先贴上雷神的一张FFmpeg关键结构体之间的关系图：
再看雷神的分析：
- 每个AVStream存储一个视频/音频流的相关数据；
- 每个AVStream对应一个AVCodecContext，存储该视频/音频流使用解码方式的相关数据；
- 每个AVCodecContext中对应一个AVCodec，包含该视频/音频对应的解码器。
- 每种解码器都对应一个AVCodec结构。
看过雷神的分析，再加上自己的使用经验，总结如下：
AVCodec是存储编解码器信息的结构体，一般来说使用如下两个函数获得：
/**
* Find a registered decoder with a matching codec ID.
*
* @param id AVCodecID of the requested decoder
* @return A decoder if one was found, NULL otherwise.
*/
AVCodec *avcodec_find_decoder(enum AVCodecID id); /**
* Find a registered encoder with a matching codec ID.
*
* @param id AVCodecID of the requested encoder
* @return An encoder if one was found, NULL otherwise.
*/
AVCodec *avcodec_find_encoder(enum AVCodecID id);
AVCodecContex结构体除了包含解码器之外还包含视音频流相关的信息，如宽高，比特率，声道数等信息。一般通过如下函数获得：
/**
* Allocate an AVCodecContext and set its fields to default values. The
* resulting struct should be freed with avcodec_free_context().
*
* @param codec if non-NULL, allocate private data and initialize defaults
* for the given codec. It is illegal to then call avcodec_open2()
* with a different codec.
* If NULL, then the codec-specific defaults won't be initialized,
* which may result in suboptimal default settings (this is
* important mainly for encoders, e.g. libx264).
*
* @return An AVCodecContext filled with default values or NULL on failure.
*/
AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);
通过以上函数获得之后通常要将视音频流的信息赋值过去，使用如下函数：
/**
* Fill the codec context based on the values from the supplied codec
* parameters. Any allocated fields in codec that have a corresponding field in
* par are freed and replaced with duplicates of the corresponding field in par.
* Fields in codec that do not have a counterpart in par are not touched.
*
* @return >= 0 on success, a negative AVERROR code on failure.
*/
int avcodec_parameters_to_context(AVCodecContext *codec,
const AVCodecParameters *par);
其中的参数 AVCodecParameters为编解码器的相关参数，是从AVCodecContext分离出来，其结构体中没有函数。
/**
* This struct describes the properties of an encoded stream.
*
* sizeof(AVCodecParameters) is not a part of the public ABI, this struct must
* be allocated with avcodec_parameters_alloc() and freed with
* avcodec_parameters_free().
*/
typedef struct AVCodecParameters {
/**
* General type of the encoded data.
*/
enum AVMediaType codec_type;
/**
* Specific type of the encoded data (the codec used).
*/
enum AVCodecID codec_id;
/**
* Additional information about the codec (corresponds to the AVI FOURCC).
*/
uint32_t codec_tag; /**
* Extra binary data needed for initializing the decoder, codec-dependent.
*
* Must be allocated with av_malloc() and will be freed by
* avcodec_parameters_free(). The allocated size of extradata must be at
* least extradata_size + AV_INPUT_BUFFER_PADDING_SIZE, with the padding
* bytes zeroed.
*/
uint8_t *extradata;
/**
* Size of the extradata content in bytes.
*/
int extradata_size; /**
* - video: the pixel format, the value corresponds to enum AVPixelFormat.
* - audio: the sample format, the value corresponds to enum AVSampleFormat.
*/
int format; /**
* The average bitrate of the encoded data (in bits per second).
*/
int64_t bit_rate; /**
* The number of bits per sample in the codedwords.
*
* This is basically the bitrate per sample. It is mandatory for a bunch of
* formats to actually decode them. It's the number of bits for one sample in
* the actual coded bitstream.
*
* This could be for example 4 for ADPCM
* For PCM formats this matches bits_per_raw_sample
* Can be 0
*/
int bits_per_coded_sample; /**
* This is the number of valid bits in each output sample. If the
* sample format has more bits, the least significant bits are additional
* padding bits, which are always 0. Use right shifts to reduce the sample
* to its actual size. For example, audio formats with 24 bit samples will
* have bits_per_raw_sample set to 24, and format set to AV_SAMPLE_FMT_S32.
* To get the original sample use "(int32_t)sample >> 8"."
*
* For ADPCM this might be 12 or 16 or similar
* Can be 0
*/
int bits_per_raw_sample; /**
* Codec-specific bitstream restrictions that the stream conforms to.
*/
int profile;
int level; /**
* Video only. The dimensions of the video frame in pixels.
*/
int width;
int height; /**
* Video only. The aspect ratio (width / height) which a single pixel
* should have when displayed.
*
* When the aspect ratio is unknown / undefined, the numerator should be
* set to 0 (the denominator may have any value).
*/
AVRational sample_aspect_ratio; /**
* Video only. The order of the fields in interlaced video.
*/
enum AVFieldOrder field_order; /**
* Video only. Additional colorspace characteristics.
*/
enum AVColorRange color_range;
enum AVColorPrimaries color_primaries;
enum AVColorTransferCharacteristic color_trc;
enum AVColorSpace color_space;
enum AVChromaLocation chroma_location; /**
* Video only. Number of delayed frames.
*/
int video_delay; /**
* Audio only. The channel layout bitmask. May be 0 if the channel layout is
* unknown or unspecified, otherwise the number of bits set must be equal to
* the channels field.
*/
uint64_t channel_layout;
/**
* Audio only. The number of audio channels.
*/
int channels;
/**
* Audio only. The number of audio samples per second.
*/
int sample_rate;
/**
* Audio only. The number of bytes per coded audio frame, required by some
* formats.
*
* Corresponds to nBlockAlign in WAVEFORMATEX.
*/
int block_align;
/**
* Audio only. Audio frame size, if known. Required by some formats to be static.
*/
int frame_size; /**
* Audio only. The amount of padding (in samples) 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.
*/
int initial_padding;
/**
* 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.
*/
int trailing_padding;
/**
* Audio only. Number of samples to skip after a discontinuity.
*/
int seek_preroll;
} AVCodecParameters;
这么做是因为AVCodecContext这个结构体实在是太大了。
我们可以从 AVStream 的 codecpar 成员中直接获得AVCodecParameters。
最后附上参考雷神的链接：
https://blog.csdn.net/leixiaohua1020/article/details/11693997
缅怀