"Two-triangle" shaders; programs which run as pixel shaders full-screen to generate video.
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By treating lights as additively blended billboards, they can be arranged into very "moody" scenes.
In addition, a simple single-sample raytrace of a solid scene is performed to create a depth sample; if that depth sample is in front of the billboard, the light is sampled there instead.
This allows silhouettes to be defined in the scene.
By writing functions which map screen-space coordinates to UV coordinates and a depth, 3D scenes can be constructed.
This is quite limited however; the camera cannot move in most cases and complicated masking must be performed to construct geometry.
Scenes created by raymarching or otherwise sampling signed distance fields, allowing for (relatively) cheap pseudo-raytracing of complex procedural geometry.