r/GIMP • u/BKNRED • 2d ago

Colour to Grayscale conversion while maintaining contrast/perceptual brightness

I am looking for a method to maintain the contrast and perceptual luminosity of an original colored picture/image. Sorry I had to use an external host to maintain pixel accuracy.

I did a little test with a sprite from Brackey's Godot tutorial. Can you give me your advice on the closest to the result I described ?

To me none of them has got exactly the same contrast/luminosity as the original picture.

1 - IrfanView - Convert To Grayscale option (included just cause)

2 - Gimp 3.0.2 - Desaturate -> luminance

3 - Gimp 3.0.2 - Desaturate -> HSL lightness

4 - Gimp 3.0.2 - Hue-Saturation - Saturation slider down to -100 Blending Mode set to HSV Saturation

Original

Ensemble

For the closest I think it's luminance

Is there a better way to achieve the result I am seeking please ?

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u/Scallact 1d ago

In the case of an icon, you should not look for perceptual accuracy, just for which one looks better.

For full size images, there are techniques to judge the rendered values. One trick painters use is to nearly close your eyes and compare the two. That's obviously not feasible with such tiny icon.

BTW, any color>BW conversion has some subjective interpretation involved, that's not an exact science.

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u/BKNRED 11h ago

It's not really an icon.

The aim is to use the grayscale version as a basis and reapply colors in a different way.

That's why my focus is on preserving the original brightness of the art and not beauty.

Many thanks for your help.

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u/Scallact 9h ago

So, you are trying to keep the perceptual brightness across two color operations? One color > desaturated, then desaturated > another color?

The problem is, colors interact in other ways than just brightness. I get the idea, but I'm not sure it works well in practice. And the question you asked here is only half of the problem, and the simplest part of it. Recoloring while keeping the perceptual brighness is a little more complicated. It involves some channels decomposition, as the color fusion modes on layers wont cut it.