Making ffmpeg work

To learn how to make this video, read on! (Except I scaled this down for web; full resolution)

When you have proteins in space evolving over time, you have to make a movie. VMD is a love-hate program that is very capable of nice renders of biophysical systems. I like to have more control over the transformation of each rendered frame into a movie file.

ffmpeg is a command-line program that can do this. If you try with the default options, your movie will have lots of compression scarring and may not play in most contexts.

Getting `ffmpeg`

There are some confusing politics with ffmpeg being forked into avconv and that getting picked up by debian-based distros, but then they were meanies so you shouldn’t use it (???). The safest thing is to get the latest version of ffmpeg and build from source.

aptitude install libx264-dev
./configure --enable-libx264 --enable-gpl

Good quality movie from individual frames

ffmpeg \
    -framerate 24 -i frames/final.protein.%05d.tga \
    -c:v libx264 \
    -preset slow \
    -crf 18 \
    -r 24 \
    movie.mp4

This will make a movie from VMD frames. You can change the filename format string (%05d means zero-padded integers to width 5 (default from VMD)). The file indices have to be contiguous.

-framerate 24: You can change this to speed up or slow down your movie. The output framerate is given by the -r option.
-c:v libx264: Use H.264 to encode your movie. This is a good, well-supported encoding that seems to be everyone’s favorite.
-preset slow: Try hard to reduce the file size. It’s not that slow.
-crf 18: “Quality”. This means essentially lossless. Higher is worse. ffmpeg documentation here and explanation here.
-r 24: Output frame rate. If you are speeding up your rendered frames with -framerate > 24, make sure you set this or your movie will look jittery in Powerpoint.

Stack two movies side-by-side

I’ll often want two rotations of a protein both shown in a movie. You can stack them side by side using the at-first-intimidating-but-then-not-so-bad “filter language” in ffmpeg

ffmpeg \
    -framerate 24 -i frames/final.frontview.%05d.tga \
    -framerate 24 -i frames/final.sideview.%05d.tga \
    -c:v libx264 \
    -preset slow \
    -crf 18
    -filter_complex "[0:v][1:v] hstack"
    -r 24 movie.mp4

This is the important line

    -filter_complex "[0:v][1:v] hstack"

The general form of this language is inputs command output. Here, our inputs are the 0th and 1st video streams. The command is hstack, which stacks two streams horizontally. The implicit output is the final movie.

Stack and overlay

Using your favorite plotting software, you can make an animated plot by saving a bunch of frames with zero-padded indices in the filename. Add it as another -i option. Now you can hstack and overlay your plot.

-filter_complex "\
    [0:v][1:v] hstack [stacked] ;\
    [stacked][2:v] overlay=eval='init':x=W/2-w/2:y=H/2-h/2"

Now the output of hstack is a stream named stacked. It’s used as input for the second command, along with the 2nd video stream (our plot frames). The command here is overlay. It takes some parameters.

eval='init': Compute x and y only once. This means our overlay doesn’t move around.
x=W/2-w/2: The x position is given in terms of the base video’s width (W) and the overlay’s width (w). This centers the overlay.

The parameters are of the form key=value and delimited with :. The implicit output is once again our output.

Stack, overlay, reverse

This does the above and then shows the movie in reverse.

ffmpeg \
    -framerate 40 -i frames/final.side.%05d.tga \
    -framerate 40 -i frames/final.bottom.%05d.tga \
    -framerate 40 -i frames/plot.%05d.png \
    -c:v libx264 \
    -preset slow \
    -crf 18 \
    -filter_complex "\
            [0:v][1:v] hstack [stacked] ;\
            [stacked][2:v] overlay=eval='init':x=W/2-w/2:y=H/2-h/2 [layed] ;\
            [layed] split [layed1][layed2] ;\
            [layed2] reverse [revved] ;\
            [layed1][revved] concat" \
    -r 24 nav.ionpull.slow.small.mp4