Legacy ImageMagick Discussions Archive

This follows from viewtopic.php?f=1&t=24122

When an image has an alpha channel that is fully opaque, "-alpha off" before a resize shouldn't make a difference, but it does.

Here is a test case. Create a black and white image with alpha channel, but fully opaque: Resize it two ways (with or without "-alpha off"): What is the difference? The difference is "1762.49 (0.0268939)", and occurs at the black/white boundary. Why is there a difference, and which version is correct?

EDIT: Sorry, forgot to say: tested in 6.8.6-10 on Windows 7.

You should not use '-alpha on', you want '-alpha set'. This will initialize the alpha channel with the correct values. With '-alpha on' the alpha channel will contain 'garbage' and give strange results. More information is here: http://www.imagemagick.org/Usage/masking/#alpha_on. I did not test if that fixes your problem so please let us know.

Fair comment that I should have used "-alpha set", not "-alpha on".

However, I did check that all pixels were opaque, and using "-alpha set" doesn't fix the problem.

I finally figured out what is happening. The resize filter is different when an image has an alpha channel. With alpha on imagemagick uses a Mitchell filter and when it is turned off it uses an Lanczos filter. Detailed information about both filters can be found here: http://www.imagemagick.org/Usage/filter

Ah, fair enough. By explicitly setting the filter and comparing results, I can confirm you are correct: st1.png (alpha switched off) used Lanczos while st2.png used Mitchell.

At this point, I would request that http://www.imagemagick.org/script/comma ... php#filter be changed to say this, but it already does.

So: not a bug, not a problem. Thanks.