以 MPEG-1 为例的视频解码例子，不含封装，只解析裸流

流程：

1. 视频数据包初始化，av_packet_alloc

2. 查找 MPEG-1 解码器， avcodec_find_decoder

3. 数据解析器初始化，av_parser_init

4. 解码结构初始化，avcodec_alloc_context3

5. 打开解码器，avcodec_open2

6. 打开视频文件，fopen

7. 解码视频帧初始化，av_frame_alloc

8. 循环
   读文件，fread
   解析数据包，av_parser_parse2
   解码视频帧，avcodec_send_packet，avcodec_receive_frame
   写文件（PGM图，p5），fopen，fwrite，fclose

9. 关闭视频文件，fclose

10. 关闭解析器，av_parser_close

11. 释放解码器，avcodec_free_context

12. 释放视频帧，av_frame_free

13. 释放视频数据包，av_packet_free

升级方案：

1. avcodec_find_decoder 可以调用其他解码器，解析其他裸流，如：h264、h265等
2. 解码后的输出帧，可以按 yuv 分量保存成 yuv 视频文件

//Y, U, V (YUV420P)
FILE *fp = fopen(filepath_out, "wba");
for(int i=0;i<frame->height;i++){fwrite(frame->data[0]+pFrame->linesize[0]*i,1,frame->width,fp);
}
for(int i=0;i<frame->height/2;i++){fwrite(frame->data[1]+frame->linesize[1]*i,1,frame->width/2,fp);
}
for(int i=0;i<frame->height/2;i++){fwrite(frame->data[2]+frame->linesize[2]*i,1,frame->width/2,fp);
}
flcose(fp);

decode_video.c

/*** @file* video decoding with libavcodec API example** @example decode_video.c*/#include <stdio.h>
#include <stdlib.h>
#include <string.h>#include <libavcodec/avcodec.h>#define INBUF_SIZE 4096static void pgm_save(unsigned char *buf, int wrap, int xsize, int ysize,char *filename)
{FILE *f;int i;f = fopen(filename,"w");fprintf(f, "P5\n%d %d\n%d\n", xsize, ysize, 255);for (i = 0; i < ysize; i++)fwrite(buf + i * wrap, 1, xsize, f);fclose(f);
}static void decode(AVCodecContext *dec_ctx, AVFrame *frame, AVPacket *pkt,const char *filename)
{char buf[1024];int ret;ret = avcodec_send_packet(dec_ctx, pkt);if (ret < 0) {fprintf(stderr, "Error sending a packet for decoding\n");exit(1);}while (ret >= 0) {ret = avcodec_receive_frame(dec_ctx, frame);if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)return;else if (ret < 0) {fprintf(stderr, "Error during decoding\n");exit(1);}printf("saving frame %3d\n", dec_ctx->frame_number);fflush(stdout);/* the picture is allocated by the decoder. no need tofree it */snprintf(buf, sizeof(buf), "%s-%d", filename, dec_ctx->frame_number);pgm_save(frame->data[0], frame->linesize[0],frame->width, frame->height, buf);}
}int main(int argc, char **argv)
{const char *filename, *outfilename;const AVCodec *codec;AVCodecParserContext *parser;AVCodecContext *c= NULL;FILE *f;AVFrame *frame;uint8_t inbuf[INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];uint8_t *data;size_t   data_size;int ret;AVPacket *pkt;if (argc <= 2) {fprintf(stderr, "Usage: %s <input file> <output file>\n""And check your input file is encoded by mpeg1video please.\n", argv[0]);exit(0);}filename    = argv[1];outfilename = argv[2];pkt = av_packet_alloc();if (!pkt)exit(1);/* set end of buffer to 0 (this ensures that no overreading happens for damaged MPEG streams) */memset(inbuf + INBUF_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);/* find the MPEG-1 video decoder */codec = avcodec_find_decoder(AV_CODEC_ID_MPEG1VIDEO);if (!codec) {fprintf(stderr, "Codec not found\n");exit(1);}parser = av_parser_init(codec->id);if (!parser) {fprintf(stderr, "parser not found\n");exit(1);}c = avcodec_alloc_context3(codec);if (!c) {fprintf(stderr, "Could not allocate video codec context\n");exit(1);}/* For some codecs, such as msmpeg4 and mpeg4, width and heightMUST be initialized there because this information is notavailable in the bitstream. *//* open it */if (avcodec_open2(c, codec, NULL) < 0) {fprintf(stderr, "Could not open codec\n");exit(1);}f = fopen(filename, "rb");if (!f) {fprintf(stderr, "Could not open %s\n", filename);exit(1);}frame = av_frame_alloc();if (!frame) {fprintf(stderr, "Could not allocate video frame\n");exit(1);}while (!feof(f)) {/* read raw data from the input file */data_size = fread(inbuf, 1, INBUF_SIZE, f);if (!data_size)break;/* use the parser to split the data into frames */data = inbuf;while (data_size > 0) {ret = av_parser_parse2(parser, c, &pkt->data, &pkt->size,data, data_size, AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);if (ret < 0) {fprintf(stderr, "Error while parsing\n");exit(1);}data      += ret;data_size -= ret;if (pkt->size)decode(c, frame, pkt, outfilename);}}/* flush the decoder */decode(c, frame, NULL, outfilename);fclose(f);av_parser_close(parser);avcodec_free_context(&c);av_frame_free(&frame);av_packet_free(&pkt);return 0;
}