Image darkening when converting from 8-bit grayscale to Indexed color modes

Question

I'm working on a heightmap image and part of it requires switching between grayscale and indexed color modes. The image begins in indexed color mode, and when I switch to grey scale, none of the values change. When I switch back to indexed color mode however (using the grayscale color map) the image visibly darkens. This is unacceptable as that lowers all heights in the heightmap, making it pointless. My understanding that since both are 8 bit the conversion should be seamless? Converting my image to indexed color via GIMP does not throw these same errors.

I've attached a video of the effect. The floating menu didn't capture unfortunately, but the effect is just toggling between the two color modes.

Thanks for your help!

D Fosse · Answer

Short answer:Change your working gray to Gray Gamma 2.2 or sGray. Long answer:Indexed color doesn't support icc profiles. That's a problem in grayscale because untagged data will be treated as the working space - and Photoshop's default working gray is Dot Gain 15%. The dot gain profiles have a tone response curve very different from a display - and the display tone curve is what you see in indexed color. This would all be handled by icc grayscale profiles, but you don't have that here unless you assign one when in grayscale mode. That's what changing working gray does here. (Dot gain are generic profiles for offset print, but really obsolete and not used any more. Today you do that with the black ink component in the appropriate CMYK profile).

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