/**
* 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與AVCodecContext里的參數有很多相同的，但是沒有函數。看來偉大的極客們，是想把一些編解碼器的參數從AVCodecContext分離出來，因為AVCodecContext這個結構體實在是太大了。
enum AVMediaType codec_type：編解碼器的類型（視頻，音頻...）
enum AVCodecID codec_id：標示特定的編解碼器
int format：像素格式/采樣數據格式
int bit_rate：平均比特率
uint8_t *extradata; int extradata_size：針對特定編碼器包含的附加信息（例如對於H.264解碼器來說，存儲SPS，PPS等）
int width, height：如果是視頻的話，代表寬和高
int refs：運動估計參考幀的個數（H.264的話會有多幀，MPEG2這類的一般就沒有了）
int sample_rate：采樣率（音頻）
uint64_t channel_layout：聲道格式
int channels：聲道數（音頻）
int profile：型（H.264里面就有，其他編碼標准應該也有）
int level：級（和profile差不太多）