FFmpeg常用结构体分析：AVCodecParameters

原创链接

AVSream中我们可以使用codec得到AVCodecContext指针，该结构体代表了AVStream中持有的codec相关的上下文，包含了众多编解码器需要的参数信息。一个AVStream对应一个AVCodecContext指针。但是，如果你使用较高的FFmpeg版本就应该发现，codec成员已经不受支持，也就是在AVStream中该成员已经被废弃，而且，AVCodecContext由于太过复杂，FFmpeg的维护者也开始逐渐拆分这个结构体的功能，准备废弃AVCodecContext。目前，官方推荐使用AVCodecParameters这个结构体来替代它。

注意：
FFmpeg3.3中 AVCodecContext *codec;被舍弃，用 AVCodecParameters *codecpar;代替。

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。

其中，我们常用到的成员有：

//(1)编解码器类型，音频/视频/字幕
enum AVMediaType codec_type;
//其中，codec_type定义如下：
enum AVMediaType {
    AVMEDIA_TYPE_UNKNOWN = -1,  ///< Usually treated as AVMEDIA_TYPE_DATA
    AVMEDIA_TYPE_VIDEO,
    AVMEDIA_TYPE_AUDIO,
    AVMEDIA_TYPE_DATA,          ///< Opaque data information usually continuous
    AVMEDIA_TYPE_SUBTITLE,
    AVMEDIA_TYPE_ATTACHMENT,    ///< Opaque data information usually sparse
    AVMEDIA_TYPE_NB
};

//(2)像素格式(YUV420等)/音频采样格式
int format;

//(3)编解码器的id
//使用函数avcodec_get_name(codec_id)可以获得视频/音频编码格式
enum AVCodecID codec_id;

//(4)平均码率/比特率
int64_t bit_rate;

//(5)视频宽度/高度
int width;
int height;

//(6) 音频通道数
int sample_rate;

//(7)音频帧率
int frame_size;

对于AVCodecParameters结构体，其定义如下：

/**
 * 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;