ffmpeg 之avcodec_find_decoder_by_name分析

布满荆棘的人生 2023-01-15 02:25 1阅读 0赞

avcodec_find_decoder_by_name（“h264_cuvid”）；
       std::string name{ };
        if (id == AV_CODEC_ID_H264)
          name = ((m_gpu < 0) ? "h264" : "h264_cuvid");
        else if (id == AV_CODEC_ID_HEVC)
          name = ((m_gpu < 0) ? "hevc" : "hevc_cuvid");

从这里开始分析：  
libavcodec codec.h

/** * Find a registered decoder with the specified name. * * @param name name of the requested decoder * @return A decoder if one was found, NULL otherwise. */
    AVCodec *avcodec_find_decoder_by_name(const char *name);
    
    AVCodec *avcodec_find_decoder_by_name(const char *name)
    { 
        return find_codec_by_name(name, av_codec_is_decoder);
    }
    
    int av_codec_is_decoder(const AVCodec *codec)
    { 
        return codec && (codec->decode || codec->receive_frame);
    }
        /** * Encode data to an AVPacket. * * @param avctx codec context * @param avpkt output AVPacket (may contain a user-provided buffer) * @param[in] frame AVFrame containing the raw data to be encoded * @param[out] got_packet_ptr encoder sets to 0 or 1 to indicate that a * non-empty packet was returned in avpkt. * @return 0 on success, negative error code on failure */
    int (*decode)(struct AVCodecContext *, void *outdata, int *outdata_size, struct AVPacket *avpkt);
        /** * Decode API with decoupled packet/frame dataflow. This function is called * to get one output frame. It should call ff_decode_get_packet() to obtain * input data. */
    int (*receive_frame)(struct AVCodecContext *avctx, struct AVFrame *frame);
    
    static AVCodec *find_codec_by_name(const char *name, int (*x)(const AVCodec *))
    { 
        void *i = 0;
        const AVCodec *p;
    
        if (!name)
            return NULL;
    
        while ((p = av_codec_iterate(&i))) { 
            if (!x(p))
                continue;
                //对比字符串,p就是返回的解码器
            if (strcmp(name, p->name) == 0)
                return (AVCodec*)p;
        }
    
        return NULL;
    }
    一部分解码器：
    extern AVCodec ff_h264_cuvid_decoder;
    extern AVCodec ff_h264_mf_encoder;
    extern AVCodec ff_h264_nvenc_encoder;
    extern AVCodec ff_hevc_cuvid_decoder;
    extern AVCodec ff_hevc_mediacodec_decoder;
    extern AVCodec ff_hevc_mf_encoder;
    extern AVCodec ff_hevc_nvenc_encoder;
    
    
    static AVOnce av_codec_static_init = AV_ONCE_INIT;
    static void av_codec_init_static(void)
    { 
        for (int i = 0; codec_list[i]; i++) { 
            if (codec_list[i]->init_static_data)
            //这里我们执行具体的某一个解码器的函数，比如h264
                codec_list[i]->init_static_data((AVCodec*)codec_list[i]);
        }
    }
    const AVCodec *av_codec_iterate(void **opaque)
    { 
        uintptr_t i = (uintptr_t)*opaque;
        const AVCodec *c = codec_list[i];
    
        ff_thread_once(&av_codec_static_init, av_codec_init_static);
    
        if (c)
            *opaque = (void*)(i + 1);
    
        return c;
    }
    
    
    AVCodec ff_h264_decoder = { 
        .name                  = "h264",
        .long_name             = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
        .type                  = AVMEDIA_TYPE_VIDEO,
        .id                    = AV_CODEC_ID_H264,
        .priv_data_size        = sizeof(H264Context),
        .init                  = h264_decode_init,
        .close                 = h264_decode_end,
        .decode                = h264_decode_frame,
        .capabilities          = /*AV_CODEC_CAP_DRAW_HORIZ_BAND |*/ AV_CODEC_CAP_DR1 |
                                 AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS |
                                 AV_CODEC_CAP_FRAME_THREADS,
        .hw_configs            = (const AVCodecHWConfigInternal*[]) 
    .....
    }

接下来看一下  
avcodec\_find\_decoder（）

AVCodec *avcodec_find_decoder(enum AVCodecID id)
    { 
        return find_codec(id, av_codec_is_decoder);
    }
    
    
    static AVCodec *find_codec(enum AVCodecID id, int (*x)(const AVCodec *))
    { 
        const AVCodec *p, *experimental = NULL;
        void *i = 0;
    
        id = remap_deprecated_codec_id(id);
    
        while ((p = av_codec_iterate(&i))) { 
            if (!x(p))
                continue;
                //这里跟上面一样
            if (p->id == id) { 
                if (p->capabilities & AV_CODEC_CAP_EXPERIMENTAL && !experimental) { 
                    experimental = p;
                } else
                    return (AVCodec*)p;
            }
        }
    
        return (AVCodec*)experimental;
    }