FFmpeg入门(4)－An ffmpeg and SDL Tutorial 2

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An ffmpeg and SDL Tutorial 2 中将1中的保存图片改为用SDL显示，在显示之前要讲本地帧改为YUV420P格式。

对照1把2 的代码进行相应修改得到：

#include <libavcodec/avcodec.h>#include <libavformat/avformat.h>#include <libswscale/swscale.h>#include <SDL/SDL.h>#include <SDL/SDL_thread.h>#include <unistd.h>//slqstatic AVFormatContext *pFormatCtx = NULL;int main(int argc, char *argv[]) {//    AVFormatContext *pFormatCtx;//    static AVFormatContext *pFormatCtx = NULL;    AVCodecContext *pCodecCtx;    AVCodec *pCodec;    AVFrame *pFrame;    AVFrame *pFrameRGB;    AVPacket packet;    SDL_Surface *screen;    SDL_Overlay *bmp;    uint8_t *buffer;    int numBytes;    int i;    av_register_all();    //slq//reads the file header and stores information about the file format in the AVFormatContext structure pFormatCtx    if (avformat_open_input( &pFormatCtx, argv[1], NULL, NULL) < 0)    //if (avformat_open_input( &input_fmt_ctx, input_file, NULL, NULL) < 0) {    //if (av_open_input_file(&pFormatCtx, argv[1], NULL, 0, NULL) != 0)        return -1;// Retrieve stream information    if (av_find_stream_info(pFormatCtx) < 0)        return -1;// Dump information about file onto standard error    //    dump_format(pFormatCtx, 0, argv[1], 0);    av_dump_format(pFormatCtx, 0, argv[1], 0);// Find the first video stream    int videoStream = -1;    for (i = 0; i < pFormatCtx->nb_streams; ++i) {        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO) {            videoStream = i;            break;        }    }// 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");    }    // Open codec    //slq    if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0)    //if (avcodec_open(pCodecCtx, pCodec) < 0)        return -1;    // Allocate video frame (native format)    pFrame = avcodec_alloc_frame();    // Allocate an AVFrame structure ( RGB )    pFrameRGB = avcodec_alloc_frame();    if (pFrameRGB == NULL)        return -1;    // we still need a place to put the raw data when we convert it.    //We use avpicture_get_size to get the size we need,    //and allocate the space manually:    // Determine required buffer size and allocate buffer    numBytes = avpicture_get_size(PIX_FMT_RGB24, pCodecCtx->width,            pCodecCtx->height);    //av_malloc is ffmpeg's malloc that is just a simple wrapper around malloc that makes sure the memory addresses are aligned and such.    buffer = (uint8_t *) av_malloc(numBytes * sizeof(uint8_t));    // Assign appropriate parts of buffer to image planes n. 图像平面；映像平面 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);    /*     * SDL     */    //SDL_Init() essentially tells the library what features we're going to use    if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER)) {        fprintf(stderr, "Could not initialize SDL - %s\n", SDL_GetError());        exit(1);    }    //This sets up a screen with the given width and height.    //The next option is the bit depth of the screen - 0 is a special value that means "same as the current display".    screen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 0, 0);    if (!screen) {        fprintf(stderr, "SDL: could not set video mode - exiting\n");        exit(1);    }    //using YV12 to display the image    bmp = SDL_CreateYUVOverlay(pCodecCtx->width, pCodecCtx->height,            SDL_YV12_OVERLAY, screen);    i = 0;    while (av_read_frame(pFormatCtx, &packet) >= 0) {        // Is this a packet from the video stream?        if (packet.stream_index == videoStream) {            int frameFinished;            /*             avcodec_decode_video(pCodecCtx, pFrame, &frameFinished, packet.data, packet.size);             */            avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);            // Did we get a video frame?            if (frameFinished) {#ifdef SAVEFILE                /*保存到文件*/                /*                 *此函数已经不用                 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, 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);                if(++i <= 5)                SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);#else                /*显示 */                AVPicture pict;                static struct SwsContext *img_convert_ctx;                SDL_Rect rect;                SDL_LockYUVOverlay(bmp);                pict.data[0] = bmp->pixels[0];                pict.data[1] = bmp->pixels[2];                pict.data[2] = bmp->pixels[1];                pict.linesize[0] = bmp->pitches[0];                pict.linesize[1] = bmp->pitches[2];                pict.linesize[2] = bmp->pitches[1];                /*                 // Convert the image into YUV format that SDL uses                 img_convert(&pict, PIX_FMT_YUV420P, (AVPicture *) pFrame,                 pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height);                 */                img_convert_ctx = sws_getContext(pCodecCtx->width,                        pCodecCtx->height, pCodecCtx->pix_fmt,                        pCodecCtx->width, pCodecCtx->height, PIX_FMT_YUV420P,                        SWS_BICUBIC, NULL, NULL, NULL);                sws_scale(img_convert_ctx,                        (const uint8_t* const *) pFrame->data,                        pFrame->linesize, 0, pCodecCtx->height, pict.data,                        pict.linesize);                SDL_UnlockYUVOverlay(bmp);                rect.x = 0;                rect.y = 0;                rect.w = pCodecCtx->width;                rect.h = pCodecCtx->height;                SDL_DisplayYUVOverlay(bmp, &rect);#endif                usleep(25000);            }        }        // Free the packet that was allocated by av_read_frame        av_free_packet(&packet);    }    av_free(buffer);    av_free(pFrameRGB);    av_free(pFrame);    avcodec_close(pCodecCtx);    av_close_input_file(pFormatCtx);    return 0;}

程序２编译　运行

gcc -o tutorial02 tutorial02.1.c -lavutil -lavformat -lavcodec -lz -lm `sdl-config --cflags --libs`

gcc -g -o tutorial02 tutorial02.1.c -lavutil -lavformat -lavcodec -lswscale -lz -lm -lpthread `sdl-config --cflags --libs`

YUV420P格式分析

http://my.oschina.net/u/589963/blog/167766

YUV格式通常有两大类：打包（packed）格式和平面（planar）格式。前者将YUV分量存放在同一个数组中，通常是几个相邻的像素组成一个宏像素（macro-pixel）；而后者使用三个数组分开存放YUV三个分量

YUV420P(planar格式)在ffmpeg中存储是在struct AVFrame的data[]数组中

data[0]-------Y分量 data[1]------U分量 data[2]-------V分量

YUV420P的内存结构:

4个Y分量对应1个UV分量

YUV420格式是指，每个像素都保留一个Y（亮度）分量，而在水平方向上，不是每行都取U和V分量，而是一行只取U分量，则其接着一行就只取V分量，以此重复(即4:2:0, 4:0:2, 4:2:0, 4:0:2 .......)，所以420不是指没有V，而是指一行采样只取U，另一行采样只取V。在取U和V时，每两个Y之间取一个U或V。但从4x4矩阵列来看，每4个矩阵点Y区域中，只有一个U和V，所以它们的比值是4:1。所以对于一个像素，RGB需要8 * 3 = 24位，即占3个字节；而YUV420P，8 + 8/4 + 8/4 = 12位，即占2个字节，其中8指Y分量，8/4指U和V分量。

YUV420内存格式:

YYYYYYYY UU VV ------最常见的

YYYYYYYY UU VV ------也有，不过我开发中没遇到过

我们用ffmpeg把yuv420P 存储到文件中，然后利用一些yuv播放器就可以播放YUV原始数据，貌似VLC不能播放YUV原始数据:

data[]中存放着YUV原始数据(在struct AVFrame中的，位于frame.h中----ffmpeg)

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