FFmpeg avcodec_parameters_to_context函数剖析

函数说明
    将AVCodecParameters结构体中码流参数拷贝到AVCodecContext结构体中，
并且重新拷贝一份extradata内容，涉及到的视频的关键参数有
format, width, height, codec_type等，这些参数在优化avformat_find_stream_info
函数的时候，手动指定该参数通过InitDecoder函数
解码统一指定H264，分辨率是1920*1080

int avcodec_parameters_to_context(AVCodecContext *codec,
                                  const AVCodecParameters *par)
{
    codec->codec_type = par->codec_type;//AVMEDIA_TYPE_VIDEO
    codec->codec_id   = par->codec_id;//AV_CODEC_ID_H264
    codec->codec_tag  = par->codec_tag;//0

codec->bit_rate              = par->bit_rate;//0
    codec->bits_per_coded_sample = par->bits_per_coded_sample;//0
    codec->bits_per_raw_sample   = par->bits_per_raw_sample;//8
    codec->profile               = par->profile;//66
    codec->level                 = par->level;//42

switch (par->codec_type) {
    case AVMEDIA_TYPE_VIDEO:
        codec->pix_fmt                = par->format;//12
        codec->width                  = par->width;//1920
        codec->height                 = par->height;//1080
        codec->field_order            = par->field_order;//AV_FIELD_PROGRESSIVE
        codec->color_range            = par->color_range;//AVCOL_RANGE_JPEG
        codec->color_primaries        = par->color_primaries;//AVCOL_PRI_BT709
        codec->color_trc              = par->color_trc;//AVCOL_TRC_BT709
        codec->colorspace             = par->color_space;//AVCOL_SPC_BT709
        codec->chroma_sample_location = par->chroma_location;//AVCHROMA_LOC_LEFT
        codec->sample_aspect_ratio    = par->sample_aspect_ratio;//num=0,den=1
        codec->has_b_frames           = par->video_delay;//0
        break;
    case AVMEDIA_TYPE_AUDIO:
        codec->sample_fmt       = par->format;
        codec->channel_layout   = par->channel_layout;
        codec->channels         = par->channels;
        codec->sample_rate      = par->sample_rate;
        codec->block_align      = par->block_align;
        codec->frame_size       = par->frame_size;
        codec->delay            =
        codec->initial_padding  = par->initial_padding;
        codec->trailing_padding = par->trailing_padding;
        codec->seek_preroll     = par->seek_preroll;
        break;
    case AVMEDIA_TYPE_SUBTITLE:
        codec->width  = par->width;
        codec->height = par->height;
        break;
    }

//extradata保存SPS/PPS信息
    if (par->extradata) {
        av_freep(&codec->extradata);
        codec->extradata = av_mallocz(par->extradata_size + AV_INPUT_BUFFER_PADDING_SIZE);
        if (!codec->extradata)
            return AVERROR(ENOMEM);
        memcpy(codec->extradata, par->extradata, par->extradata_size);
        codec->extradata_size = par->extradata_size;
    }

return 0;
}

说明：
当手动指定编码格式的时候，说明探测码流格式的代码是可以省略的，例如
avformat_open_input
avformat_find_stream_info

avformat_open_input缺省情况下，无法播放图像

原文地址：http://blog.51cto.com/fengyuzaitu/2059121