(ffmpeg) AVIを読み込んで先頭の5フレームをGray画像として取り出すコード

gistfile1.cpp

extern "C" { 
#include <libavcodec/avcodec.h> 
#include <libavformat/avformat.h> 
#include <libswscale/swscale.h> 
}

int get_first_video_stream_index(AVFormatContext* format_context)
{
    for (int i = 0; i < format_context->nb_streams; ++i) {
        if (format_context->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
            return i;
    }
    return -1;
}

AVCodec* get_decoder(AVStream* stream)
{
    AVCodecContext* codec_context = stream->codec;
    AVCodec* codec = avcodec_find_decoder(codec_context->codec_id);
    if (codec == nullptr)
        return nullptr;
    if (avcodec_open(codec_context, codec) < 0)
        return nullptr;
    return codec;
}

void save_frame(AVFrame* frame, int width, int height, int frameno)
{
    char filename[1024];
    sprintf(filename, "frame_%03d.pgm", frameno);

    FILE* file = fopen(filename, "wb");
    if (file == NULL) return;

    fprintf(file, "P5\n%d %d\n255\n", width, height);

    for (int y = 0; y < height; ++y)
        fwrite(frame->data[0] + y * frame->linesize[0], 1, width, file);

    fclose(file);
}

int main(int argc, char** argv)
{
    av_register_all();

    AVFormatContext* format_context = nullptr;
    if (avformat_open_input(&format_context, "sample.avi", nullptr, nullptr) != 0)
        return -1;

    int video_stream_index = get_first_video_stream_index(format_context);
    if (video_stream_index == -1)
        return -1;
    AVStream* video_stream = format_context->streams[video_stream_index];
    AVCodecContext* codec_context = video_stream->codec;
    AVCodec* codec = get_decoder(video_stream);

    AVFrame* frame = avcodec_alloc_frame();
    AVFrame* frame_rgb = avcodec_alloc_frame();
    int width = codec_context->width, height = codec_context->height;

    int num_bytes = avpicture_get_size(PIX_FMT_RGB24, width, height);
    uint8_t* buffer = static_cast<uint8_t*>(av_malloc(num_bytes * sizeof(uint8_t)));
    avpicture_fill(reinterpret_cast<AVPicture*>(frame_rgb), buffer, PIX_FMT_GRAY8, width, height);

    SwsContext* sws_context = sws_getContext(width, height, codec_context->pix_fmt,
                                             width, height, PIX_FMT_GRAY8,
                                             SWS_BILINEAR, nullptr, nullptr, nullptr);
    if (sws_context == nullptr)
        return -1;

    AVPacket packet;
    int frameno = 0;
    while (av_read_frame(format_context, &packet) >= 0) {
        if (packet.stream_index == video_stream_index) {
            int finished;
            avcodec_decode_video2(codec_context, frame, &finished, &packet);
            if (finished) {
                sws_scale(sws_context, frame->data, frame->linesize, 0, height,
                          frame_rgb->data, frame_rgb->linesize);
                if (++frameno <= 5)
                    save_frame(frame_rgb, width, height, frameno);
            }
        }
        av_free_packet(&packet);
    }

    av_free(buffer);
    av_free(frame_rgb);
    av_free(frame);
    avcodec_close(codec_context);
    avformat_close_input(&format_context);

    return 0;
}

参考文献

• An ffmpeg and SDL Tutorial http://dranger.com/ffmpeg/
• Using libavcodec in Visual Studio, part 1 - Neowin Forums http://www.neowin.net/forum/blog/460/entry-3634-using-libavcodec-in-visual-studio-part-1/