Speed up build_image.php

[noodlebox]

For building a sprite sheet, a single call to montage is far faster than repeated calls to composite. This gives the same result with a ~50x speedup on my machine.

[iamcal]

This is a great approach. I meant to do this back when I started, but since you can't pass them on the commandline and have to use popen() i got lazy.

However, the ordering of output here is not the same as the original, so I can't merge it yet:

[iamcal]

(build time per sheet is now ~30s on my build box, instead of multiple minutes)

[noodlebox]

That's very strange, I get the correct order on mine. What version of imagemagick are you using? Mine is ImageMagick 6.8.9-9 Q16 x86_64 2016-06-01.

The issue seems to be that it's simply dropping the null: images instead of leaving a blank space for them.

[iamcal]

It's older:

# montage -version
Version: ImageMagick 6.5.4-7 2014-02-10 Q16 OpenMP http://www.imagemagick.org
Copyright: Copyright (C) 1999-2009 ImageMagick Studio LLC

After a yum upgrade I got:

# montage -version
Version: ImageMagick 6.7.2-7 2016-06-16 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2011 ImageMagick Studio LLC
Features: OpenMP

That built the sheets correctly. I'll add a note to make sure the ImageMagick version is recent 👍

[iamcal]

note added in 1c5928f