ffmpeg thumbnailer - extract frame, scale and save it to disk

avcodec_sample.c

// avcodec_sample.0.5.0.c

// Original source:
// http://web.me.com/dhoerl/Home/Tech_Blog/Entries/2009/1/22_Revised_avcodec_sample.c.html
//
// A small sample program that shows how to use libavformat and libavcodec to
// read video from a file.
//
// This version is for the 0.4.9+ release of ffmpeg. This release adds the
// av_read_frame() API call, which simplifies the reading of video frames 
// considerably. 
//
// Use
//
// gcc -o avcodec_sample.0.5.0 avcodec_sample.0.5.0.c -lavformat -lavcodec -lavutil -lswscale -lz -lbz2
//
// to build (assuming libavformat, libavcodec, libavutil, and swscale are correctly installed on
// your system).
//
// Run using
//
// avcodec_sample.0.5.0 myvideofile.mpg
//
// to write the first five frames from "myvideofile.mpg" to disk in PPM
// format.

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

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

static void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame);

int main (int argc, const char * argv[])
{
    AVFormatContext *pFormatCtx;
    int             i, videoStream;
    AVCodecContext  *pCodecCtx;
    AVCodec         *pCodec;
    AVFrame         *pFrame; 
    AVFrame         *pFrameRGB;
    AVPacket        packet;
    int             frameFinished;
    int             numBytes;
    uint8_t         *buffer;

    // Register all formats and codecs
    av_register_all();

    // Open video file
    if(av_open_input_file(&pFormatCtx, argv[1], NULL, 0, NULL)!=0)
        return -1; // Couldn't open file

    // Retrieve stream information
    if(av_find_stream_info(pFormatCtx)<0)
        return -1; // Couldn't find stream information

    // Dump information about file onto standard error
    dump_format(pFormatCtx, 0, argv[1], false);

    // Find the first video stream
    videoStream=-1;
    for(i=0; i<pFormatCtx->nb_streams; i++)
        if(pFormatCtx->streams[i]->codec->codec_type==CODEC_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)
        return -1; // Codec not found

    // Open codec
    if(avcodec_open(pCodecCtx, pCodec)<0)
        return -1; // Could not open codec

    // Hack to correct wrong frame rates that seem to be generated by some codecs
    if(pCodecCtx->time_base.num>1000 && pCodecCtx->time_base.den==1)
		pCodecCtx->time_base.den=1000;
		
    // Allocate video frame
    pFrame=avcodec_alloc_frame();

    // Allocate an AVFrame structure
    pFrameRGB=avcodec_alloc_frame();
    if(pFrameRGB==NULL)
        return -1;

    // Determine required buffer size and allocate buffer
    numBytes=avpicture_get_size(PIX_FMT_RGB24, pCodecCtx->width,
        pCodecCtx->height);

    buffer=malloc(numBytes);

    // Assign appropriate parts of buffer to image planes in pFrameRGB
    avpicture_fill((AVPicture *)pFrameRGB, buffer, PIX_FMT_RGB24,
        pCodecCtx->width, pCodecCtx->height);

    // 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_video(pCodecCtx, pFrame, &frameFinished, 
                packet.data, packet.size);

            // Did we get a video frame?
            if(frameFinished)
            {
				static struct SwsContext *img_convert_ctx;

#if 0
				// Older removed code
                // Convert the image from its native format to RGB swscale
                img_convert((AVPicture *)pFrameRGB, PIX_FMT_RGB24, 
                    (AVPicture*)pFrame, pCodecCtx->pix_fmt, pCodecCtx->width, 
                    pCodecCtx->height);
				
				// function template, for reference
				int sws_scale(struct SwsContext *context, uint8_t* src[], int srcStride[], int srcSliceY,
							  int srcSliceH, uint8_t* dst[], int dstStride[]);
#endif
				// Convert the image into YUV format that SDL uses
				if(img_convert_ctx == NULL) {
					int w = pCodecCtx->width;
					int h = pCodecCtx->height;
					
					img_convert_ctx = sws_getContext(w, h, 
									pCodecCtx->pix_fmt, 
									w, h, PIX_FMT_RGB24, SWS_BICUBIC,
									NULL, NULL, NULL);
					if(img_convert_ctx == NULL) {
						fprintf(stderr, "Cannot initialize the conversion context!\n");
						exit(1);
					}
				}
				int ret = sws_scale(img_convert_ctx, pFrame->data, pFrame->linesize, 0, 
						  pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);
#if 0 // this use to be true, as of 1/2009, but apparently it is no longer true in 3/2009
				if(ret) {
					fprintf(stderr, "SWS_Scale failed [%d]!\n", ret);
					exit(-1);
				}
#endif
                // Save the frame to disk
                if(i++<=5)
                    SaveFrame(pFrameRGB, pCodecCtx->width, pCodecCtx->height, i);
            }
        }

        // Free the packet that was allocated by av_read_frame
        av_free_packet(&packet);
    }

    // Free the RGB image
    free(buffer);
    av_free(pFrameRGB);

    // Free the YUV frame
    av_free(pFrame);

    // Close the codec
    avcodec_close(pCodecCtx);

    // Close the video file
    av_close_input_file(pFormatCtx);

    return 0;
}

static void SaveFrame(AVFrame *pFrame, int width, int height, int iFrame)
{
    FILE *pFile;
    char szFilename[32];
    int  y;

    // Open file
    sprintf(szFilename, "frame%d.ppm", iFrame);
    pFile=fopen(szFilename, "wb");
    if(pFile==NULL)
        return;

    // Write header
    fprintf(pFile, "P6\n%d %d\n255\n", width, height);

    // Write pixel data
    for(y=0; y<height; y++)
        fwrite(pFrame->data[0]+y*pFrame->linesize[0], 1, width*3, pFile);

    // Close file
    fclose(pFile);
}