#pseudopupil

For those unaware, pseudopupils are what those pupil-like dots on the eyes of animals like mantises and crabs are. You might be aware that both mantises and crabs have compound eyes--that is, each eye is formed of thousands of little eyes (ommatidia). A real pupil is a hole in your eye that constricts to concentrate the light that hits your retina. A pseudopupil is something entirely different. It works based on the polarization of light--that is, where the compound eyes reflect the light that is aimed directly at you, the ommatidia turn black. As a result, like the Mona Lisa, mantises will always seem to be looking at you.

As you can imagine, this was a pain in the ass to set up on a 3D model. So, I had to get creative.

Basically, in 3D you have what are called "shaders." These are what make all the difference between something having realistic shading, and something having a more cartoony cel shading kind of look (e.g. Shrek vs. Okami).

Setting up the shader for the compound eye itself wasn't a big deal--I had a dot texture, and I made it very reflective with high contrasts for light and shadow. The pseudopupil presented all kinds of problems, though--for one, the problem of wrapping a texture around a sphere (notice the way a globe gets squished on an atlas--the poles are not actually nearly that big relative to the other land masses!). Add to that the fact that a mantis's eyes aren't perfect spheres, and you ended up with an effect where the pseudopupils are warping drastically in size and shape whenever they move around the eye. I painstakingly edited the UVs (that is, the way the texture maps to the grid representing the model's surface), and it still looked wrong. So, using an image texture for the pseudopupil was a no go. (I did eventually figure out some methods that didn't warp across the eye like that, but I ended up not liking them as well as the method below.)

There was also the problem of moving the pupil around the eye to begin with. The obvious method was to make an empty (an invisible object) to tell the texture which coordinates in a 3D space it should move to, parent that to a bone, and then just rotate that bone around the eye. I was using a similar method for the cartoony Sophodra model, and figured that would transfer over. The texture coordinates wouldn't behave right, though--the way they transferred the coordinates from a 3D system to a 2D system was unpredictable at best. (Again, I did eventually figure out a way around this, unrelated to the method I settled on.)

There was also the fact that the texture didn't look right. Light behaves a certain way that even a moving sticker doesn't capture, even if the sticker works right.

And that's when I realized that, like an actual pseudopupil, I should make it respond to light!

So, I went into the shader, and modified it so that the very brightest light, instead of being white, would show up as pitch black! Then, instead of empties, I parented a couple of very tiny, very bright lights to the eyes. I was amazed at how well it worked! I have to be careful with the lighting in the scene, and correct for angles that make the lights start moving off the eyes onto the face, but it gives her that gleam that makes a mantis eye really look like a mantis eye. (You can spot the artifacts of this method in the way there's a glow around the pseudopupil in darker lighting, but I ended up liking that too!)

As for making the pseudopupil follow the viewer, that was easy--Blender (the program I use) has a modifier for making objects rotate to follow another object, so I just rigged the bone up to follow the camera and that was that! (I haven't had a whole lot of opportunity to show that off yet, though--hope to soon!)