AVCodecContext，编码器上下文结构体，保存了视频（音频）编解码相关信息
AVCodecContext 结构体定义在文件ffmpeg/libavcodec/avcodec.h中
/**
* 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.
* 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;
//编解码器的类型（视频，音频...）
enum AVMediaType codec_type; /* see AVMEDIA_TYPE_xxx */
// 采用的编解码器AVCodec（H.264,MPEG2...）
const struct AVCodec *codec;
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;
void *priv_data;
/**
* Private context used for internal data.
*
* Unlike priv_data, this is not codec-specific. It is used in general
* libavcodec functions.
*/
//保存了AVCodecContext的各种状态信息
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;
/**
* 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.
* Must be allocated with the av_malloc() family of functions.
* - 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_receive_frame() 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.
*/
//如果是视频的话，代表宽和高
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;
/**
* 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 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 pixel format. Decoding only, may be set by the
* caller before avcodec_open2().
*
* Called by some decoders to select the pixel format that will be used for
* the output frames. This is mainly used to set up hardware acceleration,
* then the provided format list contains the corresponding hwaccel pixel
* formats alongside the "software" one. The software pixel format may also
* be retrieved from \ref sw_pix_fmt.
*
* This callback will be called when the coded frame properties (such as
* resolution, pixel format, etc.) change and more than one output format is
* supported for those new properties. If a hardware pixel format is chosen
* and initialization for it fails, the callback may be called again
* immediately.
*
* This callback may be called from different threads if the decoder is
* multi-threaded, but not from more than one thread simultaneously.
*
* @param fmt list of formats which may be used in the current
* configuration, terminated by AV_PIX_FMT_NONE.
* @warning Behavior is undefined if the callback returns a value other
* than one of the formats in fmt or AV_PIX_FMT_NONE.
* @return the chosen format or AV_PIX_FMT_NONE
*/
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;
/**
* 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;
/**
* 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;
/**
* 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;
/**
* 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;
/**
* 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;
/**
* maximum motion estimation search range in subpel units
* If 0 then no limit.
*
* - encoding: Set by user.
* - decoding: unused
*/
int me_range;
/**
* slice flags
* - encoding: unused
* - decoding: Set by user.
*/
int slice_flags;
///< draw_horiz_band() is called in coded order instead of display
///< allow draw_horiz_band() with field slices (MPEG-2 field pics)
///< allow draw_horiz_band() with 1 component at a time (SVQ1)
/**
* macroblock decision mode
* - encoding: Set by user.
* - decoding: unused
*/
int mb_decision;
///< uses mb_cmp
///< chooses the one which needs the fewest bits
///< rate distortion
/**
* custom intra quantization matrix
* Must be allocated with the av_malloc() family of functions, and will be freed in
* avcodec_free_context().
* - encoding: Set/allocated by user, freed by libavcodec. Can be NULL.
* - decoding: Set/allocated/freed by libavcodec.
*/
uint16_t *intra_matrix;
/**
* custom inter quantization matrix
* Must be allocated with the av_malloc() family of functions, and will be freed in
* avcodec_free_context().
* - encoding: Set/allocated by user, freed by libavcodec. Can be NULL.
* - decoding: Set/allocated/freed by libavcodec.
*/
uint16_t *inter_matrix;
/**
* 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;
/**
* 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;
/**
* - encoding: Set by user.
* - decoding: unused
*/
int bidir_refine;
/**
* 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;
/**
* Note: Value depends upon the compare function used for fullpel ME.
* - encoding: Set by user.
* - decoding: unused
*/
int mv0_threshold;
/**
* 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 */
//采样率（音频）
int sample_rate; ///< samples per second
//声道数（音频）
int channels; ///< number of audio channels
/**
* audio sample format
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
//采样格式(音频)
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, 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);
/* - 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;
/**
* 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;
/**
* 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;
/**
* 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;
/**
* trellis RD quantization
* - encoding: Set by user.
* - decoding: unused
*/
int trellis;
/**
* 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;
///< autodetection
///< Work around various bugs in Microsoft's broken decoders.
/**
* 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;
///< Strictly conform to an older more strict version of the spec or reference software.
///< Strictly conform to all the things in the spec no matter what consequences.
///< Allow unofficial extensions
///< Allow nonstandardized experimental things.
/**
* error concealment flags
* - encoding: unused
* - decoding: Set by user.
*/
int error_concealment;
/**
* debug
* - encoding: Set by user.
* - decoding: Set by user.
*/
int debug;
/**
* Error recognition; may misdetect some more or less valid parts as errors.
* - encoding: Set by user.
* - 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.
*/
///< detect bitstream specification deviations
///< detect improper bitstream length
///< abort decoding on minor error detection
///< ignore errors and continue
///< consider things that violate the spec, are fast to calculate and have not been seen in the wild as errors
///< consider all spec non compliances as errors
///< 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: Set by libavcodec to the reordered_opaque of the input
* frame corresponding to the last returned packet. Only
* supported by encoders with the
* AV_CODEC_CAP_ENCODER_REORDERED_OPAQUE capability.
* - decoding: Set by user.
*/
int64_t reordered_opaque;
/**
* Hardware accelerator in use
* - encoding: unused.
* - decoding: Set by libavcodec
*/
const 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.
* - 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;
/**
* IDCT algorithm, see FF_IDCT_* below.
* - encoding: Set by user.
* - decoding: Set by user.
*/
int idct_algo;
/* Used by XvMC to extract IDCT coefficients with FF_IDCT_PERM_NONE */
/**
* 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;
/**
* 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;
///< Decode more than one frame at once
///< 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.
*
* @deprecated the custom get_buffer2() callback should always be
* thread-safe. Thread-unsafe get_buffer2() implementations will be
* invalid starting with LIBAVCODEC_VERSION_MAJOR=60; in other words,
* libavcodec will behave as if this field was always set to 1.
* Callers that want to be forward compatible with future libavcodec
* versions should wrap access to this field in
* #if LIBAVCODEC_VERSION_MAJOR < 60
*/
attribute_deprecated
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;
// 8+1; constraint_set1_flag
// 8+3; constraint_set3_flag
/**
* level
* - encoding: Set by user.
* - decoding: Set by libavcodec.
*/
int level;
/**
* 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;
/**
* 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.
* - encoding unused.
* - decoding set by user.
*/
AVRational pkt_timebase;
/**
* AVCodecDescriptor
* - encoding: unused.
* - decoding: set by libavcodec.
*/
const AVCodecDescriptor *codec_descriptor;
/**
* Current statistics for PTS correction.
* - decoding: maintained and used by libavcodec, not intended to be used by user apps
* - encoding: unused
*/
int64_t pts_correction_num_faulty_pts; /// Number of incorrect PTS values so far
int64_t pts_correction_num_faulty_dts; /// Number of incorrect DTS values so far
int64_t pts_correction_last_pts; /// PTS of the last frame
int64_t pts_correction_last_dts; /// DTS of the last frame
/**
* 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;
///< do nothing (demuxer outputs a stream supposed to be already in UTF-8, or the codec is bitmap for instance)
///< libavcodec will select the mode itself
///< the AVPacket data needs to be recoded to UTF-8 before being fed to the decoder, requires iconv
///< neither convert the subtitles, nor check them for valid UTF-8
/**
* 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).
*
* - 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;
/**
* @deprecated unused
*/
attribute_deprecated
int debug_mv;
//visualize forward predicted MVs of P frames
//visualize forward predicted MVs of B frames
//visualize backward predicted MVs of B frames
/**
* custom intra quantization 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
* - 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
*/
char *codec_whitelist;
/**
* Properties of the stream that gets decoded
* - encoding: unused
* - decoding: set by libavcodec
*/
unsigned properties;
/**
* 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;
/**
* A reference to the AVHWFramesContext describing the input (for encoding)
* or output (decoding) frames. The reference is set by the caller and
* afterwards owned (and freed) by libavcodec - it should never be read by
* the caller after being set.
*
* - decoding: This field should be set by the caller from the get_format()
* callback. The previous reference (if any) will always be
* unreffed by libavcodec before the get_format() call.
*
* If the default get_buffer2() is used with a hwaccel pixel
* format, then this AVHWFramesContext will be used for
* allocating the frame buffers.
*
* - encoding: 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.
*/
AVBufferRef *hw_frames_ctx;
/**
* Control the form of AVSubtitle.rects[N]->ass
* - decoding: set by user
* - encoding: unused
*/
int sub_text_format;
/**
* Audio only. The amount of padding (in samples) appended by the encoder to
* the end of the audio. I.e. this number of decoded samples must be
* discarded by the caller from the end of the stream to get the original
* audio without any trailing padding.
*
* - decoding: unused
* - encoding: unused
*/
int trailing_padding;
/**
* The number of pixels per image to maximally accept.
*
* - decoding: set by user
* - encoding: set by user
*/
int64_t max_pixels;
/**
* A reference to the AVHWDeviceContext describing the device which will
* be used by a hardware encoder/decoder. The reference is set by the
* caller and afterwards owned (and freed) by libavcodec.
*
* This should be used if either the codec device does not require
* hardware frames or any that are used are to be allocated internally by
* libavcodec. If the user wishes to supply any of the frames used as
* encoder input or decoder output then hw_frames_ctx should be used
* instead. When hw_frames_ctx is set in get_format() for a decoder, this
* field will be ignored while decoding the associated stream segment, but
* may again be used on a following one after another get_format() call.
*
* For both encoders and decoders this field should be set before
* avcodec_open2() is called and must not be written to thereafter.
*
* Note that some decoders may require this field to be set initially in
* order to support hw_frames_ctx at all - in that case, all frames
* contexts used must be created on the same device.
*/
AVBufferRef *hw_device_ctx;
/**
* Bit set of AV_HWACCEL_FLAG_* flags, which affect hardware accelerated
* decoding (if active).
* - encoding: unused
* - decoding: Set by user (either before avcodec_open2(), or in the
* AVCodecContext.get_format callback)
*/
int hwaccel_flags;
/**
* Video decoding only. Certain video codecs support cropping, meaning that
* only a sub-rectangle of the decoded frame is intended for display. This
* option controls how cropping is handled by libavcodec.
*
* When set to 1 (the default), libavcodec will apply cropping internally.
* I.e. it will modify the output frame width/height fields and offset the
* data pointers (only by as much as possible while preserving alignment, or
* by the full amount if the AV_CODEC_FLAG_UNALIGNED flag is set) so that
* the frames output by the decoder refer only to the cropped area. The
* crop_* fields of the output frames will be zero.
*
* When set to 0, the width/height fields of the output frames will be set
* to the coded dimensions and the crop_* fields will describe the cropping
* rectangle. Applying the cropping is left to the caller.
*
* @warning When hardware acceleration with opaque output frames is used,
* libavcodec is unable to apply cropping from the top/left border.
*
* @note when this option is set to zero, the width/height fields of the
* AVCodecContext and output AVFrames have different meanings. The codec
* context fields store display dimensions (with the coded dimensions in
* coded_width/height), while the frame fields store the coded dimensions
* (with the display dimensions being determined by the crop_* fields).
*/
int apply_cropping;
/*
* Video decoding only. Sets the number of extra hardware frames which
* the decoder will allocate for use by the caller. This must be set
* before avcodec_open2() is called.
*
* Some hardware decoders require all frames that they will use for
* output to be defined in advance before decoding starts. For such
* decoders, the hardware frame pool must therefore be of a fixed size.
* The extra frames set here are on top of any number that the decoder
* needs internally in order to operate normally (for example, frames
* used as reference pictures).
*/
int extra_hw_frames;
/**
* The percentage of damaged samples to discard a frame.
*
* - decoding: set by user
* - encoding: unused
*/
int discard_damaged_percentage;
/**
* The number of samples per frame to maximally accept.
*
* - decoding: set by user
* - encoding: set by user
*/
int64_t max_samples;
/**
* Bit set of AV_CODEC_EXPORT_DATA_* flags, which affects the kind of
* metadata exported in frame, packet, or coded stream side data by
* decoders and encoders.
*
* - decoding: set by user
* - encoding: set by user
*/
int export_side_data;
/**
* This callback is called at the beginning of each packet to get a data
* buffer for it.
*
* The following field will be set in the packet before this callback is
* called:
* - size
* This callback must use the above value to calculate the required buffer size,
* which must padded by at least AV_INPUT_BUFFER_PADDING_SIZE bytes.
*
* In some specific cases, the encoder may not use the entire buffer allocated by this
* callback. This will be reflected in the size value in the packet once returned by
* avcodec_receive_packet().
*
* This callback must fill the following fields in the packet:
* - data: alignment requirements for AVPacket apply, if any. Some architectures and
* encoders may benefit from having aligned data.
* - buf: must contain a pointer to an AVBufferRef structure. The packet's
* data pointer must be contained in it. See: av_buffer_create(), av_buffer_alloc(),
* and av_buffer_ref().
*
* If AV_CODEC_CAP_DR1 is not set then get_encode_buffer() must call
* avcodec_default_get_encode_buffer() instead of providing a buffer allocated by
* some other means.
*
* The flags field may contain a combination of AV_GET_ENCODE_BUFFER_FLAG_ flags.
* They may be used for example to hint what use the buffer may get after being
* created.
* Implementations of this callback may ignore flags they don't understand.
* If AV_GET_ENCODE_BUFFER_FLAG_REF is set in flags then the packet may be reused
* (read and/or written to if it is writable) later by libavcodec.
*
* This callback must be thread-safe, as when frame threading is used, it may
* be called from multiple threads simultaneously.
*
* @see avcodec_default_get_encode_buffer()
*
* - encoding: Set by libavcodec, user can override.
* - decoding: unused
*/
int (*get_encode_buffer)(struct AVCodecContext *s, AVPacket *pkt, int flags);
} AVCodecContext;
该结构体中主要的变量：
