Unable to add image using cv::videowriter with CAP_FFMPEG

[Mephiro]

Test config:

• OpenCV compiled with FFMPEG.
• code used to create the VideoWriter object:

cv::VideoWriter video;
int fourcc = cv::VideoWriter::fourcc('a', 'v', 'c', '1');
cv::Size videoSize(xcrop.size(),ycrop.size());
if(canvas.timelapse()){
    video.open("timelapse.mp4",cv::CAP_FFMPEG,fourcc,10,videoSize,true);
}

• code used to write a cv::Mat image to the video file:

video.write(out_img);

Result:

• Video file "timelapse.mp4" is created
• no error neither from FFMPEG or OpenCV
• cv::VideoWriter do not write the images to the video file