Here's a neat visual trick: This shader program (by yx) renders a fully illuminated Reutersvard*/Penrose impossible triangle: https://www.shadertoy.com/view/ttXGWr All of the GLSL code used to generate the image on the left (minus Shadertoy's framework) is listed and editable on the right, and you can edit the code on the right to see what it does. Note how the geometry is locally consistent, and how the shadows between cubes, other cubes, and the ground all match up without any hard seams. Here's some context behind what makes this especially interesting. This program simulates global illumination using Monte Carlo path tracing: For each on- screen pixel, we trace a ray from a virtual camera into the scene, find the first place where the ray intersects the scene geometry, choose a new direction for the ray, trace the ray until it intersects the scene geometry again, and repeat until the ray reaches a light source (in this case, the sky) or reaches some maximum number of bounces (in this case, 4). (There's a lot I'm leaving out here, but this is sort of the basis behind it.) For geometry in R^3, this makes sense; finding ray-object intersections can sometimes be difficult, but are usually feasible. But here's the impossibility: how can you find the intersection of a ray and an object that can't exist? Left-click and drag over the image on the left (ideally while the shader is playing) to see a hint of how the shader works. *With a diaresis over the a: Reutersvärd --Neil Bickford (encoded partial solution (swaps a<->z): FromCharacterCode[219 - ToCharacterCode["zeviztv{tvlnvgib{levi{grnv"]].)