converting images to mp4 using ffmpeg on iphone
av_register_all();
printf("Video encodingn");

/// find the mpeg video encoder
//codec=avcodec_find_encoder(CODEC_ID_MPEG1VIDEO);
codec = avcodec_find_encoder(CODEC_ID_MPEG4);

if (!codec) {
    fprintf(stderr, "codec not foundn");
    exit(1);
}

c = avcodec_alloc_context();
picture = avcodec_alloc_frame();

// put sample parameters
c->bit_rate = 400000;
/// resolution must be a multiple of two
c->width = 240;
c->height = 320;
//c->codec_id = fmt->video_codec;
//frames per second
c->time_base= (AVRational){1,25};
c->gop_size = 10; /// emit one intra frame every ten frames
c->max_b_frames=1;
c->pix_fmt =PIX_FMT_YUV420P; // PIX_FMT_YUV420P

if (avcodec_open(c, codec) < 0) {
    fprintf(stderr, "could not open codecn");
    exit(1);
}

f = fopen([[NSHomeDirectory() stringByAppendingPathComponent:@"test.mp4"] UTF8String], "wb");

if (!f) {
    fprintf(stderr, "could not open %sn",[@"test.mp4" UTF8String]);
    exit(1);
}

// alloc image and output buffer
outbuf_size = 100000;
outbuf = malloc(outbuf_size);
size = c->width * c->height;

#pragma mark -

AVFrame* outpic = avcodec_alloc_frame();
int nbytes = avpicture_get_size(PIX_FMT_YUV420P, c->width, c->height); //this is half size of numbytes.

//create buffer for the output image
uint8_t* outbuffer = (uint8_t*)av_malloc(nbytes);

#pragma mark -
for(k=0;k<1;k++) {
    for(i=0;i<25;i++) {
        fflush(stdout);

        int numBytes = avpicture_get_size(PIX_FMT_RGBA, c->width, c->height);
        uint8_t *buffer = (uint8_t *)av_malloc(numBytes*sizeof(uint8_t));

        UIImage *image = [UIImage imageNamed:[NSString stringWithFormat:@"%d.png", i+1]];
        CGImageRef newCgImage = [image CGImage];

        CGDataProviderRef dataProvider = CGImageGetDataProvider(newCgImage);
        CFDataRef bitmapData = CGDataProviderCopyData(dataProvider);
        buffer = (uint8_t *)CFDataGetBytePtr(bitmapData);
        ///////////////////////////
        //outbuffer=(uint8_t *)CFDataGetBytePtr(bitmapData);
        //////////////////////////
        avpicture_fill((AVPicture*)picture, buffer, PIX_FMT_RGBA, c->width, c->height);
        avpicture_fill((AVPicture*)outpic, outbuffer, PIX_FMT_YUV420P, c->width, c->height);//does not have image data.

        struct SwsContext* fooContext = sws_getContext(c->width, c->height,
                                                      PIX_FMT_RGBA,
                                                      c->width, c->height,
                                                      PIX_FMT_YUV420P,
                                                      SWS_FAST_BILINEAR, NULL, NULL, NULL);

        //perform the conversion
        sws_scale(fooContext, picture->data, picture->linesize, 0, c->height, outpic->data, outpic->linesize);
        // Here is where I try to convert to YUV

        // encode the image
        out_size = avcodec_encode_video(c, outbuf, outbuf_size, outpic);
        printf("encoding frame %3d (size=%5d)n", i, out_size);
        fwrite(outbuf, 1, out_size, f);

        free(buffer);
        buffer = NULL;
    }

    // get the delayed frames
    for(; out_size; i++) {
        fflush(stdout);

        out_size = avcodec_encode_video(c, outbuf, outbuf_size, NULL);
        printf("write frame %3d (size=%5d)n", i, out_size);
        fwrite(outbuf, 1, outbuf_size, f);
    }
}

// add sequence end code to have a real mpeg file
outbuf[0] = 0x00;
outbuf[1] = 0x00;
outbuf[2] = 0x01;
outbuf[3] = 0xb7;
fwrite(outbuf, 1, 4, f);
fclose(f);
free(picture_buf);
free(outbuf);

avcodec_close(c);
av_free(c);
av_free(picture);
//av_free(outpic);
printf("n");