Resizing images with correct gamma using PHP and GD

“A short while ago Ty W posted an interesting question on
StackOverflow. Apparently, most graphics software cannot scale
images the right way. Usually it’s hard to notice the flaw but the
linked article does a great job of explaining the problem.

“PHP’s GD library suffers from the same issue, but Ty discovered
that the sample PHP program provided with the article did not work
on partially transparent images. After a couple of hours of
fiddling I managed to get a working solution.

“Apparently, the imagegammacorrect() function in PHP deals badly
with images that have an alpha channel. I suspect that it tries to
apply the same calculation to the alpha channel that it applies to
the red, green and blue channels. To work around this, my solution
splits the aplha channel from the original image. The alpha channel
is resampled regularly while the red, green and blue channels are
resampled using gamma correction.”

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