Gobos? Cucoloris? Whatever you call them, they're one of my favorite lighting tricks. A gobo is a cutout placed in front of a light to create a shaped light (think the Bat-Light in Batman). They can be used in both CG and live-action to create illusions, like light from a window that doesn't exist or dappled tree light even when there are no trees.
In CG, they can be created by adding a texture to a light or by adding a card in front of a light. The gobo itself is a black and white mask. Some lighting artists create them in Nuke by drawing vector shapes, but I usually make mine in Photoshop.
On Rugrats, gobos helped me to speed up renders and stylize the lighting. In this jungle scene, I hid all of the off-camera trees in order to make the file lighter and speed up render times. Foliage in CG can cause files to become very heavy. To create the illusion of a lush jungle, I used a gobo to add dappled light on the characters and tree trunk.
Here are some tree dapple gobos I used. Depending on how close or far the light is to the characters and how angular the rays of light are, the pattern will appear more or less blurred or stretched.
In Tommy Pickle's house, the modelers forgot to model a window behind the curtain, so there was no cut-out for light to come through. Oops! I solved the problem by placing a rectangular light behind the curtain with a gobo in the shape of a window.
I made dozens of gobos while working on this show and I didn't save them all, but you can watch the episodes on Paramount+ and try to spot them! One of my favorites is the light pattern on the wall in Cynthia's futuristic loft.