用FFmpeg5.0 + SDL3 播放视频

上个月看完了讲H.264视频编解码的经典书：“The H.264 Advanced Video Compression Standard”，看完后感觉也不是那么难。记得大约十年前第一次看到预测(prediction), 运动补偿(motion compensation)之类的术语和示意图，感觉是非常晦涩难懂的知识。而现在补了基础信号理论的知识，也扫除了很多畏难心理。

后来了解到比较常用的开源H.264编码器是X264，而FFmpeg则自带一个广泛使用的开源H.264解码器。于是，我就想对照着开源代码，再过一遍理论知识。那就选FFmpeg吧～毕竟名气太大。

首先，得用FFmpeg整一个简单的播放器跑起来才能调试。但是，网络上虽然有大量FFmpeg编写简单播放器的代码，绝大部分代码却是无法用最新的FFmpeg通过编译的。毕竟FFmpeg在持续开发中，连上层API也会有变化。我下载了目前(2022/Feb/13)最新的代码，版本号是5.0。旧FFmpeg播放器代码跑不起来。

好在我在youtube.com上找到一位FFmpeg核心维护者Matt Szatmary做的presentation，An Introduction to Building tools with FFmpeg libraries and APIs – Matt Szatmary | August 2019，结合它的介绍，把简单的播放功能跑起来了。这里有个小遗憾没能找到Matt Szatmary的源代码，虽然他在视频中说是开源的。本来还想学习一下大佬的写法。

代码贴在这里。用这个小DEMO可以用调试器跟到FFmpeg的代码里看H264解码器的具体实现了。

#include <stdio.h>
#define __STDC_CONSTANT_MACROS
// Linux...
#ifdef __cplusplus
extern "C"
{
#endif
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswscale/swscale.h>
#include <libavutil/imgutils.h>
#include <SDL2/SDL.h>
#ifdef __cplusplus
};
#endif

int sfp_refresh_thread(void *opaque);

// Refresh Event
#define SFM_REFRESH_EVENT (SDL_USEREVENT + 1)
#define SFM_BREAK_EVENT (SDL_USEREVENT + 2)
int thread_exit = 0;
int thread_pause = 0;

int sfp_refresh_thread(void *opaque)
{
    SDL_Event event;

    thread_exit = 0;
    thread_pause = 0;
    while (!thread_exit)
    {
        if (!thread_pause)
        {
            SDL_Event event;
            event.type = SFM_REFRESH_EVENT;
            SDL_PushEvent(&event);
        }
        SDL_Delay(40);
    }
    thread_exit = 0;
    thread_pause = 0;
    // Break
    event.type = SFM_BREAK_EVENT;
    SDL_PushEvent(&event);
    return 0;
}

int main(int argc, char *argv[])
{
    AVFormatContext *pFormatCtx;
    int i, videoindex;
    AVCodecParameters *pCodecPar;
    AVCodecContext *pCodecCtx;
    const AVCodec *pCodec;
    AVFrame *pFrame, *pFrameYUV;
    unsigned char *out_buffer;
    AVPacket *packet;
    int ret, got_picture;
    //------------SDL----------------
    int screen_w, screen_h;
    SDL_Window *screen;
    SDL_Renderer *sdlRenderer;
    SDL_Texture *sdlTexture;
    SDL_Rect sdlRect;
    SDL_Thread *video_tid;
    SDL_Event event;
    struct SwsContext *img_convert_ctx;
    char filepath[] = "video";

    // avformat_network_init();
    // pFormatCtx = avformat_alloc_context();
    pFormatCtx = NULL;    // MYNOTE: if pFormatCtx is not allocated explicitly using avformat_alloc_context()
    // pFormatCtx will be allocated implicitly in avformat_open_input.
    if (avformat_open_input(&pFormatCtx, filepath, NULL, NULL) != 0)
    {
        printf("Couldn't open input stream.\n");
        return -1;
    }
    if (avformat_find_stream_info(pFormatCtx, NULL) < 0)
    {
        printf("Couldn't find stream information.\n");
        return -1;
    }
    videoindex = -1;
    for (i = 0; i < pFormatCtx->nb_streams; i++)
        if (pFormatCtx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)
        {
            videoindex = i;
            pCodecPar = pFormatCtx->streams[i]->codecpar;
            break;
        }
    if (videoindex == -1)
    {
        printf("Didn't find a video stream.\n");
        return -1;
    }

    pCodec = avcodec_find_decoder(pCodecPar->codec_id);
    if (pCodec == NULL)
    {
        printf("Codec not found.\n");
        return -1;
    }
    pCodecCtx = avcodec_alloc_context3(pCodec);
    if (!pCodecCtx)
    {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }
    // 这一步不能少，否则会崩溃。把CodecPar的参数再复制到codecCtx中
    ret = avcodec_parameters_to_context(pCodecCtx, pCodecPar);
    if (ret < 0)
    {
        printf("avcodec_parameters_to_context failed...");
        return -1;
    }
    // pCodecCtx = pFormatCtx->streams[videoindex]->codecpar;
    if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0)
    {
        printf("Could not open codec.\n");
        return -1;
    }
    pFrame = av_frame_alloc();
    pFrameYUV = av_frame_alloc();
    size_t sz = av_image_get_buffer_size(AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height, 1);
    out_buffer = (unsigned char *)av_malloc(sz);
    av_image_fill_arrays(pFrameYUV->data, pFrameYUV->linesize, out_buffer,
                         AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height, 1);
    // Output Info-----------------------------
    printf("---------------- File Information ---------------\n");
    av_dump_format(pFormatCtx, 0, filepath, 0);
    printf("-------------------------------------------------\n");
    img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt,
                                     pCodecCtx->width, pCodecCtx->height, AV_PIX_FMT_YUV420P,
                                     SWS_BICUBIC, NULL, NULL, NULL);
    if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO |
                 SDL_INIT_TIMER))
    {
        printf("Could not initialize SDL - %s\n", SDL_GetError());
        return -1;
    }
    // SDL 2.0 Support for multiple windows
    screen_w = pCodecCtx->width;
    screen_h = pCodecCtx->height;
    screen = SDL_CreateWindow("Simplest ffmpeg player's Window",
                              SDL_WINDOWPOS_UNDEFINED,
                              SDL_WINDOWPOS_UNDEFINED,
                              screen_w, screen_h, SDL_WINDOW_OPENGL);
    if (!screen)
    {
        printf("SDL: could not create window - exiting:%s\n", SDL_GetError());
        return -1;
    }
    sdlRenderer = SDL_CreateRenderer(screen, -1, 0);
    // IYUV: Y + U + V (3 planes)
    // YV12: Y + V + U (3 planes)
    sdlTexture = SDL_CreateTexture(sdlRenderer,
                                   SDL_PIXELFORMAT_IYUV,
                                   SDL_TEXTUREACCESS_STREAMING, pCodecCtx->width, pCodecCtx->height);
    sdlRect.x = 0;
    sdlRect.y = 0;
    sdlRect.w = screen_w;
    sdlRect.h = screen_h;
    packet = (AVPacket *)av_malloc(sizeof(AVPacket));
    video_tid = SDL_CreateThread(sfp_refresh_thread, NULL, NULL);
    //------------SDL End------------
    // Event Loop
    for (;;)
    {
        char buf[1024];
        int ret;
        // Wait
        SDL_WaitEvent(&event);
        if (event.type == SFM_REFRESH_EVENT)
        {
            while (1)
            {
                if (av_read_frame(pFormatCtx, packet) < 0)
                    thread_exit = 1;
                if (packet->stream_index == videoindex)
                    break;
            }

            // ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, packet);
            // ret = my_decode();
            ret = avcodec_send_packet(pCodecCtx, packet);
            if (ret < 0)
            {
                fprintf(stderr, "Error sending a packet for decoding\n");
                exit(1);
            }

            while (ret >= 0)
            {
                ret = avcodec_receive_frame(pCodecCtx, pFrame);
                if (ret == AVERROR(EAGAIN))
                {
                    fprintf(stderr, "EAGAIN occurred...\n");
                    break;
                }
                if (ret == AVERROR_EOF)
                {
                    printf("byebye...\n");
                    exit(0);
                }
                if (ret < 0)
                {
                    fprintf(stderr, "Error during decoding\n");
                    exit(1);
                }

                printf("Got frame %3d\n", pCodecCtx->frame_number);
                fflush(stdout);

                /* the picture is allocated by the decoder. no need to
                   free it */
                // snprintf(buf, sizeof(buf), "%s-%d", "player.c", dec_ctx->frame_number);
                // pgm_save(frame->data[0], frame->linesize[0], frame->width, frame->height, buf);

                sws_scale(img_convert_ctx, (const unsigned char *const *)pFrame->data,
                          pFrame->linesize, 0, pCodecCtx->height, pFrameYUV->data, pFrameYUV->linesize);
                // SDL---------------------------
                SDL_UpdateTexture(sdlTexture, NULL, pFrameYUV->data[0],
                                  pFrameYUV->linesize[0]);
                SDL_RenderClear(sdlRenderer);
                SDL_RenderCopy(sdlRenderer, sdlTexture, NULL, NULL);
                SDL_RenderPresent(sdlRenderer);
                // SDL End-----------------------
            }

            // av_free_packet(packet);
            av_packet_unref(packet);
        }
        else if (event.type == SDL_KEYDOWN)
        {
            // Pause
            if (event.key.keysym.sym == SDLK_SPACE)
                thread_pause = !thread_pause;
        }
        else if (event.type == SDL_QUIT)
        {
            thread_exit = 1;
        }
        else if (event.type == SFM_BREAK_EVENT)
        {
            break;
        }
    }
    sws_freeContext(img_convert_ctx);
    SDL_Quit();
    av_frame_free(&pFrameYUV);
    av_frame_free(&pFrame);
    avcodec_free_context(&pCodecCtx);
    avcodec_close(pCodecCtx);
    avformat_close_input(&pFormatCtx);

    return 0;
}