Pareidolia in 3D Terrain: The "Face on Mars" Effect

This simulation renders a procedurally generated 3D rock terrain and lets you control the angle of a single directional light while freely rotating the camera. As the light's azimuth and elevation change, the shadows cast by the terrain's bumps and hollows shift, and at a narrow band of angles two shadow pockets and a shadow crease can momentarily align into a face-like pattern — then vanish completely as you keep sweeping the light. This is a direct, controllable demonstration of why the real 1976 Viking "Face on Mars" image on the Cydonia mesa looked face-like from one sun angle and camera position, yet turned out to be ordinary eroded terrain once photographed at higher resolution under different lighting. Regenerate the terrain for a new random rock shape, or adjust roughness to change how deep and frequent the surface's features are, and see how easily — and how narrowly — the brain's face-detection circuitry can be fooled by nothing more than geometry and light.