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The Blender manual page says about Diffuse direct and indirect as follows:

Diffuse Direct

Direct lighting from diffuse BSDFs. We define direct lighting as coming from lamps, emitting surfaces, the background, or ambient occlusion after a single reflection or transmission off a surface. BSDF color is not included in this pass.

Diffuse Indirect

Indirect lighting from diffuse BSDFs. We define indirect lighting as coming from lamps, emitting surfaces or the background after more than one reflection or transmission off a surface. BSDF color is not included in this pass.

What I expect from lighting passes is a gray-scale image, or RGB image with color reflects the light source's color. However, the Diffuse direct/indirect images made by Cycles gives weird colors and I cannot explain it.

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I'll try to explain. Here is an example. We have an orange ball on the gray plane, lit from the side.

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What contains the Diffuse Direct pass? A reflection of light coming from the lamp on the side. As you can see, the layer does not contain information about that the ball is orange.

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To get the orange color you need to multiply Diffuse Direct and Diffuse Color.

What contains the Diffuse Indirect pass? light reflected from the ball becomes orange and this light illuminates the plane. And the plane reflects light almost unchanged, so the ball looks white:

enter image description here

Finally, connect all layers together

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You can see, gray plane have orange reflect from sphere. This reflection goes from indirect pass.

So if Diffuse Direct and Diffuse Indirect are just gray-scale images, where were no color reflexes in scene

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  • $\begingroup$ Actually, if look closely at the Direct Image, I think I can see a tiny bit of orange on the top-right boundary of the ball (between the ball and the plane). Can you please check it, maybe it's the effect of my screen. $\endgroup$ – AugLe Mar 6 '18 at 13:35
  • $\begingroup$ Hm, it's default meshlight 5500K light, and filmic color management, it can be a little worm. $\endgroup$ – Crantisz Mar 6 '18 at 13:46

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