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！