Uhhh - I can trivially blend any iris with any sclera, even if there is only one material group for the whole eye. But as odf says, why do you want to change iris but not sclera? And when there are two groups, if you want a smooth blend you cannot use two disparate images. Whether you choose to bleed the sclera into the iris, or the iris into the sclera, either way you have two zones that must match color exactly.

Having a sclera that overlaps the iris and has transparency near the joint is way too complicated and will slow down rendering with ray-tracing, especially with IDL. And it really isn't going to produce any more flexibility than simply blending two image maps in one shader. You don't even have to make a mask. I calculate the radius using nodes. In fact, the image I'm using (think I said this before) has junk in the pupil and I masked it out with nodes.

Here are some comparisons in a much different lighting setup. The differences are not entirely due to shaders. Look at the edge of the iris. This is what I complained about many months ago - the hard edge. This edge was due to geometry, not to the texture. The new eye uses the same texture, but has a soft edge to the iris. And the iris-pupil transition is much more realistic.

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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)