The rim shadow is part of the shader, not the texture. The default is .4 shadow strength. Here I set them all to 0 (none) or 1 (maximum), alternating.

When you work on textures for the iris for this shader, you do not have to get the edges right at all. The shader never lets the edge be rendered. It will take care of how the edge looks. Just draw (or cut and paste) a decent chunk of iris and the shader takes care of the edge.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)