reflection, bounces = 1.

Instead of using black, one ball has green in the background slot of the reflect node, the other has yellow. Background color (or image) will show when the render-pixel can't get a color from the scene itself because the light ray involved does not hit an object before it dies / gets killed. Either because there is no object, or because the ray gets killed early.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though