FFmpeg最常调用的外部编码库就是libx264和libx265了。我们知道在FFMPEG内部，可以通过结构体AVCodecContext设置一些通用的编码参数，结构体定义见下方代码段。
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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;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.*/struct AVCodecInternal *internal;/*** Private data of the user, can be used to carry app specific stuff.* - encoding: Set by user.* - decoding: Set by user.*/void *opaque;/*** the average bitrate* - encoding: Set by user; unused for constant quantizer encoding.* - decoding: Set by user, may be overwritten by libavcodec* if this info is available in the stream*/int64_t bit_rate;/*** number of bits the bitstream is allowed to diverge from the reference.* the reference can be CBR (for CBR pass1) or VBR (for pass2)* - encoding: Set by user; unused for constant quantizer encoding.* - decoding: unused*/int bit_rate_tolerance;/*** Global quality for codecs which cannot change it per frame.* This should be proportional to MPEG-1/2/4 qscale.* - encoding: Set by user.* - decoding: unused*/int global_quality;/*** - encoding: Set by user.* - decoding: unused*/int compression_level;
#define FF_COMPRESSION_DEFAULT -1/*** AV_CODEC_FLAG_*.* - encoding: Set by user.* - decoding: Set by user.*/int flags;/*** AV_CODEC_FLAG2_** - encoding: Set by user.* - decoding: Set by user.*/int flags2;/*** some codecs need / can use extradata like Huffman tables.* MJPEG: Huffman tables* rv10: additional flags* MPEG-4: global headers (they can be in the bitstream or here)* The allocated memory should be AV_INPUT_BUFFER_PADDING_SIZE bytes larger* than extradata_size to avoid problems if it is read with the bitstream reader.* The bytewise contents of extradata must not depend on the architecture or CPU endianness.* - encoding: Set/allocated/freed by libavcodec.* - decoding: Set/allocated/freed by user.*/uint8_t *extradata;int extradata_size;/*** This is the fundamental unit of time (in seconds) in terms* of which frame timestamps are represented. For fixed-fps content,* timebase should be 1/framerate and timestamp increments should be* identically 1.* This often, but not always is the inverse of the frame rate or field rate* for video. 1/time_base is not the average frame rate if the frame rate is not* constant.** Like containers, elementary streams also can store timestamps, 1/time_base* is the unit in which these timestamps are specified.* As example of such codec time base see ISO/IEC 14496-2:2001(E)* vop_time_increment_resolution and fixed_vop_rate* (fixed_vop_rate == 0 implies that it is different from the framerate)** - encoding: MUST be set by user.* - decoding: the use of this field for decoding is deprecated.* Use framerate instead.*/AVRational time_base;/*** For some codecs, the time base is closer to the field rate than the frame rate.* Most notably, H.264 and MPEG-2 specify time_base as half of frame duration* if no telecine is used ...** Set to time_base ticks per frame. Default 1, e.g., H.264/MPEG-2 set it to 2.*/int ticks_per_frame;/*** Codec delay.** Encoding: Number of frames delay there will be from the encoder input to* the decoder output. (we assume the decoder matches the spec)* Decoding: Number of frames delay in addition to what a standard decoder* as specified in the spec would produce.** Video:* Number of frames the decoded output will be delayed relative to the* encoded input.** Audio:* For encoding, this field is unused (see initial_padding).** For decoding, this is the number of samples the decoder needs to* output before the decoder's output is valid. When seeking, you should* start decoding this many samples prior to your desired seek point.** - encoding: Set by libavcodec.* - decoding: Set by libavcodec.*/int delay;/* video only *//*** picture width / height.** @note Those fields may not match the values of the last* AVFrame output by avcodec_decode_video2 due frame* reordering.** - encoding: MUST be set by user.* - decoding: May be set by the user before opening the decoder if known e.g.* from the container. Some decoders will require the dimensions* to be set by the caller. During decoding, the decoder may* overwrite those values as required while parsing the data.*/int width, height;/*** Bitstream width / height, may be different from width/height e.g. when* the decoded frame is cropped before being output or lowres is enabled.** @note Those field may not match the value of the last* AVFrame output by avcodec_receive_frame() due frame* reordering.** - encoding: unused* - decoding: May be set by the user before opening the decoder if known* e.g. from the container. During decoding, the decoder may* overwrite those values as required while parsing the data.*/int coded_width, coded_height;/*** 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 pixelFormat* @param fmt is the list of formats which are supported by the codec,* it is terminated by -1 as 0 is a valid format, the formats are ordered by quality.* The first is always the native one.* @note The callback may be called again immediately if initialization for* the selected (hardware-accelerated) pixel format failed.* @warning Behavior is undefined if the callback returns a value not* in the fmt list of formats.* @return the chosen format* - encoding: unused* - decoding: Set by user, if not set the native format will be chosen.*/enum AVPixelFormat (*get_format)(struct AVCodecContext *s, const enum AVPixelFormat * fmt);/*** maximum number of B-frames between non-B-frames* Note: The output will be delayed by max_b_frames+1 relative to the input.* - encoding: Set by user.* - decoding: unused*/int max_b_frames;/*** qscale factor between IP and B-frames* If > 0 then the last P-frame quantizer will be used (q= lastp_q*factor+offset).* If < 0 then normal ratecontrol will be done (q= -normal_q*factor+offset).* - encoding: Set by user.* - decoding: unused*/float b_quant_factor;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint b_frame_strategy;
#endif/*** qscale offset between IP and B-frames* - encoding: Set by user.* - decoding: unused*/float b_quant_offset;/*** Size of the frame reordering buffer in the decoder.* For MPEG-2 it is 1 IPB or 0 low delay IP.* - encoding: Set by libavcodec.* - decoding: Set by libavcodec.*/int has_b_frames;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint mpeg_quant;
#endif/*** qscale factor between P- and I-frames* If > 0 then the last P-frame quantizer will be used (q = lastp_q * factor + offset).* If < 0 then normal ratecontrol will be done (q= -normal_q*factor+offset).* - encoding: Set by user.* - decoding: unused*/float i_quant_factor;/*** qscale offset between P and I-frames* - encoding: Set by user.* - decoding: unused*/float i_quant_offset;/*** luminance masking (0-> disabled)* - encoding: Set by user.* - decoding: unused*/float lumi_masking;/*** temporary complexity masking (0-> disabled)* - encoding: Set by user.* - decoding: unused*/float temporal_cplx_masking;/*** spatial complexity masking (0-> disabled)* - encoding: Set by user.* - decoding: unused*/float spatial_cplx_masking;/*** p block masking (0-> disabled)* - encoding: Set by user.* - decoding: unused*/float p_masking;/*** darkness masking (0-> disabled)* - encoding: Set by user.* - decoding: unused*/float dark_masking;/*** slice count* - encoding: Set by libavcodec.* - decoding: Set by user (or 0).*/int slice_count;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint prediction_method;
#define FF_PRED_LEFT 0
#define FF_PRED_PLANE 1
#define FF_PRED_MEDIAN 2
#endif/*** slice offsets in the frame in bytes* - encoding: Set/allocated by libavcodec.* - decoding: Set/allocated by user (or NULL).*/int *slice_offset;/*** sample aspect ratio (0 if unknown)* That is the width of a pixel divided by the height of the pixel.* Numerator and denominator must be relatively prime and smaller than 256 for some video standards.* - encoding: Set by user.* - decoding: Set by libavcodec.*/AVRational sample_aspect_ratio;/*** motion estimation comparison function* - encoding: Set by user.* - decoding: unused*/int me_cmp;/*** subpixel motion estimation comparison function* - encoding: Set by user.* - decoding: unused*/int me_sub_cmp;/*** macroblock comparison function (not supported yet)* - encoding: Set by user.* - decoding: unused*/int mb_cmp;/*** interlaced DCT comparison function* - encoding: Set by user.* - decoding: unused*/int ildct_cmp;
#define FF_CMP_SAD 0
#define FF_CMP_SSE 1
#define FF_CMP_SATD 2
#define FF_CMP_DCT 3
#define FF_CMP_PSNR 4
#define FF_CMP_BIT 5
#define FF_CMP_RD 6
#define FF_CMP_ZERO 7
#define FF_CMP_VSAD 8
#define FF_CMP_VSSE 9
#define FF_CMP_NSSE 10
#define FF_CMP_W53 11
#define FF_CMP_W97 12
#define FF_CMP_DCTMAX 13
#define FF_CMP_DCT264 14
#define FF_CMP_MEDIAN_SAD 15
#define FF_CMP_CHROMA 256/*** ME diamond size & shape* - encoding: Set by user.* - decoding: unused*/int dia_size;/*** amount of previous MV predictors (2a+1 x 2a+1 square)* - encoding: Set by user.* - decoding: unused*/int last_predictor_count;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint pre_me;
#endif/*** motion estimation prepass comparison function* - encoding: Set by user.* - decoding: unused*/int me_pre_cmp;/*** ME prepass diamond size & shape* - encoding: Set by user.* - decoding: unused*/int pre_dia_size;/*** subpel ME quality* - encoding: Set by user.* - decoding: unused*/int me_subpel_quality;/*** 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;
#define SLICE_FLAG_CODED_ORDER 0x0001 ///< draw_horiz_band() is called in coded order instead of display
#define SLICE_FLAG_ALLOW_FIELD 0x0002 ///< allow draw_horiz_band() with field slices (MPEG-2 field pics)
#define SLICE_FLAG_ALLOW_PLANE 0x0004 ///< allow draw_horiz_band() with 1 component at a time (SVQ1)/*** macroblock decision mode* - encoding: Set by user.* - decoding: unused*/int mb_decision;
#define FF_MB_DECISION_SIMPLE 0 ///< uses mb_cmp
#define FF_MB_DECISION_BITS 1 ///< chooses the one which needs the fewest bits
#define FF_MB_DECISION_RD 2 ///< rate distortion/*** custom intra quantization matrix* - encoding: Set by user, can be NULL.* - decoding: Set by libavcodec.*/uint16_t *intra_matrix;/*** custom inter quantization matrix* - encoding: Set by user, can be NULL.* - decoding: Set by libavcodec.*/uint16_t *inter_matrix;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint scenechange_threshold;/** @deprecated use encoder private options instead */attribute_deprecatedint noise_reduction;
#endif/*** precision of the intra DC coefficient - 8* - encoding: Set by user.* - decoding: Set by libavcodec*/int intra_dc_precision;/*** Number of macroblock rows at the top which are skipped.* - encoding: unused* - decoding: Set by user.*/int skip_top;/*** Number of macroblock rows at the bottom which are skipped.* - encoding: unused* - decoding: Set by user.*/int skip_bottom;/*** minimum MB Lagrange multiplier* - encoding: Set by user.* - decoding: unused*/int mb_lmin;/*** maximum MB Lagrange multiplier* - encoding: Set by user.* - decoding: unused*/int mb_lmax;#if FF_API_PRIVATE_OPT/*** @deprecated use encoder private options instead*/attribute_deprecatedint me_penalty_compensation;
#endif/*** - encoding: Set by user.* - decoding: unused*/int bidir_refine;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint brd_scale;
#endif/*** minimum GOP size* - encoding: Set by user.* - decoding: unused*/int keyint_min;/*** number of reference frames* - encoding: Set by user.* - decoding: Set by lavc.*/int refs;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint chromaoffset;
#endif/*** Note: Value depends upon the compare function used for fullpel ME.* - encoding: Set by user.* - decoding: unused*/int mv0_threshold;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint b_sensitivity;
#endif/*** Chromaticity coordinates of the source primaries.* - encoding: Set by user* - decoding: Set by libavcodec*/enum AVColorPrimaries color_primaries;/*** Color Transfer Characteristic.* - encoding: Set by user* - decoding: Set by libavcodec*/enum AVColorTransferCharacteristic color_trc;/*** YUV colorspace type.* - encoding: Set by user* - decoding: Set by libavcodec*/enum AVColorSpace colorspace;/*** MPEG vs JPEG YUV range.* - encoding: Set by user* - decoding: Set by libavcodec*/enum AVColorRange color_range;/*** This defines the location of chroma samples.* - encoding: Set by user* - decoding: Set by libavcodec*/enum AVChromaLocation chroma_sample_location;/*** Number of slices.* Indicates number of picture subdivisions. Used for parallelized* decoding.* - encoding: Set by user* - decoding: unused*/int slices;/** Field order* - encoding: set by libavcodec* - decoding: Set by user.*/enum AVFieldOrder field_order;/* audio only */int sample_rate; ///< samples per secondint channels; ///< number of audio channels/*** audio sample format* - encoding: Set by user.* - decoding: Set by libavcodec.*/enum AVSampleFormat sample_fmt; ///< sample format/* The following data should not be initialized. *//*** Number of samples per channel in an audio frame.** - encoding: set by libavcodec in avcodec_open2(). Each submitted frame* except the last must contain exactly frame_size samples per channel.* May be 0 when the codec has AV_CODEC_CAP_VARIABLE_FRAME_SIZE set, then the* frame size is not restricted.* - decoding: may be set by some decoders to indicate constant frame size*/int frame_size;/*** Frame counter, set by libavcodec.** - decoding: total number of frames returned from the decoder so far.* - encoding: total number of frames passed to the encoder so far.** @note the counter is not incremented if encoding/decoding resulted in* an error.*/int frame_number;/*** number of bytes per packet if constant and known or 0* Used by some WAV based audio codecs.*/int block_align;/*** Audio cutoff bandwidth (0 means "automatic")* - encoding: Set by user.* - decoding: unused*/int cutoff;/*** Audio channel layout.* - encoding: set by user.* - decoding: set by user, may be overwritten by libavcodec.*/uint64_t channel_layout;/*** Request decoder to use this channel layout if it can (0 for default)* - encoding: unused* - decoding: Set by user.*/uint64_t request_channel_layout;/*** Type of service that the audio stream conveys.* - encoding: Set by user.* - decoding: Set by libavcodec.*/enum AVAudioServiceType audio_service_type;/*** desired sample format* - encoding: Not used.* - decoding: Set by user.* Decoder will decode to this format if it can.*/enum AVSampleFormat request_sample_fmt;/*** This callback is called at the beginning of each frame to get data* buffer(s) for it. There may be one contiguous buffer for all the data or* there may be a buffer per each data plane or anything in between. What* this means is, you may set however many entries in buf[] you feel necessary.* Each buffer must be reference-counted using the AVBuffer API (see description* of buf[] below).** The following fields will be set in the frame before this callback is* called:* - format* - width, height (video only)* - sample_rate, channel_layout, nb_samples (audio only)* Their values may differ from the corresponding values in* AVCodecContext. This callback must use the frame values, not the codec* context values, to calculate the required buffer size.** This callback must fill the following fields in the frame:* - data[]* - linesize[]* - extended_data:* * if the data is planar audio with more than 8 channels, then this* callback must allocate and fill extended_data to contain all pointers* to all data planes. data[] must hold as many pointers as it can.* extended_data must be allocated with av_malloc() and will be freed in* av_frame_unref().* * otherwise extended_data must point to data* - buf[] must contain one or more pointers to AVBufferRef structures. Each of* the frame's data and extended_data pointers must be contained in these. That* is, one AVBufferRef for each allocated chunk of memory, not necessarily one* AVBufferRef per data[] entry. See: av_buffer_create(), av_buffer_alloc(),* and av_buffer_ref().* - extended_buf and nb_extended_buf must be allocated with av_malloc() by* this callback and filled with the extra buffers if there are more* buffers than buf[] can hold. extended_buf will be freed in* av_frame_unref().** If AV_CODEC_CAP_DR1 is not set then get_buffer2() must call* avcodec_default_get_buffer2() instead of providing buffers allocated by* some other means.** Each data plane must be aligned to the maximum required by the target* CPU.** @see avcodec_default_get_buffer2()** Video:** If AV_GET_BUFFER_FLAG_REF is set in flags then the frame may be reused* (read and/or written to if it is writable) later by libavcodec.** avcodec_align_dimensions2() should be used to find the required width and* height, as they normally need to be rounded up to the next multiple of 16.** Some decoders do not support linesizes changing between frames.** If frame multithreading is used and thread_safe_callbacks is set,* this callback may be called from a different thread, but not from more* than one at once. Does not need to be reentrant.** @see avcodec_align_dimensions2()** Audio:** Decoders request a buffer of a particular size by setting* AVFrame.nb_samples prior to calling get_buffer2(). The decoder may,* however, utilize only part of the buffer by setting AVFrame.nb_samples* to a smaller value in the output frame.** As a convenience, av_samples_get_buffer_size() and* av_samples_fill_arrays() in libavutil may be used by custom get_buffer2()* functions to find the required data size and to fill data pointers and* linesize. In AVFrame.linesize, only linesize[0] may be set for audio* since all planes must be the same size.** @see av_samples_get_buffer_size(), av_samples_fill_arrays()** - encoding: unused* - decoding: Set by libavcodec, user can override.*/int (*get_buffer2)(struct AVCodecContext *s, AVFrame *frame, int flags);/*** If non-zero, the decoded audio and video frames returned from* avcodec_decode_video2() and avcodec_decode_audio4() are reference-counted* and are valid indefinitely. The caller must free them with* av_frame_unref() when they are not needed anymore.* Otherwise, the decoded frames must not be freed by the caller and are* only valid until the next decode call.** This is always automatically enabled if avcodec_receive_frame() is used.** - encoding: unused* - decoding: set by the caller before avcodec_open2().*/attribute_deprecatedint refcounted_frames;/* - encoding parameters */float qcompress; ///< amount of qscale change between easy & hard scenes (0.0-1.0)float qblur; ///< amount of qscale smoothing over time (0.0-1.0)/*** minimum quantizer* - encoding: Set by user.* - decoding: unused*/int qmin;/*** maximum quantizer* - encoding: Set by user.* - decoding: unused*/int qmax;/*** maximum quantizer difference between frames* - encoding: Set by user.* - decoding: unused*/int max_qdiff;/*** 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;#if FF_API_CODER_TYPE
#define FF_CODER_TYPE_VLC 0
#define FF_CODER_TYPE_AC 1
#define FF_CODER_TYPE_RAW 2
#define FF_CODER_TYPE_RLE 3/*** @deprecated use encoder private options instead*/attribute_deprecatedint coder_type;
#endif /* FF_API_CODER_TYPE */#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint context_model;
#endif#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint frame_skip_threshold;/** @deprecated use encoder private options instead */attribute_deprecatedint frame_skip_factor;/** @deprecated use encoder private options instead */attribute_deprecatedint frame_skip_exp;/** @deprecated use encoder private options instead */attribute_deprecatedint frame_skip_cmp;
#endif /* FF_API_PRIVATE_OPT *//*** trellis RD quantization* - encoding: Set by user.* - decoding: unused*/int trellis;#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint min_prediction_order;/** @deprecated use encoder private options instead */attribute_deprecatedint max_prediction_order;/** @deprecated use encoder private options instead */attribute_deprecatedint64_t timecode_frame_start;
#endif#if FF_API_RTP_CALLBACK/*** @deprecated unused*//* The RTP callback: This function is called *//* every time the encoder has a packet to send. *//* It depends on the encoder if the data starts *//* with a Start Code (it should). H.263 does. *//* mb_nb contains the number of macroblocks *//* encoded in the RTP payload. */attribute_deprecatedvoid (*rtp_callback)(struct AVCodecContext *avctx, void *data, int size, int mb_nb);
#endif#if FF_API_PRIVATE_OPT/** @deprecated use encoder private options instead */attribute_deprecatedint rtp_payload_size; /* The size of the RTP payload: the coder will *//* do its best to deliver a chunk with size *//* below rtp_payload_size, the chunk will start *//* with a start code on some codecs like H.263. *//* This doesn't take account of any particular *//* headers inside the transmitted RTP payload. */
#endif#if FF_API_STAT_BITS/* statistics, used for 2-pass encoding */attribute_deprecatedint mv_bits;attribute_deprecatedint header_bits;attribute_deprecatedint i_tex_bits;attribute_deprecatedint p_tex_bits;attribute_deprecatedint i_count;attribute_deprecatedint p_count;attribute_deprecatedint skip_count;attribute_deprecatedint misc_bits;/** @deprecated this field is unused */attribute_deprecatedint frame_bits;
#endif/*** pass1 encoding statistics output buffer* - encoding: Set by libavcodec.* - decoding: unused*/char *stats_out;/*** pass2 encoding statistics input buffer* Concatenated stuff from stats_out of pass1 should be placed here.* - encoding: Allocated/set/freed by user.* - decoding: unused*/char *stats_in;/*** Work around bugs in encoders which sometimes cannot be detected automatically.* - encoding: Set by user* - decoding: Set by user*/int workaround_bugs;
#define FF_BUG_AUTODETECT 1 ///< autodetection
#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_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
#define FF_BUG_IEDGE 32768/*** strictly follow the standard (MPEG-4, ...).* - encoding: Set by user.* - decoding: Set by user.* Setting this to STRICT or higher means the encoder and decoder will* generally do stupid things, whereas setting it to unofficial or lower* will mean the encoder might produce output that is not supported by all* spec-compliant decoders. Decoders don't differentiate between normal,* unofficial and experimental (that is, they always try to decode things* when they can) unless they are explicitly asked to behave stupidly* (=strictly conform to the specs)*/int strict_std_compliance;
#define FF_COMPLIANCE_VERY_STRICT 2 ///< Strictly conform to an older more strict version of the spec or reference software.
#define FF_COMPLIANCE_STRICT 1 ///< Strictly conform to all the things in the spec no matter what consequences.
#define FF_COMPLIANCE_NORMAL 0
#define FF_COMPLIANCE_UNOFFICIAL -1 ///< Allow unofficial extensions
#define FF_COMPLIANCE_EXPERIMENTAL -2 ///< Allow nonstandardized experimental things./*** error concealment flags* - encoding: unused* - decoding: Set by user.*/int error_concealment;
#define FF_EC_GUESS_MVS 1
#define FF_EC_DEBLOCK 2
#define FF_EC_FAVOR_INTER 256/*** debug* - encoding: Set by user.* - decoding: Set by user.*/int debug;
#define FF_DEBUG_PICT_INFO 1
#define FF_DEBUG_RC 2
#define FF_DEBUG_BITSTREAM 4
#define FF_DEBUG_MB_TYPE 8
#define FF_DEBUG_QP 16
#if FF_API_DEBUG_MV
/*** @deprecated this option does nothing*/
#define FF_DEBUG_MV 32
#endif
#define FF_DEBUG_DCT_COEFF 0x00000040
#define FF_DEBUG_SKIP 0x00000080
#define FF_DEBUG_STARTCODE 0x00000100
#define FF_DEBUG_ER 0x00000400
#define FF_DEBUG_MMCO 0x00000800
#define FF_DEBUG_BUGS 0x00001000
#if FF_API_DEBUG_MV
#define FF_DEBUG_VIS_QP 0x00002000
#define FF_DEBUG_VIS_MB_TYPE 0x00004000
#endif
#define FF_DEBUG_BUFFERS 0x00008000
#define FF_DEBUG_THREADS 0x00010000
#define FF_DEBUG_GREEN_MD 0x00800000
#define FF_DEBUG_NOMC 0x01000000#if FF_API_DEBUG_MV/*** debug* - encoding: Set by user.* - decoding: Set by user.*/int debug_mv;
#define FF_DEBUG_VIS_MV_P_FOR 0x00000001 // visualize forward predicted MVs of P-frames
#define FF_DEBUG_VIS_MV_B_FOR 0x00000002 // visualize forward predicted MVs of B-frames
#define FF_DEBUG_VIS_MV_B_BACK 0x00000004 // visualize backward predicted MVs of B-frames
#endif/*** Error recognition; may misdetect some more or less valid parts as errors.* - encoding: unused* - decoding: Set by user.*/int err_recognition;/*** Verify checksums embedded in the bitstream (could be of either encoded or* decoded data, depending on the codec) and print an error message on mismatch.* If AV_EF_EXPLODE is also set, a mismatching checksum will result in the* decoder returning an error.*/
#define AV_EF_CRCCHECK (1<<0)
#define AV_EF_BITSTREAM (1<<1) ///< detect bitstream specification deviations
#define AV_EF_BUFFER (1<<2) ///< detect improper bitstream length
#define AV_EF_EXPLODE (1<<3) ///< abort decoding on minor error detection#define AV_EF_IGNORE_ERR (1<<15) ///< ignore errors and continue
#define AV_EF_CAREFUL (1<<16) ///< consider things that violate the spec, are fast to calculate and have not been seen in the wild as errors
#define AV_EF_COMPLIANT (1<<17) ///< consider all spec non compliances as errors
#define AV_EF_AGGRESSIVE (1<<18) ///< consider things that a sane encoder should not do as an error/*** opaque 64-bit number (generally a PTS) that will be reordered and* output in AVFrame.reordered_opaque* - encoding: unused* - decoding: Set by user.*/int64_t reordered_opaque;/*** Hardware accelerator in use* - encoding: unused.* - decoding: Set by libavcodec*/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* is. e.g. for VA API, this is a struct vaapi_context.* - encoding: unused* - decoding: Set by user*/void *hwaccel_context;/*** error* - encoding: Set by libavcodec if flags & AV_CODEC_FLAG_PSNR.* - decoding: unused*/uint64_t error[AV_NUM_DATA_POINTERS];/*** DCT algorithm, see FF_DCT_* below* - encoding: Set by user.* - decoding: unused*/int dct_algo;
#define FF_DCT_AUTO 0
#define FF_DCT_FASTINT 1
#define FF_DCT_INT 2
#define FF_DCT_MMX 3
#define FF_DCT_ALTIVEC 5
#define FF_DCT_FAAN 6/*** IDCT algorithm, see FF_IDCT_* below.* - encoding: Set by user.* - decoding: Set by user.*/int idct_algo;
#define FF_IDCT_AUTO 0
#define FF_IDCT_INT 1
#define FF_IDCT_SIMPLE 2
#define FF_IDCT_SIMPLEMMX 3
#define FF_IDCT_ARM 7
#define FF_IDCT_ALTIVEC 8
#define FF_IDCT_SIMPLEARM 10
#define FF_IDCT_XVID 14
#define FF_IDCT_SIMPLEARMV5TE 16
#define FF_IDCT_SIMPLEARMV6 17
#define FF_IDCT_FAAN 20
#define FF_IDCT_SIMPLENEON 22
#define FF_IDCT_NONE 24 /* Used by XvMC to extract IDCT coefficients with FF_IDCT_PERM_NONE */
#define FF_IDCT_SIMPLEAUTO 128/*** bits per sample/pixel from the demuxer (needed for huffyuv).* - encoding: Set by libavcodec.* - decoding: Set by user.*/int bits_per_coded_sample;/*** Bits per sample/pixel of internal libavcodec pixel/sample format.* - encoding: set by user.* - decoding: set by libavcodec.*/int bits_per_raw_sample;#if FF_API_LOWRES/*** low resolution decoding, 1-> 1/2 size, 2->1/4 size* - encoding: unused* - decoding: Set by user.*/int lowres;
#endif#if FF_API_CODED_FRAME/*** the picture in the bitstream* - encoding: Set by libavcodec.* - decoding: unused** @deprecated use the quality factor packet side data instead*/attribute_deprecated AVFrame *coded_frame;
#endif/*** thread count* is used to decide how many independent tasks should be passed to execute()* - encoding: Set by user.* - decoding: Set by user.*/int thread_count;/*** Which multithreading methods to use.* Use of FF_THREAD_FRAME will increase decoding delay by one frame per thread,* so clients which cannot provide future frames should not use it.** - encoding: Set by user, otherwise the default is used.* - decoding: Set by user, otherwise the default is used.*/int thread_type;
#define FF_THREAD_FRAME 1 ///< Decode more than one frame at once
#define FF_THREAD_SLICE 2 ///< Decode more than one part of a single frame at once/*** Which multithreading methods are in use by the codec.* - encoding: Set by libavcodec.* - decoding: Set by libavcodec.*/int active_thread_type;/*** Set by the client if its custom get_buffer() callback can be called* synchronously from another thread, which allows faster multithreaded decoding.* draw_horiz_band() will be called from other threads regardless of this setting.* Ignored if the default get_buffer() is used.* - encoding: Set by user.* - decoding: Set by user.*/int thread_safe_callbacks;/*** The codec may call this to execute several independent things.* It will return only after finishing all tasks.* The user may replace this with some multithreaded implementation,* the default implementation will execute the parts serially.* @param count the number of things to execute* - encoding: Set by libavcodec, user can override.* - decoding: Set by libavcodec, user can override.*/int (*execute)(struct AVCodecContext *c, int (*func)(struct AVCodecContext *c2, void *arg), void *arg2, int *ret, int count, int size);/*** The codec may call this to execute several independent things.* It will return only after finishing all tasks.* The user may replace this with some multithreaded implementation,* the default implementation will execute the parts serially.* Also see avcodec_thread_init and e.g. the --enable-pthread configure option.* @param c context passed also to func* @param count the number of things to execute* @param arg2 argument passed unchanged to func* @param ret return values of executed functions, must have space for "count" values. May be NULL.* @param func function that will be called count times, with jobnr from 0 to count-1.* threadnr will be in the range 0 to c->thread_count-1 < MAX_THREADS and so that no* two instances of func executing at the same time will have the same threadnr.* @return always 0 currently, but code should handle a future improvement where when any call to func* returns < 0 no further calls to func may be done and < 0 is returned.* - encoding: Set by libavcodec, user can override.* - decoding: Set by libavcodec, user can override.*/int (*execute2)(struct AVCodecContext *c, int (*func)(struct AVCodecContext *c2, void *arg, int jobnr, int threadnr), void *arg2, int *ret, int count);/*** noise vs. sse weight for the nsse comparison function* - encoding: Set by user.* - decoding: unused*/int nsse_weight;/*** profile* - encoding: Set by user.* - decoding: Set by libavcodec.*/int profile;
#define FF_PROFILE_UNKNOWN -99
#define FF_PROFILE_RESERVED -100#define FF_PROFILE_AAC_MAIN 0
#define FF_PROFILE_AAC_LOW 1
#define FF_PROFILE_AAC_SSR 2
#define FF_PROFILE_AAC_LTP 3
#define FF_PROFILE_AAC_HE 4
#define FF_PROFILE_AAC_HE_V2 28
#define FF_PROFILE_AAC_LD 22
#define FF_PROFILE_AAC_ELD 38
#define FF_PROFILE_MPEG2_AAC_LOW 128
#define FF_PROFILE_MPEG2_AAC_HE 131#define FF_PROFILE_DNXHD 0
#define FF_PROFILE_DNXHR_LB 1
#define FF_PROFILE_DNXHR_SQ 2
#define FF_PROFILE_DNXHR_HQ 3
#define FF_PROFILE_DNXHR_HQX 4
#define FF_PROFILE_DNXHR_444 5#define FF_PROFILE_DTS 20
#define FF_PROFILE_DTS_ES 30
#define FF_PROFILE_DTS_96_24 40
#define FF_PROFILE_DTS_HD_HRA 50
#define FF_PROFILE_DTS_HD_MA 60
#define FF_PROFILE_DTS_EXPRESS 70#define FF_PROFILE_MPEG2_422 0
#define FF_PROFILE_MPEG2_HIGH 1
#define FF_PROFILE_MPEG2_SS 2
#define FF_PROFILE_MPEG2_SNR_SCALABLE 3
#define FF_PROFILE_MPEG2_MAIN 4
#define FF_PROFILE_MPEG2_SIMPLE 5#define FF_PROFILE_H264_CONSTRAINED (1<<9) // 8+1; constraint_set1_flag
#define FF_PROFILE_H264_INTRA (1<<11) // 8+3; constraint_set3_flag#define FF_PROFILE_H264_BASELINE 66
#define FF_PROFILE_H264_CONSTRAINED_BASELINE (66|FF_PROFILE_H264_CONSTRAINED)
#define FF_PROFILE_H264_MAIN 77
#define FF_PROFILE_H264_EXTENDED 88
#define FF_PROFILE_H264_HIGH 100
#define FF_PROFILE_H264_HIGH_10 110
#define FF_PROFILE_H264_HIGH_10_INTRA (110|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_MULTIVIEW_HIGH 118
#define FF_PROFILE_H264_HIGH_422 122
#define FF_PROFILE_H264_HIGH_422_INTRA (122|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_STEREO_HIGH 128
#define FF_PROFILE_H264_HIGH_444 144
#define FF_PROFILE_H264_HIGH_444_PREDICTIVE 244
#define FF_PROFILE_H264_HIGH_444_INTRA (244|FF_PROFILE_H264_INTRA)
#define FF_PROFILE_H264_CAVLC_444 44#define FF_PROFILE_VC1_SIMPLE 0
#define FF_PROFILE_VC1_MAIN 1
#define FF_PROFILE_VC1_COMPLEX 2
#define FF_PROFILE_VC1_ADVANCED 3#define FF_PROFILE_MPEG4_SIMPLE 0
#define FF_PROFILE_MPEG4_SIMPLE_SCALABLE 1
#define FF_PROFILE_MPEG4_CORE 2
#define FF_PROFILE_MPEG4_MAIN 3
#define FF_PROFILE_MPEG4_N_BIT 4
#define FF_PROFILE_MPEG4_SCALABLE_TEXTURE 5
#define FF_PROFILE_MPEG4_SIMPLE_FACE_ANIMATION 6
#define FF_PROFILE_MPEG4_BASIC_ANIMATED_TEXTURE 7
#define FF_PROFILE_MPEG4_HYBRID 8
#define FF_PROFILE_MPEG4_ADVANCED_REAL_TIME 9
#define FF_PROFILE_MPEG4_CORE_SCALABLE 10
#define FF_PROFILE_MPEG4_ADVANCED_CODING 11
#define FF_PROFILE_MPEG4_ADVANCED_CORE 12
#define FF_PROFILE_MPEG4_ADVANCED_SCALABLE_TEXTURE 13
#define FF_PROFILE_MPEG4_SIMPLE_STUDIO 14
#define FF_PROFILE_MPEG4_ADVANCED_SIMPLE 15#define FF_PROFILE_JPEG2000_CSTREAM_RESTRICTION_0 1
#define FF_PROFILE_JPEG2000_CSTREAM_RESTRICTION_1 2
#define FF_PROFILE_JPEG2000_CSTREAM_NO_RESTRICTION 32768
#define FF_PROFILE_JPEG2000_DCINEMA_2K 3
#define FF_PROFILE_JPEG2000_DCINEMA_4K 4#define FF_PROFILE_VP9_0 0
#define FF_PROFILE_VP9_1 1
#define FF_PROFILE_VP9_2 2
#define FF_PROFILE_VP9_3 3#define FF_PROFILE_HEVC_MAIN 1
#define FF_PROFILE_HEVC_MAIN_10 2
#define FF_PROFILE_HEVC_MAIN_STILL_PICTURE 3
#define FF_PROFILE_HEVC_REXT 4#define FF_PROFILE_AV1_MAIN 0
#define FF_PROFILE_AV1_HIGH 1
#define FF_PROFILE_AV1_PROFESSIONAL 2#define FF_PROFILE_MJPEG_HUFFMAN_BASELINE_DCT 0xc0
#define FF_PROFILE_MJPEG_HUFFMAN_EXTENDED_SEQUENTIAL_DCT 0xc1
#define FF_PROFILE_MJPEG_HUFFMAN_PROGRESSIVE_DCT 0xc2
#define FF_PROFILE_MJPEG_HUFFMAN_LOSSLESS 0xc3
#define FF_PROFILE_MJPEG_JPEG_LS 0xf7#define FF_PROFILE_SBC_MSBC 1/*** level* - encoding: Set by user.* - decoding: Set by libavcodec.*/int level;
#define FF_LEVEL_UNKNOWN -99/*** Skip loop filtering for selected frames.* - encoding: unused* - decoding: Set by user.*/enum AVDiscard skip_loop_filter;/*** Skip IDCT/dequantization for selected frames.* - encoding: unused* - decoding: Set by user.*/enum AVDiscard skip_idct;/*** Skip decoding for selected frames.* - encoding: unused* - decoding: Set by user.*/enum AVDiscard skip_frame;/*** Header containing style information for text subtitles.* For SUBTITLE_ASS subtitle type, it should contain the whole ASS* [Script Info] and [V4+ Styles] section, plus the [Events] line and* the Format line following. It shouldn't include any Dialogue line.* - encoding: Set/allocated/freed by user (before avcodec_open2())* - decoding: Set/allocated/freed by libavcodec (by avcodec_open2())*/uint8_t *subtitle_header;int subtitle_header_size;#if FF_API_VBV_DELAY/*** VBV delay coded in the last frame (in periods of a 27 MHz clock).* Used for compliant TS muxing.* - encoding: Set by libavcodec.* - decoding: unused.* @deprecated this value is now exported as a part of* AV_PKT_DATA_CPB_PROPERTIES packet side data*/attribute_deprecateduint64_t vbv_delay;
#endif#if FF_API_SIDEDATA_ONLY_PKT/*** Encoding only and set by default. Allow encoders to output packets* that do not contain any encoded data, only side data.** Some encoders need to output such packets, e.g. to update some stream* parameters at the end of encoding.** @deprecated this field disables the default behaviour and* it is kept only for compatibility.*/attribute_deprecatedint side_data_only_packets;
#endif/*** Audio only. The number of "priming" samples (padding) inserted by the* encoder at the beginning of the audio. I.e. this number of leading* decoded samples must be discarded by the caller to get the original audio* without leading padding.** - decoding: unused* - encoding: Set by libavcodec. The timestamps on the output packets are* adjusted by the encoder so that they always refer to the* first sample of the data actually contained in the packet,* including any added padding. E.g. if the timebase is* 1/samplerate and the timestamp of the first input sample is* 0, the timestamp of the first output packet will be* -initial_padding.*/int initial_padding;/*** - decoding: For codecs that store a framerate value in the compressed* bitstream, the decoder may export it here. { 0, 1} when* unknown.* - encoding: May be used to signal the framerate of CFR content to an* encoder.*/AVRational framerate;/*** Nominal unaccelerated pixel format, see AV_PIX_FMT_xxx.* - encoding: unused.* - decoding: Set by libavcodec before calling get_format()*/enum AVPixelFormat sw_pix_fmt;/*** Timebase in which pkt_dts/pts and AVPacket.dts/pts are.* - encoding unused.* - decoding set by user.*/AVRational pkt_timebase;/*** AVCodecDescriptor* - encoding: unused.* - decoding: set by libavcodec.*/const AVCodecDescriptor *codec_descriptor;#if !FF_API_LOWRES/*** low resolution decoding, 1-> 1/2 size, 2->1/4 size* - encoding: unused* - decoding: Set by user.*/int lowres;
#endif/*** Current statistics for PTS correction.* - decoding: maintained and used by libavcodec, not intended to be used by user apps* - encoding: unused*/int64_t pts_correction_num_faulty_pts; /// Number of incorrect PTS values so farint64_t pts_correction_num_faulty_dts; /// Number of incorrect DTS values so farint64_t pts_correction_last_pts; /// PTS of the last frameint64_t pts_correction_last_dts; /// DTS of the last frame/*** Character encoding of the input subtitles file.* - decoding: set by user* - encoding: unused*/char *sub_charenc;/*** Subtitles character encoding mode. Formats or codecs might be adjusting* this setting (if they are doing the conversion themselves for instance).* - decoding: set by libavcodec* - encoding: unused*/int sub_charenc_mode;
#define FF_SUB_CHARENC_MODE_DO_NOTHING -1 ///< do nothing (demuxer outputs a stream supposed to be already in UTF-8, or the codec is bitmap for instance)
#define FF_SUB_CHARENC_MODE_AUTOMATIC 0 ///< libavcodec will select the mode itself
#define FF_SUB_CHARENC_MODE_PRE_DECODER 1 ///< the AVPacket data needs to be recoded to UTF-8 before being fed to the decoder, requires iconv
#define FF_SUB_CHARENC_MODE_IGNORE 2 ///< 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;#if !FF_API_DEBUG_MV/*** debug motion vectors* - encoding: Set by user.* - decoding: Set by user.*/int debug_mv;
#define FF_DEBUG_VIS_MV_P_FOR 0x00000001 //visualize forward predicted MVs of P frames
#define FF_DEBUG_VIS_MV_B_FOR 0x00000002 //visualize forward predicted MVs of B frames
#define FF_DEBUG_VIS_MV_B_BACK 0x00000004 //visualize backward predicted MVs of B frames
#endif/*** custom intra quantization matrix* - 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;
#define FF_CODEC_PROPERTY_LOSSLESS 0x00000001
#define FF_CODEC_PROPERTY_CLOSED_CAPTIONS 0x00000002/*** Additional data associated with the entire coded stream.** - decoding: unused* - encoding: may be set by libavcodec after avcodec_open2().*/AVPacketSideData *coded_side_data;int nb_coded_side_data;/*** 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;
#define FF_SUB_TEXT_FMT_ASS 0
#if FF_API_ASS_TIMING
#define FF_SUB_TEXT_FMT_ASS_WITH_TIMINGS 1
#endif/*** 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;
}可以看到，AVCodecContext非常庞大，涵盖了大部分编码器需要的参数，通常，我们通过设置这些参数也够用了，但也有部分编码器私有参数是未涵盖到的，拿x264举例，下面命令行中的-preset fast和-tune zerolatency就是libx264的私有编码参数。
ffmpeg -i input.mp4 -f mpegts -c:v libx264 -preset fast -tune zerolatency http://:8080
那么，在我们使用FFMPEG和libx264提供的接口进行编码的时候，在代码中该如何设置这类参数呢？目前我本人使用到的方式有两种，一是通过设置字典的方式，二是通过设置AVCodecContext中priv_data的options来实现。
一、通过av_dict_set()的方式设置编码参数：
依然用libx264的参数来举例，看下面代码段：
AVCodecContext *pCodecCtx;
AVCodec* pCodec;//通用编码参数的设置
pCodecCtx->codec_type = AVMEDIA_TYPE_VIDEO;
pCodecCtx->pix_fmt = AV_PIX_FMT_YUV420P;
pCodecCtx->width = 640;
pCodecCtx->height = 480;pCodecCtx->time_base.num = 1;
pCodecCtx->time_base.den = 25;
pCodecCtx->bit_rate = 4000000;
pCodecCtx->gop_size = 50;
pCodecCtx->codec_id = AV_CODEC_ID_H264;AVDictionary *param;if(pCodecCtx->codec_id == AV_CODEC_ID_H264)
{av_dict_set(¶m, "preset", "veryfast", 0); //设置字典参数"preset"，键值为"veryfast" av_dict_set(¶m, "tune", "zerolatency", 0); //设置字典参数"tune"，键值为"zerolatency"
}//查找编码器
pCodec = avcodec_find_encoder(pCodecCtx->codec_id);
if(!pCodec)
{printf("Can not find encoder! \n");return -1;
}// 打开编码器
//注意该函数的最后一个参数就是我们刚才设置的字典型结构参数param的地址
if(avcodec_open2(pCodecCtx, pCodec, ¶m) < 0)
{printf("Can not open encoder! \n");return -1
}二、通过av_opt_set()的方式设置AVCodecContext中的priv_data
从AVCodecContext结构体我们可以看到，priv_data是void*型的，在使用时，可以转化为编码器的内部结构体，如libx264中，priv_data要转化为X264Context。
static av_cold int X264_init(AVCodecContext *avctx)
{X264Context *x4 = avctx->priv_data; //priv_data向X264Context结构体的转换AVCPBProperties *cpb_props;int sw,sh;所以，我们可以直接设置priv_data来达到设置编码器参数的目的。具体实现见下面代码片段：
AVCodecContext *pCodecCtx;
AVCodec* pCodec;//通用编码参数的设置
pCodecCtx->codec_type = AVMEDIA_TYPE_VIDEO;
pCodecCtx->pix_fmt = AV_PIX_FMT_YUV420P;
pCodecCtx->width = 640;
pCodecCtx->height = 480;pCodecCtx->time_base.num = 1;
pCodecCtx->time_base.den = 25;
pCodecCtx->bit_rate = 4000000;
pCodecCtx->gop_size = 50;
pCodecCtx->codec_id = AV_CODEC_ID_H264;//AVDictionary *param;if(pCodecCtx->codec_id == AV_CODEC_ID_H264)
{//av_dict_set(¶m, "preset", "veryfast", 0);//av_dict_set(¶m, "tune", "zerolatency", 0);av_opt_set(pCodecCtx->priv_data, "preset", "superfast", 0); //设置priv_data的optionav_opt_set(pCodecCtx->priv_data, "tune", "zerolatency", 0); //设置priv_data的option
}//查找编码器
pCodec = avcodec_find_encoder(pCodecCtx->codec_id);
if(!pCodec)
{printf("Can not find encoder! \n");return -1;
}// 打开编码器
//if(avcodec_open2(pCodecCtx, pCodec, ¶m) < 0)
if(avcodec_open2(pCodecCtx, pCodec, NULL) < 0) //没有AVDictionary的有效参数，最后一个参数传NULL即可
{printf("Can not open encoder! \n");return -1
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