FFMPEG结构体分析：AVCodecParameters

/**
 * 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;

    int bits_per_coded_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;

乍眼一看，AVCodecParameters与AVCodecContext里的参数有很多相同的，但是没有函数。看来伟大的极客们，是想把一些编解码器的参数从AVCodecContext分离出来，因为AVCodecContext这个结构体实在是太大了。

enum AVMediaType codec_type：编解码器的类型（视频，音频...）

enum AVCodecID   codec_id：标示特定的编解码器

int format：像素格式/采样数据格式

int bit_rate：平均比特率

uint8_t *extradata; int extradata_size：针对特定编码器包含的附加信息（例如对于H.264解码器来说，存储SPS，PPS等）

int width, height：如果是视频的话，代表宽和高

int refs：运动估计参考帧的个数（H.264的话会有多帧，MPEG2这类的一般就没有了）

int sample_rate：采样率（音频）

uint64_t channel_layout：声道格式

int channels：声道数（音频）

int profile：型（H.264里面就有，其他编码标准应该也有）

int level：级（和profile差不太多）