I am working on a render of Saturn as part of a scale model of the solar system, and have run into a problem with the transparency of the rings. I am using an alpha-mapped png image which seems to work fine, except for the break in Saturn's horizon, where it is crossed by the rings. (See image) It looks like refraction, which I certainly don't want in this case. How would I get rid of that?
Or, since this is a scale model where 1BU = 10,000km, could this be a precision error miscalculating the re-transmission angle? The scene is lit with a Sun model, 143,350 BU away...
EDIT: Upon further research, this aberration is directly linked to the distance from the origin. I built a test scene to try to reproduce the problem, and was successful. The test rig is an emission plane over a striped sphere that is intersected by a fully transparent horizontal plane just above the equator. The brick floor is just a backdrop, and is not as severely distorted.
My guess is that this is a precision error from trying to calculate the exact angle of re-transmission through the clear plane with such high-magnitude inputs.
I tried just scaling everything down, but the distortion remains the same size relative to the geometry. Is this simply a practical limitation of the software that I should learn to live with, or might there be some way to get around it?