I came across an effect challenging my understanding of ambient colour. In my understanding of the Phong Reflection Model (see: http://en.wikipedia.org/wiki/Phong_reflection_model ) ambient colour is the colour of those parts of an object which are not exposed to light. And ambient colour blends smoothly into diffuse colour with decreasing light intensity.
In order to illustrate my issue let’s have a look at a simple scene which contains just a default camera, a plane, a cylinder and a single distant light illuminating the cylinder from the right side of the camera. See attached picture Ambient-all_default.png. In the next step I applied ambient colours to the plane (RGB= 255 0 0) and the cylinder (RGB= 0 0 255) leaving diffuse at its default plain white. This renders to Ambient-d-white-a-colours.png.
So far the resulting renders are fully consistent with my understanding. But the trouble comes when I alter the diffuse colours for instance to (RGB= 0 255 0) for the plane and (RGB=255 0 0) for the cylinder. In preview mode the cylinder is still looking as expected. It shows a smooth gradient from diffuse colour where it is exposed to the light to ambient colour in the shadow. See Ambient-d-colours-a-colours-cyl-screen.jpg
However when I render this the shadows turn black. See Ambient-d-colours-a-colours.jpg
Can anybody please explain to me whether my understanding is wrong or if this is a bug. Most of all, why is preview, which is consistent with my understanding, different from the render output, which seems to be wrong as far as I understand the Wikipedia article referenced above. Did I get it wrong?