ffmpeg

FFmpeg is a useful program for stitching together separate image files into a movie file. In particular, this is useful for producing movies of simulation. If you create a series of image files (png, mpeg, etc) for your simulation outputs, ffmpeg can then combine them into a movie file (mp4, avi, etc).

Of course, ffmpeg can do quite a bit more, and is actually a pretty powerful tool, but here we will focus on converting a collection of images into a movie.

Usage / Example

Suppose you have a directory frames containing files frame_0000.png, frame_0001.png, frame_0002.png, ... and want to make movie.mp4.

ffmpeg -framerate 12 -r 12 -i frames/frame_%04d.png -q 1 -pix_fmt yuv420p -vf scale=-1:600 movie.mp4

In the above line:

• -framerate 12 -r 12 sets the frame rate is set to 12 frames per second
• -i frames/frame_%04d.png sets the input (-i) to the the png files. %04d means that the file names have four-digit integers.
• -q 1 tells ffmpeg to maintain full image quality
• -pix_fmt yuv420p sets the image format
• movie.mp4 specifies the name of the output file
• -vf scale=-1:600 re-scales the images to be 600 pixels tall, while maintaining the aspect ratio.

Calling within MATLAB

The following code snippet outlines how to call ffmpeg from within MATLAB.

frame rate = 3;
first_out = 0;
filename = 'name_of_file';

status = system(['ffmpeg -r ',num2str(framerate),...
    ' -start_number ',num2str(first_out),' -i tmp_%03d.png',...
    ' -r ',num2str(framerate),' -y -pix_fmt yuv420p -q 1',...
    ' -vf scale=-1:600 ../movies/',filename,'.mp4']);
if status ~= 0
    disp([filename,'.mp4 was possibly not rendered correctly.'])
end

Calling within Python

The following code snippet outlines how to call ffmpeg from within Python.

import subprocess
mov_fps = 12
in_name = 'frames/frame_%04d.png'
out_name = 'movie.mp4'
cmd = ['ffmpeg', '-framerate', str(mov_fps), '-r', str(mov_fps),
    '-i', in_name, '-y', '-q', '1', '-pix_fmt', 'yuv420p', 
    '-vf', 'scale=-1:600', out_name]
subprocess.call(cmd)