Antonia Tiger,

You are absolutely correct in your interpretation of how bump maps and displacement maps work. Bump maps only change the orientation of the surface normals causing the light to reflect least for the black and most for the white areas with the center or neutral grey value being what you would see without a bump map applied. These are simply two-dimensional patterns. There is no change to the actual physical mesh, flat is flat and round is round and no shadows are cast by the bumps. Think of the fresnel effect where the edges of a sphere reflect less light and go dark while the front of the sphere reflects almost all of the light.

Displacement maps alter both the orientation of the normals and the position of the surface itself resulting in a texture that actually has three-dimensionality, in combination with the two-dimensional patterns mapped on it. Here again, black is least reflective and lowest, white is most reflective and highest. These surfaces will cast shadows. Displacement maps alleviate the bump map only problem that can occur when you see a highly irregular surface yet the edge appears perfectly smooth. Obviously displacement maps are a lot heavier come render time than bump maps, so it pays to save them for closeups or terrain generation and use bumps for less prominent objects.

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