FFmpeg浅尝辄止（三）——小试视频解码
上一篇介绍了视频编码的小例子，视频解码跟编码差不多，只是要在视频文件中寻找视频流，找到后对流逐帧解码，就这样简单。闲言少叙，上code：
int main(int argc, char *argv[]){AVFormatContext *pFormatCtx;int i, videoStream;AVCodecContext *pCodecCtx;AVCodec *pCodec;AVFrame *pFrame; AVFrame *pFrameRGB;AVPacket packet;int frameFinished;int numBytes;uint8_t *buffer;struct SwsContext *img_convert_ctx;if(argc < 2){printf("Please provide a movie file\n");return -1;}// Register all formats and codecs// 初始化ffmpeg库av_register_all();// Open video fileif(av_open_input_file(&pFormatCtx, argv[1], NULL, 0, NULL)!=0)return -1; // Couldn't open file// Retrieve stream information// 查找文件的流信息if(av_find_stream_info(pFormatCtx)<0)return -1; // Couldn't find stream information// Dump information about file onto standard error// dump只是一个调试函数，输出文件的音、视频流的基本信息：帧率、分辨率、音频采样等等dump_format(pFormatCtx, 0, argv[1], 0);// Find the first video stream// 遍历文件的流，找到第一个视频流，并记录流的编码信息videoStream=-1;for(i=0; i<pFormatCtx->nb_streams; i++){if(pFormatCtx->streams[i]->codec->codec_type==CODEC_TYPE_VIDEO){videoStream=i;break;}}if(videoStream==-1)return -1; // Didn't find a video stream// Get a pointer to the codec context for the video stream// 得到视频流编码的上下文指针pCodecCtx=pFormatCtx->streams[videoStream]->codec;// construct the scale context, conversing to PIX_FMT_RGB24// 根据编码信息设置渲染格式img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height, PIX_FMT_RGB24, SWS_BICUBIC, NULL, NULL, NULL);if(img_convert_ctx == NULL){fprintf(stderr, "Cannot initialize the conversion context!\n");// exit(1);return -1;}// Find the decoder for the video stream// 在库里面查找支持该格式的解码器pCodec=avcodec_find_decoder(pCodecCtx->codec_id);if(pCodec==NULL){fprintf(stderr, "Unsupported codec!\n");return -1; // Codec not found}// Open codec// 打开解码器if(avcodec_open(pCodecCtx, pCodec)<0)return -1; // Could not open codec// Allocate video frame// 分配一个帧指针，指向解码后的原始帧pFrame=avcodec_alloc_frame();// Allocate an AVFrame structure// 分配一个帧指针，指向存放转换成rgb后的帧pFrameRGB=avcodec_alloc_frame();if(pFrameRGB==NULL)return -1;// Determine required buffer size and allocate buffernumBytes=avpicture_get_size(PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height);buffer=(uint8_t *)av_malloc(numBytes*sizeof(uint8_t));// buffer = new uint8_t[numBytes];// Assign appropriate parts of buffer to image planes in pFrameRGB// Note that pFrameRGB is an AVFrame, but AVFrame is a superset// of AVPicture// 给pFrameRGB帧附加上分配的内存avpicture_fill((AVPicture *)pFrameRGB, buffer, PIX_FMT_RGB24, pCodecCtx->width, pCodecCtx->height);// Read frames and save first five frames to diski=0;while(av_read_frame(pFormatCtx, &packet)>=0)// 读取一个帧{// Is this a packet from the video stream?if(packet.stream_index==videoStream){// Decode video frame// 解码该帧avcodec_decode_video(pCodecCtx, pFrame, &frameFinished, packet.data, packet.size);// Did we get a video frame?if(frameFinished){// Convert the image from its native format to RGB// img_convert((AVPicture *)pFrameRGB, PIX_FMT_RGB24, // (AVPicture*)pFrame, pCodecCtx->pix_fmt, pCodecCtx->width, // pCodecCtx->height);// 把该帧转换成rgb// 如果只提取关键帧，加上这句// if (pFrame->key_frame == 1)sws_scale(img_convert_ctx, pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);// Save the frame to disk// 保存前5帧if(++i<=5){// char pic[200];// sprintf(pic,"pic%d.bmp",i);// av_create_bmp(pic, pFrameRGB->data[0], pCodecCtx->width, pCodecCtx->height, 24);SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);}}}// Free the packet that was allocated by av_read_frame// 释放读取的帧内存av_free_packet(&packet);}// Free the RGB imageav_free(buffer);av_free(pFrameRGB);// Free the YUV frameav_free(pFrame);// Close the codecavcodec_close(pCodecCtx);// Close the video fileav_close_input_file(pFormatCtx);return 0;}除了saveFrame函数都贴上来啦。感觉注释应该够了，不用我多费口舌了，so easy！
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