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I'm doing some compositor effects on my Cycles passes before outputting them to Krita and recombining them. But I'm running into a problem: Cycles stores a bunch of indirect light color in the Direct and Indirect passes, which complicates some of my setup. I want my shading passes to be totally black and white, and all the color to be in the Color pass. I'd like to transfer the color from the Shading passes to my color pass.

The normal way passes are combined is (Direct+Indirect)*Color. So I want to somehow alter the Color pass based on the colors in the Direct and Indirect pass, so that when it's all combined, it looks the same as it does now. I'm not really sure where to start with this. Is this something that can be done?

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Why there is color in other passes than Color

The direct and indirect stands for direct or indirect lighting so if the light has color, it will show in direct passes and if objects in the scene have color and light bounces off them, the light color will change accordingly in the indirect passes. These passes are supposed to have color. You can change the colors of your objects without changing lighting by changing the Color passes. Unfortunately this means indirect light that bounces off those objects has to be recalculated during rendering for it to change.

Separating color can be done

If for whatever artistic purposes you would like to separate all the color, you can do that in Blender or in any other software that supports 32bit image editing just by manipulating the images.

In this case you could make a copy of the image you wish to separate with a RGB to BW node and simply divide the colored image by it. You would have to multiply them together to get the colors back, so now you can multiply only the color 'passes' together and have the greyscale image for the separated monochromatic part of lighting:

enter image description here

Should you do it?

Obviously, you can do this separation at any moment in the compositing, even with the final image, but then passes are not needed. This can be done inside most image editing software. In many cases the purposes of rendering passes includes separation of the lighting from the colors and textures of the objects.

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