r/GIMP • u/BKNRED • 21h 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/ofnuts 10h ago

"Luminance", "lightness" and such are just combinations of the RGB channels with accepted coefficients. You can make up your own combination using Colors > Components > Mono mixer.

But on such small images, the perception is very dependent on the surroundings.

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u/Scallact 15h 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.