ffmpeg学习---1Making Screencaps

     想根据教程An ffmpeg and SDL Tutorial  学习音视频解码，这个作为入门教程实在是太爽了，唯一有点不好的是，这个教程太老了使用现在的ffmpeg1.0的库是编译不过的（不过SDL变化比较小）。所以修改代码使之符合现有的库，这样印象比较深刻。目前可以下到的ffmpeg的最新版本是ffmpeg-1.0，SDL的最新版本是SDL-1.2.15.tar.gz，在ubuntu 12.04上进行开发。

 一、环境搭建

    下载ffmpeg-1.0.tar.bz2与SDL-1.2.15.tar.gz，并分别解压到/home/sun/code/目录下

     ffmpeg       --> 源码: /home/sun/code/ffmpeg-1.0   --> 安装: /home/sun/code/ffmpeg-1.0/install

     SDL-1.2.15 -->源码: /home/sun/code/SDL-1.2.15  --> 安装: /home/sun/code/SDL-1.2.15/install 

1.1 编译

SDL编译：

sun@ubuntu:~/code/SDL-1.2.15$ ./configure --prefix=/home/sun/code/SDL-1.2.15/install
sun@ubuntu:~/code/SDL-1.2.15$ make && make install

ffmpeg编译：

sun@ubuntu:~/code/ffmpeg-1.0$ ./configure --prefix=/home/sun/code/ffmpeg-1.0/install --enable-memalign-hack --enable-shared --disable-yasm  ；
sun@ubuntu:~/code/ffmpeg-1.0$ make && make install
如果想在ffmpeg编译的同时也编译出ffplay，需要先安装SDL库

注意:av_read_frame读出的完整的帧，不是包，在函数内部已经保证了帧的完整性。对于视频，av_read_frame读出的就是完整的一帧视频，不会是半帧或多帧；对于音频，av_read_frame读出的可能是多帧，但也是完整的，不存在半帧的情况。
[参考]

二、Tutorial 01: Making Screencaps

1. 1screencaps.c

// tutorial01.c
// Code based on a tutorial by Martin Bohme ([e-mail])
// Tested on Gentoo, CVS version 5/01/07 compiled with GCC 4.1.1

// A small sample program that shows how to use libavformat and libavcodec to
// read video from a file.
//
// Use
//
// gcc -o tutorial01 tutorial01.c -lavformat -lavcodec -lz
//
// to build (assuming libavformat and libavcodec are correctly installed
// your system).
//
// Run using
//
// tutorial01 myvideofile.mpg
//
// to write the first five frames from "myvideofile.mpg" to disk in PPM
// format.
#include <stdio.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>

void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame) {
    FILE *pFile;
    char szFilename[32];
    int y;

    // Open file
    sprintf(szFilename, "frame%d.ppm", iFrame);
    pFile=fopen(szFilename, "wb");
    if(pFile==NULL)
        return;

    // Write header
    fprintf(pFile, "P6\n%d %d\n255\n", width, height);

    // Write pixel data
    for(y=0; y<height; y++)
        fwrite(pFrame->data[0]+y*pFrame->linesize[0], 1, width*3, pFile);

    // Close file
    fclose(pFile);
}

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;

    if(argc < 2) {
        printf("Please provide a movie file\n");
        return -1;
    }
    // Register all formats and codecs
    av_register_all();

    pFormatCtx = avformat_alloc_context();
    // Open video file
    //if(av_open_input_file(&pFormatCtx, argv[1], NULL, 0, NULL)!=0)
    if(avformat_open_input(&pFormatCtx, argv[1], NULL, NULL)!=0)
        return -1; // Couldn't open file

    // Retrieve stream information
    if(avformat_find_stream_info(pFormatCtx,NULL)<0)
        return -1; // Couldn't find stream information

    // Dump information about file onto standard error
    av_dump_format(pFormatCtx, 0, argv[1], 0);

    // Find the first video stream
    videoStream=-1;
    for(i=0; i<pFormatCtx->nb_streams; i++)
        if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_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
    pCodecCtx=pFormatCtx->streams[videoStream]->codec;

    // Find the decoder for the video stream
    pCodec=avcodec_find_decoder(pCodecCtx->codec_id);
    if(pCodec==NULL) {
        fprintf(stderr, "Unsupported codec!\n");
        return -1; // Codec not found
    }
    // Open codec
    if(avcodec_open2(pCodecCtx, pCodec, NULL)<0)
        return -1; // Could not open codec

    // Allocate video frame
    pFrame=avcodec_alloc_frame();

    // Allocate an AVFrame structure
    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));

    // Assign appropriate parts of buffer to image planes in pFrameRGB
    // Note that pFrameRGB is an AVFrame, but AVFrame is a superset
    // of AVPicture
    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) {
        // Is this a packet from the video stream?
        if(packet.stream_index==videoStream) {
            // Decode video frame
            avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);

            // 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);
                static struct SwsContext * img_convert_ctx;
                //img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height, 聽PIX_FMT_RGB24, SWS_BICUBIC, NULL, NULL, NULL);
                //sws_scale(img_convert_ctx, (const uint8_t* const*)pFrame->data, pFrame->linesize, 聽0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);

                img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height, PIX_FMT_RGB24, SWS_BICUBIC, NULL, NULL, NULL);
                sws_scale(img_convert_ctx, (const uint8_t* const*)pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);
                // Save the frame to disk
                if(++i<=5)
                    SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height,
                            i);
            }
        }

        // Free the packet that was allocated by av_read_frame
        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
    avformat_close_input(&pFormatCtx);

    return 0;
}

2. Makefile

CC=gcc
CFLAGS=-g -I/home/sun/code/ffmpeg-1.0/install/include
LDFLAGS = -L/home/sun/code/ffmpeg-1.0/install/lib/ -lavutil -lavformat -lavcodec -lavutil -lm -lswscale
TARGETS=1screencaps
all: $(TARGETS)
1screencaps:1screencaps.c
    $(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)

clean:
    rm -rf $(TARGETS)

3. 要想运行还需要把运行时库的搜索路径加上去

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/home/sun/code/ffmpeg-1.0/install/lib/:/home/sun/code/SDL-1.2.15/install/lib