoded output will be delayed by this
      *   many samples relative to the input to the encoder. Note that this
      *   field is purely informational and does not directly affect the pts
      *   output by the encoder, which should always be based on the actual
      *   presentation time, including any delay.
      *   For decoding, this is the number of samples the decoder needs to
      *   output before the decoder's output is valid. When seeking, you should
      *   start decoding this many samples prior to your desired seek point.
      *
      * - encoding: Set by libavcodec.
      * - decoding: Set by libavcodec.
      */
     int delay;


     /* video only */
     /**
      * picture width / height.
      * - encoding: MUST be set by user.
      * - decoding: Set by libavcodec.
      * Note: For compatibility it is possible to set this instead of
      * coded_width/height before decoding.
      */
     int width, height;

     /**
      * Bitstream width / height, may be different from width/height if lowres enabled.
      * - encoding: unused
      * - decoding: Set by user before init if known. Codec should override / dynamically change if needed.
      */
     int coded_width, coded_height;

 #define FF_ASPECT_EXTENDED 15

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

     /**
      * Pixel format, see AV_PIX_FMT_xxx.
      * May be set by the demuxer if known from headers.
      * May be overridden by the decoder if it knows better.
      * - encoding: Set by user.
      * - decoding: Set by user if known, overridden by libavcodec if known
      */
     enum AVPixelFormat pix_fmt;

     /**
      * Motion estimation algorithm used for video coding.
      * 1 (zero), 2 (full), 3 (log), 4 (phods), 5 (epzs), 6 (x1), 7 (hex),
      * 8 (umh), 9 (iter), 10 (tesa) [7, 8, 10 are x264 specific, 9 is snow specific]
      * - encoding: MUST be set by user.
      * - decoding: unused
      */
     int me_method;

     /**
      * If non NULL, 'draw_horiz_band' is called by the libavcodec
      * decoder to draw a horizontal band. It improves cache usage. Not
      * all codecs can do that. You must check the codec capabilities
      * beforehand.
      * When multithreading is used, it may be called from multiple threads
      * at the same time; threads might draw different parts of the same AVFrame,
      * or multiple AVFrames, and there is no guarantee that slices will be drawn
      * in order.
      * The function is also used by hardware acceleration APIs.
      * It is called at least once during frame decoding to pass
      * the data needed for hardware render.
      * In that mode instead of pixel data, AVFrame points to
      * a structure specific to the acceleration API. The application
      * reads the structure and can change some fields to indicate progress
      * or mark state.
      * - encoding: unused
      * - decoding: Set by user.
      * @param height the height of the slice
      * @param y the y position of the slice
      * @param type 1->top field, 2->bottom field, 3->frame
      * @param offset offset into the AVFrame.data from which the slice should be read
      */
     void (*draw_horiz_band)(struct AVCodecContext *s,
                             const AVFrame *src, int offset[AV_NUM_DATA_POINTERS],
                             int y, int type, int height);

     /**
      * callback to negotiate the pixelFormat
      * @param fmt is the list of formats which are supported by the codec,
      * it is terminated by -1 as 0 is a valid format, the formats are ordered by quality.
      * The first is always the native one.
      * @return the chosen format
      * - encoding: unused
      * - decoding: Set by user, if not set the native format will be chosen.
      */
     enum AVPixelFormat (*get_format)(struct AVCodecContext *s, const enum AVPixelFormat * fmt);

     /**
      * maximum number of B-frames between non-B-frames
      * Note: The output will be delayed by max_b_frames+1 relative to the input.
      * - encoding: Set by user.
      * - decoding: unused
      */
     int max_b_frames;

     /**
      * qscale factor between IP and B-frames
      * If > 0 then the last P-frame quantizer will be used (q= lastp_q*factor+offset).
      * If < 0 then normal ratecontrol will be done (q= -normal_q*factor+offset).
      * - encoding: Set by user.
      * - decoding: unused
      */
     float b_quant_factor;

     /** obsolete FIXME remove */
     int rc_strategy;
 #define FF_RC_STRATEGY_XVID 1

     int b_frame_strategy;

 #if FF_API_MPV_GLOBAL_OPTS
     /**
      * luma single coefficient elimination threshold
      * - encoding: Set by user.
      * - decoding: unused
      */
     attribute_deprecated int luma_elim_threshold;

     /**
      * chroma single coeff elimination threshold
      * - encoding: Set by user.
      * - decoding: unused
      */
     attribute_deprecated int chroma_elim_threshold;
 #endif

     /**
      * qscale offset between IP and B-frames
      * - encoding: Set by user.
      * - decoding: unused
      */
     float b_quant_offset;

     /**
      * Size of the frame reordering buffer in the decoder.
      * For MPEG-2 it is 1 IPB or 0 low delay IP.
      * - encoding: Set by libavcodec.
      * - decoding: Set by libavcodec.
      */
     int has_b_frames;

     /**
      * 0-> h263 quant 1-> mpeg quant
      * - encoding: Set by user.
      * - decoding: unused
      */
     int mpeg_quant;

     /**
      * qscale factor between P and I-frames
      * If > 0 then the last p frame quantizer will be used (q= lastp_q*factor+offset).
      * If < 0 then normal ratecontrol will be done (q= -normal_q*factor+offset).
      * - encoding: Set by user.
      * - decoding: unused
      */
     float i_quant_factor;

     /**
      * qscale offset between P and I-frames
      * - encoding: Set by user.
      * - decoding: unused
      */
     float i_quant_offset;

     /**
      * luminance masking (0-> disabled)
      * - encoding: Set by user.
      * - decoding: unused
      */
     float lumi_masking;

     /**
      * temporary complexity masking (0-> disabled)
      * - encoding: Set by user.
      * - decoding: unused
      */
     float temporal_cplx_masking;

     /**
      * spatial complexity masking (0-> disabled)
      * - encoding: Set by user.
      * - decoding: unused
      */
     float spatial_cplx_masking;

     /**
      * p block masking (0-> disabled)
      * - encoding: Set by user.
      * - decoding: unused
      */
     float p_masking;

     /**
      * darkness masking (0-> disabled)
      * - encoding: Set by user.
      * - decoding: unused
      */
     float dark_masking;

     /**
      * slice count
      * - encoding: Set by libavcodec.
      * - decoding: Set by user (or 0).
      */
     int slice_count;
     /**
      * prediction method (needed for huffyuv)
      * - encoding: Set by user.
      * - decoding: unused
      */
      int prediction_method;
 #define FF_PRED_LEFT   0
 #define FF_PRED_PLANE  1
 #define FF_PRED_MEDIAN 2

     /**
      * slice offsets in the frame in bytes
      * - encoding: Set/allocated by libavcodec.
      * - decoding: Set/allocated by user (or NULL).
      */
     int *slice_offset;

     /**
      * sample aspect ratio (0 if unknown)
      * That is the width of a pixel divided by the height of the pixel.
      * Numerator and denominator must be relatively prime and smaller than 256 for some video standards.
      * - encoding: Set by user.
      * - decoding: Set by libavcodec.
      */
     AVRational sample_aspect_ratio;

     /**
      * motion estimation comparison function
      * - encoding: Set by user.
      * - decoding: unused
      */
     int me_cmp;
     /**
      * subpixel motion estimation comparison function
      * - encoding: Set by user.
      * - decoding: unused
      */
     int me_sub_cmp;
     /**
      * macroblock comparison function (not supported yet)
      * - encoding: Set by user.
      * - decoding: unused
      */
     int mb_cmp;
     /**
      * interlaced DCT comparison function
      * - encoding: Set by user.
      * - decoding: unused
      */
     int ildct_cmp;
 #define FF_CMP_SAD    0
 #define FF_CMP_SSE    1
 #define FF_CMP_SATD   2
 #define FF_CMP_DCT    3
 #define FF_CMP_PSNR   4
 #define FF_CMP_BIT    5
 #define FF_CMP_RD     6
 #define FF_CMP_ZERO   7
 #define FF_CMP_VSAD   8
 #define FF_CMP_VSSE   9
 #define FF_CMP_NSSE   10
 #define FF_CMP_W53    11
 #define FF_CMP_W97    12
 #define FF_CMP_DCTMAX 13
 #define FF_CMP_DCT264 14
 #define FF_CMP_CHROMA 256

     /**
      * ME diamond size & shape
      * - encoding: Set by user.
      * - decoding: unused
      */
     int dia_size;

     /**
      * amount of previous MV predictors (2a+1 x 2a+1 square)
      * - encoding: Set by user.
      * - decoding: unused
      */
     int last_predictor_count;

     /**
      * prepass for motion estimation
      * - encoding: Set by user.
      * - decoding: unused
      */
     int pre_me;

     /**
      * motion estimation prepass comparison function
      * - encoding: Set by user.
      * - decoding: unused
      */
     int me_pre_cmp;

     /**
      * ME prepass diamond size & shape
      * - encoding: Set by user.
      * - decoding: unused
      */
     int pre_dia_size;

     /**
      * subpel ME quality
      * - encoding: Set by user.
      * - decoding: unused
      */
     int me_subpel_quality;

     /**
      * DTG active format information (additional aspect ratio
      * information only used in DVB MPEG-2 transport streams)
      * 0 if not set.
      *
      * - encoding: unused
      * - decoding: Set by decoder.
      */
     int dtg_active_format;
 #define FF_DTG_AFD_SAME         8
 #define FF_DTG_AFD_4_3          9
 #define FF_DTG_AFD_16_9         10
 #define FF_DTG_AFD_14_9         11
 #define FF_DTG_AFD_4_3_SP_14_9  13
 #define FF_DTG_AFD_16_9_SP_14_9 14
 #define FF_DTG_AFD_SP_4_3       15

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

     /**
      * intra quantizer bias
      * - encoding: Set by user.
      * - decoding: unused
      */
     int intra_quant_bias;
 #define FF_DEFAULT_QUANT_BIAS 999999

     /**
      * inter quantizer bias
      * - encoding: Set by user.
      * - decoding: unused
      */
     int inter_quant_bias;

 #if FF_API_COLOR_TABLE_ID
     /**
      * color table ID
      * - encoding: unused
      * - decoding: Which clrtable should be used for 8bit RGB images.
      *             Tables have to be stored somewhere. FIXME
      */
     attribute_deprecated int color_table_id;
 #endif

     /**
      * slice flags
      * - encoding: unused
      * - decoding: Set by user.
      */
     int slice_flags;
 #define SLICE_FLAG_CODED_ORDER    0x0001 ///< draw_horiz_band() is called in coded order instead of display
 #define SLICE_FLAG_ALLOW_FIELD    0x0002 ///< allow draw_horiz_band() with field slices (MPEG2 field pics)
 #define SLICE_FLAG_ALLOW_PLANE    0x0004 ///< allow draw_horiz_band() with 1 component at a time (SVQ1)

     /**
      * XVideo Motion Acceleration
      * - encoding: forbidden
      * - decoding: set by decoder
      */
     int xvmc_acceleration;

     /**
      * macroblock decision mode
      * - encoding: Set by user.
      * - decoding: unused
      */
     int mb_decision;
 #define FF_MB_DECISION_SIMPLE 0        ///< uses mb_cmp
 #define FF_MB_DECISION_BITS   1        ///< chooses the one which needs the fewest bits
 #define FF_MB_DECISION_RD     2        ///< rate distortion

     /**
      * custom intra quantization matrix
      * - encoding: Set by user, can be NULL.
      * - decoding: Set by libavcodec.
      */
     uint16_t *intra_matrix;

     /**
      * custom inter quantization matrix
      * - encoding: Set by user, can be NULL.
      * - decoding: Set by libavcodec.
      */
     uint16_t *inter_matrix;

     /**
      * scene change detection threshold
      * 0 is default, larger means fewer detected scene changes.
      * - encoding: Set by user.
      * - decoding: unused
      */
     int scenechange_threshold;

     /**
      * noise reduction strength
      * - encoding: Set by user.
      * - decoding: unused
      */
     int noise_reduction;

 #if FF_API_INTER_THRESHOLD
     /**
      * @deprecated this field is unused
      */
     attribute_deprecated int inter_threshold;
 #endif

 #if FF_API_MPV_GLOBAL_OPTS
     /**
      * @deprecated use mpegvideo private options instead
      */
     attribute_deprecated int quantizer_noise_shaping;
 #endif

     /**
      * Motion estimation threshold below which no motion estimation is
      * performed, but instead the user specified motion vectors are used.
      *
      * - encoding: Set by user.
      * - decoding: unused
      */
     int me_threshold;

     /**
      * Macroblock threshold below which the user specified macroblock types will be used.
      * - encoding: Set by user.
      * - decoding: unused
      */
     int mb_threshold;

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

     /**
      * Number of macroblock rows at the top which are skipped.
      * - encoding: unused
      * - decoding: Set by user.
      */
     int skip_top;

     /**
      * Number of macroblock rows at the bottom which are skipped.
      * - encoding: unused
      * - decoding: Set by user.
      */
     int skip_bottom;

     /**
      * Border processing masking, raises the quantizer for mbs on the borders
      * of the picture.
      * - encoding: Set by user.
      * - decoding: unused
      */
     float border_masking;

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

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

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

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

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

     /**
      * minimum GOP size
      * - encoding: Set by user.
      * - decoding: unused
      */
     int keyint_min;

     /**
      * number of reference frames
      * - encoding: Set by user.
      * - decoding: Set by lavc.
      */
     int refs;

     /**
      * chroma qp offset from luma
      * - encoding: Set by user.
      * - decoding: unused
      */
     int chromaoffset;

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

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

     /**
      * Adjust sensitivity of b_frame_strategy 1.
      * - encoding: Set by user.
      * - decoding: unused
      */
     int b_sensitivity;

     /**
      * Chromaticity coordinates of the source primaries.
      * - encoding: Set by user
      * - decoding: Set by libavcodec
      */
     enum AVColorPrimaries color_primaries;

     /**
      * Color Transfer Characteristic.
      * - encoding: Set by user
      * - decoding: Set by libavcodec
      */
     enum AVColorTransferCharacteristic color_trc;

     /**
      * YUV colorspace type.
      * - encoding: Set by user
      * - decoding: Set by libavcodec
      */
     enum AVColorSpace colorspace;

     /**
      * MPEG vs JPEG YUV range.
      * - encoding: Set by user
      * - decoding: Set by libavcodec
      */
     enum AVColorRange color_range;

     /**
      * This defines the location of chroma samples.
      * - encoding: Set by user
      * - decoding: Set by libavcodec
      */
     enum AVChromaLocation chroma_sample_location;

     /**
      * Number of slices.
      * Indicates number of picture subdivisions. Used for parallelized
      * decoding.
      * - encoding: Set by user
      * - decoding: unused
      */
     int slices;

     /** Field order
      * - encoding: set by libavcodec
      * - decoding: Set by user.
      */
     enum AVFieldOrder field_order;

     /* audio only */
     int sample_rate; ///< samples per second
     int channels;    ///< number of audio channels

     /**
      * audio sample format
      * - encoding: Set by user.
      * - decoding: Set by libavcodec.
      */
     enum AVSampleFormat sample_fmt;  ///< sample format

     /* The following data should not be initialized. */
     /**
      * Samples per packet, initialized when calling 'init'.
      */
     int frame_size;

     /**
      * Frame counter, set by libavcodec.
      *
      * - decoding: total number of frames returned from the decoder so far.
      * - encoding: total number of frames passed to the encoder so far.
      *
      *   @note the counter is not incremented if encoding/decoding resulted in
      *   an error.
      */
     int frame_number;

     /**
      * number of bytes per packet if constant and known or 0
      * Used by some WAV based audio codecs.
      */
     int block_align;

     /**
      * Audio cutoff bandwidth (0 means "automatic")
      * - encoding: Set by user.
      * - decoding: unused
      */
     int cutoff;

 #if FF_API_REQUEST_CHANNELS
     /**
      * Decoder should decode to this many channels if it can (0 for default)
      * - encoding: unused
      * - decoding: Set by user.
      * @deprecated Deprecated in favor of request_channel_layout.
      */
     int request_channels;
 #endif

     /**
      * Audio channel layout.
      * - encoding: set by user.
      * - decoding: set by user, may be overwritten by libavcodec.
      */
     uint64_t channel_layout;

     /**
      * Request decoder to use this channel layout if it can (0 for default)
      * - encoding: unused
      * - decoding: Set by user.
      */
     uint64_t request_channel_layout;

     /**
      * Type of service that the audio stream conveys.
      * - encoding: Set by user.
      * - decoding: Set by libavcodec.
      */
     enum AVAudioServiceType audio_service_type;

     /**
      * desired sample format
      * - encoding: Not used.
      * - decoding: Set by user.
      * Decoder will decode to this format if it can.
      */
     enum AVSampleFormat request_sample_fmt;

     /**
      * Called at the beginning of each frame to get a buffer for it.
      *
      * The function will set AVFrame.data[], AVFrame.linesize[].
      * AVFrame.extended_data[] must also be set, but it should be the same as
      * AVFrame.data[] except for planar audio with more channels than can fit
      * in AVFrame.data[]. In that case, AVFrame.data[] shall still contain as
      * many data pointers as it can hold.
      *
      * if CODEC_CAP_DR1 is not set then get_buffer() must call
      * avcodec_default_get_buffer() instead of providing buffers allocated by
      * some other means.
      *
      * AVFrame.data[] should be 32- or 16-byte-aligned unless the CPU doesn't
      * need it. avcodec_default_get_buffer() aligns the output buffer properly,
      * but if get_buffer() is overridden then alignment considerations should
      * be taken into account.
      *
      * @see avcodec_default_get_buffer()
      *
      * Video:
      *
      * If pic.reference is set then the frame will be read later by libavcodec.
      * avcodec_align_dimensions2() should be used to find the required width and
      * height, as they normally need to be rounded up to the next multiple of 16.
      *
      * If frame multithreading is used and thread_safe_callbacks is set,
      * it may be called from a different thread, but not from more than one at
      * once. Does not need to be reentrant.
      *
      * @see release_buffer(), reget_buffer()
      * @see avcodec_align_dimensions2()
      *
      * Audio:
      *
      * Decoders request a buffer of a particular size by setting
      * AVFrame.nb_samples prior to calling get_buffer(). The decoder may,
      * however, utilize only part of the buffer by setting AVFrame.nb_samples
      * to a smaller value in the output frame.
      *
      * Decoders cannot use the buffer after returning from
      * avcodec_decode_audio4(), so they will not call release_buffer(), as it
      * is assumed to be released immediately upon return.
      *
      * As a convenience, av_samples_get_buffer_size() and
      * av_samples_fill_arrays() in libavutil may be used by custom get_buffer()
      * functions to find the required data size and to fill data pointers and
      * linesize. In AVFrame.linesize, only linesize[0] may be set for audio
      * since all planes must be the same size.
      *
      * @see av_samples_get_buffer_size(), av_samples_fill_arrays()
      *
      * - encoding: unused
      * - decoding: Set by libavcodec, user can override.
      */
     int (*get_buffer)(struct AVCodecContext *c, AVFrame *pic);

     /**
      * Called to release buffers which were allocated with get_buffer.
      * A released buffer can be reused in get_buffer().
      * pic.data[*] must be set to NULL.
      * May be called from a different thread if frame multithreading is used,
      * but not by more than one thread at once, so does not need to be reentrant.
      * - encoding: unused
      * - decoding: Set by libavcodec, user can override.
      */
     void (*release_buffer)(struct AVCodecContext *c, AVFrame *pic);

     /**
      * Called at the beginning of a frame to get cr buffer for it.
      * Buffer type (size, hints) must be the same. libavcodec won't check it.
      * libavcodec will pass previous buffer in pic, function should return
      * same buffer or new buffer with old frame "painted" into it.
      * If pic.data[0] == NULL must behave like get_buffer().
      * if CODEC_CAP_DR1 is not set then reget_buffer() must call
      * avcodec_default_reget_buffer() instead of providing buffers allocated by
      * some other means.
      * - encoding: unused
      * - decoding: Set by libavcodec, user can override.
      */
     int (*reget_buffer)(struct AVCodecContext *c, AVFrame *pic);


     /* - encoding parameters */
     float qcompress;  ///< amount of qscale change between easy & hard scenes (0.0-1.0)
     float qblur;      ///< amount of qscale smoothing over time (0.0-1.0)

     /**
      * minimum quantizer
      * - encoding: Set by user.
      * - decoding: unused
      */
     int qmin;

     /**
      * maximum quantizer
      * - encoding: Set by user.
      * - decoding: unused
      */
     int qmax;

     /**
      * maximum quantizer difference between frames
      * - encoding: Set by user.
      * - decoding: unused
      */
     int max_qdiff;

     /**
      * ratecontrol qmin qmax limiting method
      * 0-> clipping, 1-> use a nice continuous function to limit qscale wthin qmin/qmax.
      * - encoding: Set by user.
      * - decoding: unused
      */
     float rc_qsquish;

     float rc_qmod_amp;
     int rc_qmod_freq;

     /**
      * decoder bitstream buffer size
      * - encoding: Set by user.
      * - decoding: unused
      */
     int rc_buffer_size;

     /**
      * ratecontrol override, see RcOverride
      * - encoding: Allocated/set/freed by user.
      * - decoding: unused
      */
     int rc_override_count;
     RcOverride *rc_override;

     /**
      * rate control equation
      * - encoding: Set by user
      * - decoding: unused
      */
     const char *rc_eq;

     /**
      * maximum bitrate
      * - encoding: Set by user.
      * - decoding: unused
      */
     int rc_max_rate;

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

     float rc_buffer_aggressivity;

     /**
      * initial complexity for pass1 ratecontrol
      * - encoding: Set by user.
      * - decoding: unused
      */
     float rc_initial_cplx;

     /**
      * Ratecontrol attempt to use, at maximum,  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&n