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 file
	if(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 buffer
	numBytes=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 disk
	i=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 image
	av_free(buffer);
	av_free(pFrameRGB);

	// Free the YUV frame
	av_free(pFrame);

	// Close the codec
	avcodec_close(pCodecCtx);

	// Close the video file
	av_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！

下载链接：http://download.youkuaiyun.com/detail/yang_xian521/4399108