Decode H264 Frame with FFmpeg

joy.c

#include <errno.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>

#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>   

void my_log_callback(void *ptr, int level, const char *fmt, va_list vargs)
{
    vprintf(fmt, vargs);
}

// LDFLAGS='-L/usr/local/Cellar/ffmpeg/6.0/lib -lavformat -lavcodec -lavutil -lswscale -lswresample' make joy

int main(void)
{
    av_log_set_level(AV_LOG_VERBOSE);
    av_log_set_callback(my_log_callback);
    
    AVCodec *const codec = avcodec_find_decoder(AV_CODEC_ID_H264);
    AVCodecContext* ctx = avcodec_alloc_context3(codec);
    AVPacket pkt_bin;
    AVPacket *thumbnail_packet = av_packet_alloc();
    AVFrame *frame = av_frame_alloc();
    struct stat st;

    avcodec_open2(ctx, codec, NULL);
    
    /// ffmpeg -ss 00:00:31 -i sintel.mp4 -f image2 -vframes 1 -vcodec copy -bsf h264_mp4toannexb frame.bin

    stat("frame.bin", &st);

    av_new_packet(&pkt_bin, st.st_size);
    int bin_fh = open("frame.bin", O_RDONLY);
    read(bin_fh, pkt_bin.data, st.st_size);

    pkt_bin.size = st.st_size;
    pkt_bin.pts = 1;

    avcodec_send_packet(ctx, &pkt_bin);
    avcodec_receive_frame(ctx, frame);

    fprintf(stderr, "size: %dx%d\n", ctx->width, ctx->height); 

    // 

    AVCodec *thumbnail_codec = avcodec_find_encoder(AV_CODEC_ID_MJPEG);
    AVCodecContext *thumbnail_encoder_codec_ctx = avcodec_alloc_context3(thumbnail_codec);

    thumbnail_encoder_codec_ctx->width = ctx->width;
    thumbnail_encoder_codec_ctx->height = ctx->height;
    thumbnail_encoder_codec_ctx->pix_fmt = AV_PIX_FMT_YUVJ420P;
    thumbnail_encoder_codec_ctx->codec_type = AVMEDIA_TYPE_VIDEO;
    thumbnail_encoder_codec_ctx->time_base = (AVRational){1, 30};
    thumbnail_encoder_codec_ctx->framerate = (AVRational){30, 1};

    avcodec_open2(thumbnail_encoder_codec_ctx, thumbnail_codec, NULL);
    
    int error_code = avcodec_send_frame(thumbnail_encoder_codec_ctx, frame);

    if (error_code >= 0)
    {
        error_code = avcodec_receive_packet(thumbnail_encoder_codec_ctx, thumbnail_packet);

        FILE* out_JPEG = fopen("yay.jpeg", "wb");
        fwrite(thumbnail_packet->data, thumbnail_packet->size, 1, out_JPEG);
        fclose(out_JPEG);
    }

    fprintf(stderr, "error_code: %d\n", error_code);

    av_frame_free(&frame);
    av_packet_unref(&pkt_bin);
    avcodec_free_context(&ctx);
    avcodec_free_context(&thumbnail_encoder_codec_ctx);
}