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 seas with spectral cascades, coastal breakers or camera-medium
crossings, use $threejs-spectral-ocean. This skill also owns the air-side
photoreal-water shallow-water compositor.

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
for the authored displacement/normal contract, optical hierarchy, side-aware
refraction, absorption fallback, and diagnostic channels.

Read the
interactive pool volume implementation
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.

Read the photoreal water implementation
for gravity-capillary FFT waves, touch ripples, RGB caustics, refracted pebble-bed
radiance and aperture diffraction glare. Read
references/photoreal-water.md for its pass,
coordinate, filtering, integration and diagnostic contracts.

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 large spectral cascades, Jacobian breaking,
persistent ocean foam and camera-medium crossings. The photoreal-water
example uses an FFT patch for shallow-water optics viewed from air; it does
not implement underwater viewing. The analytic screen-refraction recipe also
assumes an air-side camera above a bounded volume. 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.