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Texture on SphereStim distorted

I modified the demo stim3d to put an “iris” on a sphere to get a rotatable /rotating eyeball. I changed the script only minimally, basically just exchanging the box with a sphere and the original face photo texture with an iris picture (i.e., an iris on a white background). Putting a circular iris on a sphere should not be a big thing, but surprisingly the perfectly circular flat iris (picture) projected on the sphere is not circular anymore but ellipsoid and about half as wide as high. Any suggestions are welcome.

Ok this was not very successful. Second try, with more pictures:

  1. the script that I used is stim3d from the demo, with the only difference being that I use a sphere and not a box and a different texture (not the face)
  2. Here is the texture:
  3. This is the result. I did not catch the best moment, so it is not the perfect from view of the rotating sphere but rather a little to the right. But anyway, the distortion is quite clear.

    This stimulus class would be very useful for my purpose, but I do not understand the nature of the distortion. Any idea?