Threejs Precipitation Surfaces Agent Skill

Precipitation Surfaces

Treat weather as a coupled event, particle, and surface-response system. Do not

add rain or snow particles that are visually disconnected from the ground.

Build order

weather envelope
  -> falling precipitation volume
  -> world/object surface mask
  -> displaced or optical surface response
  -> impact residue and splashes
  -> shared lighting/post presentation

Read references/precipitation-surface-systems.md

for snow accumulation, object capping, wrapped precipitation volumes, wet

puddle masks, procedural ripple normals, splash placement, debug outputs, and

licensing boundaries.

Read the

snow accumulation implementation

for camera-wrapped snowfall, shared wind/time uniforms, world-space snow masks,

single-source snow height and normals, model snow capping, and optional ice

surface composition.

Read the

wet puddle rain implementation

for rain-progress wetness, asphalt puddle masks, procedural ripple normals,

instanced rain streaks, upward-surface splash sampling, and flipbook splashes.

This example includes GPL-licensed source material;

preserve its license boundary when copying or publishing it.

Required controls

• precipitation density and speed;
• wind direction and strength;
• shared weather progress or coverage;
• wetness, snow, or puddle mask threshold and softness;
• ripple or drift normal strength;
• surface roughness response;
• particle/splash opacity;
• debug modes for masks, normals, particles, and event progress.

Failure conditions

• falling precipitation ignores the wind or timing used by surface response;
• snow height and snow normals come from different fields;
• model snow sticks to vertical faces without an upward-facing filter;
• puddles only lower roughness without a mask, normal response, or ripples;
• splashes appear on downward or hidden faces;
• rain streaks allocate per drop or fail to wrap around the camera;
• temporal wetness is faked with unrelated time noise;
• the license boundary for GPL-derived rain code is removed or obscured.

Routing boundary

Use $threejs-water-optics for bounded pool simulation, caustics, Fresnel,

refraction, and Beer-Lambert water volumes. Use $threejs-procedural-vfx for

general sparks, plasma, trails, and non-weather particles. Use

$threejs-temporal-surfaces for screen-space touch history or frost clearing.

This skill owns precipitation events and the surfaces they visibly alter.