Tutorial 02: Outputting to the Screen

这个例子主要是播放视频的，和tutorial01有很多相似的地方。

这里就不一一叙述了。

// tutorial02.c
// A pedagogical video player that will stream through every video frame as fast as it can.
//
// This tutorial was written by Stephen Dranger ([email protected]).
//
// Code based on FFplay, Copyright (c) 2003 Fabrice Bellard, 
// and a tutorial by Martin Bohme ([email protected])
// Tested on Gentoo, CVS version 5/01/07 compiled with GCC 4.1.1
//
// Use the Makefile to build all examples.
//
// Run using
// tutorial02 myvideofile.mpg
//
// to play the video stream on your screen.


extern "C"
{
#include "libavutil/avstring.h"
#include "libavutil/mathematics.h"
#include "libavutil/pixdesc.h"
#include "libavutil/imgutils.h"
#include "libavutil/dict.h"
#include "libavutil/parseutils.h"
#include "libavutil/samplefmt.h"
#include "libavutil/avassert.h"
#include "libavutil/time.h"
#include "libavformat/avformat.h"
#include "libavdevice/avdevice.h"
#include "libswscale/swscale.h"
#include "libavutil/opt.h"
#include "libavcodec/avfft.h"
#include "libswresample/swresample.h"

#include "SDL1.2/SDL.h"
#include "SDL1.2/SDL_thread.h"
}

#pragma comment(lib, "avcodec.lib")
#pragma comment(lib, "avformat.lib")
#pragma comment(lib, "avutil.lib")
#pragma comment(lib, "avdevice.lib")
#pragma comment(lib, "avfilter.lib")
#pragma comment(lib, "postproc.lib")
#pragma comment(lib, "swresample.lib")
#pragma comment(lib, "swscale.lib")
#pragma comment(lib, "SDL.lib")

#ifdef __MINGW32__
#undef main /* Prevents SDL from overriding main() */
#endif

#include <stdio.h>

int _tmain() {
    AVFormatContext *pFormatCtx = NULL;
    int             i, videoStream;
    AVCodecContext  *pCodecCtx = NULL;
    AVCodec         *pCodec = NULL;
    AVFrame         *pFrame = NULL; 
    AVPacket        packet;
    int             frameFinished;
    //float           aspect_ratio;

    AVDictionary    *optionsDict = NULL;
    struct SwsContext *sws_ctx = NULL;

    SDL_Overlay     *bmp = NULL;
    SDL_Surface     *screen = NULL;
    SDL_Rect        rect;
    SDL_Event       event;

    char szFile[] = "song.flv";

    av_register_all();

    if( SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO | SDL_INIT_TIMER) ) {
        fprintf(stderr, "Could not initialize SDL - %s\n", SDL_GetError());
        exit(1);
    }

    // Open video file
    if( avformat_open_input(&pFormatCtx, szFile, 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, szFile, 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, &optionsDict) < 0 ) {
        return -1; // Could not open codec
    }

    // Allocate video frame
    pFrame = av_frame_alloc();

    // Make a screen to put our video
#ifndef __DARWIN__
    screen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 0, 0);
#else
    screen = SDL_SetVideoMode(pCodecCtx->width, pCodecCtx->height, 24, 0);
#endif
    if( !screen ) {
        fprintf(stderr, "SDL: could not set video mode - exiting\n");
        exit(1);
    }

    // Allocate a place to put our YUV image on that screen
    bmp = SDL_CreateYUVOverlay(pCodecCtx->width,
        pCodecCtx->height,
        SDL_YV12_OVERLAY,
        screen);

    // 将解码得到的视频格式转化为YUV420格式.
    sws_ctx =
        sws_getContext
        (
        pCodecCtx->width,
        pCodecCtx->height,
        pCodecCtx->pix_fmt,
        pCodecCtx->width,
        pCodecCtx->height,
        PIX_FMT_YUV420P,
        SWS_BILINEAR,
        NULL,
        NULL,
        NULL
        );

    // 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 ) {
                SDL_LockYUVOverlay(bmp);

                AVPicture pict;
                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
                sws_scale
                    (
                    sws_ctx, 
                    (uint8_t const * 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);

                SDL_Delay(40);
            }
        }

        // Free the packet that was allocated by av_read_frame
        av_free_packet(&packet);
        SDL_PollEvent(&event);
        switch(event.type) {
        case SDL_QUIT:
            SDL_Quit();
            exit(0);
            break;
        default:
            break;
        }
    }

    // Free the YUV frame
    av_free(pFrame);

    // Close the codec
    avcodec_close(pCodecCtx);

    // Close the video file
    avformat_close_input(&pFormatCtx);

    return 0;
}

只是把有区别的地方说一下。

因为SDL能播放的是YUV420格式，所以需要将解码后得到的视频转码为YUV420格式。

另外每解码，播放一帧，需要暂停40ms，因为若不暂停，那么播放速度是等于解码速度的，视频会迅速播放完。