3D Underwater God Rays

Use this when the camera is under the water and the light comes down through it. Reach for a sibling instead in these cases:

• Sun shafts in air, cut by trees and rooflines: use 3d-sky-rays. Its screen-space radial blur of an occlusion mask is the right tool there. It cannot parallax or carry ripples, so under water keep it only as a low companion for fish shadows (Blue Hour runs it at 0.25).
• The ocean surface seen from above, with a horizon: use 3d-ultra-realistic-water.
• Authored shaft planes through one window in a lit interior: use 3d-wood-lighting-scorecard.

Source: extracted from Blue Hour, the living-ocean wallpaper of a Three.js iPhone Duo study. A school of 2,200 Blender sardines turns behind a glass device. The light had to read as sun through water across the whole 38 s camera dolly and on the device's own screen.

Verified stack: Three.js r170, WebGL2, GLSL ShaderMaterials, one HalfFloat render target, and a hand-written ACES pass. No post-processing library.

The mechanism

Sample one drifting swell field across the sun direction, and integrate it along every view ray. Each point in the water is lit by the swell value at its position measured perpendicular to the sun. The shafts are therefore parallel columns in the world. They converge on the sun the way rails converge on a vanishing point, and they slide past the fish when the camera moves. Paint the same shafts in screen space and they stay locked to the lens.

Reuse the working effect

Copy assets/underwater-light.mjs. You pass in your own THREE, and the module does not import a second copy. The demo runs this same module inside Blue Hour's renderer. demo/build.mjs rebuilds it from the Blue Hour sources.