---
name: threejs-water-optics
description: Build production analytic and bounded water in Three.js. Use for shared multi-wave displacement and normals, bounded RGBA heightfield pool simulation, local drops, object-driven ripples, differential-area caustics, ray-traced pool/water/sphere volume optics, derivative-filtered normal bands, analytic sky reflection, side-aware Fresnel, heuristic screen refraction, Beer-Lambert absorption, and crest foam.
---

# Water Optics

Treat water as geometry motion, surface orientation, and a participating optical layer. A blue transparent material is not a water system.

This skill contains exemplary examples and assets beyond descriptive guidance,
they're worth studying, referencing, or even copying. Use them sufficiently
when relevant and do NOT blindly skip them.

For large stochastic seas driven by directional spectra and GPU FFTs, use
`$threejs-spectral-ocean` instead.

## Analytic surface build order

1. Define wave bands and evaluate displacement.
2. Derive the normal analytically from the same waves.
3. Choose displaced geometry or explicitly normal-only water.
4. Establish scene-color ownership for heuristic refraction.
5. Declare whether absorption uses true depth or a fallback path-length estimate.
6. Blend analytic reflection/refraction through side-aware Fresnel.
7. Derive foam and glints from the shared wave response.
8. Filter unresolved normal bands from derivatives.

Read [references/water-surface-system.md](references/water-surface-system.md)
for the authored displacement/normal contract, optical hierarchy, side-aware
refraction, absorption fallback, and diagnostic channels.

Read the
[interactive pool volume implementation](examples/interactive-pool-volume/water-volume-system.js)
for bounded RGBA height/velocity/normal simulation, local drops, moving-sphere
displacement suitable for draggable objects, differential-area caustics, and
in-shader pool/water/sphere ray tracing against the pool bounds and sphere.

## Failure conditions

- normal texture motion does not agree with displaced crests;
- heuristic refraction can sample foreground objects but the limitation is undisclosed;
- fallback path length is presented as reconstructed scene thickness;
- bounded pool caustics are a decorative projection detached from simulated
  height normals;
- micro-waves alias into sparkling noise;
- foam is a scrolling texture unrelated to the shared crest metric;
- Fresnel is replaced by constant opacity;
- reflection, refraction, and transparency are all added without energy control.

## Routing boundary

Use `$threejs-spectral-ocean` for stochastic directional spectra, FFT
cascades, Jacobian breaking, persistent ocean foam, and any interface the camera
crosses — this skill's heuristic screen-refraction offset assumes a bounded
volume seen from air, and an open interface needs that skill's forward
projection instead. Use
`$threejs-precipitation-surfaces` for rain-driven puddle wetness, ripple masks,
and weather-coupled splashes on ground surfaces. This skill owns authored
analytic waves, bounded heightfield simulation, ray-traced pool-volume optics,
and bounded-water optics.