I was playing around with layered textures today, and on a whim, I 
decided to see what would happen if I averaged two opposed normals together.

sphere
	{
	<2, 1, 0> 1
	normal
		{
		average
		normal_map
			{
			[ granite 1 scale 1 ]
			[ granite -1 scale 1 ]
			}
		}
	}

I assumed that they would completely cancel out.. but instead, they 
produced a weird, not exactly granite-like normal that's quite noticible.

Why is this?

Incidentally, the same problem exists if you average the normals as 
textures (in a texture map), although the resulting normal looks 
somewhat different.

-Xplo

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