Legacy ImageMagick Discussions Archive

The razor which determines whether this issue is solved is as follows:

We use the test image from Eric Brasseur's page http://www.4p8.com/eric.brasseur/gamma.html :

http://www.4p8.com/eric.brasseur/gamma_dalai_lama_gray.jpg

Resize this image to 50% linear-dimension without using any special ...

Input image (alternating black and white lines)

convert lines.png -resize 50%% lines1.png
lines1.png: { A largely uniform rectangle of pixels #7F7F7F }

convert lines.png -gamma 0.45454 -resize 50%% -gamma 2.2 lines2.png
lines2.png: { A largely uniform rectangle of pixels #BABABA }

"%IMG ...

A great majority of image processing algorithms assume a linear colorspace for correct results. However, the ImageMagick policy / philosophy is to assume the user knows what they are doing and to not do anything automatically. Its up to the user to use the -colorspace option or profiles to convert ...

I thought that was the function of "-set colorspace", ie

convert in.png -set colorspace sRGB -resize 50% out.png

does the resize assuming the pixels of in.png are sRGB, so it will do (roughly) the "-gamma 0.45" before the resize and "-gamma 2.2" after.

Thank-you for replying.

OK that does ...

I've read Eric Brasseur's Gamma article at http://www.4p8.com/eric.brasseur/gamma.html, and I am now miffed with pretty much every graphics tool I have.

Eric has pointed out particularly about resizing, but similar problems would apply to any other operation which does algebraic operations on pixel ...