Reveal Mask: WebGL / GLSL Minimal walkthrough

This version keeps the integration small so the moving parts are visible. It uses the WebGL / GLSL path from `three-fluid-fx`, where the effect is built with EffectComposer passes, ShaderMaterial uniforms, and WebGL render targets.

Live demo

What this demo teaches

The fluid is not the picture; it is the stencil. The solver writes a screen-space texture every frame — density in one channel, the velocity vector in two more. Where the pointer paints density, the sealed layer gives way to the hidden one underneath; the velocity bends the boundary so the cut looks liquid instead of a hard circle. In this variant, the relevant fluid output is Texture objects: velocityTexture, densityTexture, and dyeTexture when dye is enabled.

• Read the fluid output as data: density (.b) is a mask, velocity (.rg) is a 2D offset — not something you draw.
• Composite two cover-fitted layers with mix(base, reveal, mask).
• Confine the velocity distortion to a thin edge band so the boundary ripples while the revealed interior stays crisp.
• Get a glowing "wet rim" for free from the same band, and let the reveal heal back via density dissipation.

Implementation path

1. Render the fluid, then composite fullscreen

   The solver renders into a screen-space density/velocity texture. One fullscreen pass — a FullscreenPass + ShaderMaterial in WebGL, an output node in TSL — reads that texture together with the two image layers and writes the final pixel. No 3D scene is needed.

2. Cover-fit both layers

   Each image is mapped with a background-size: cover UV so it fills the canvas without stretching; base and reveal can even be different sizes (the full demo lets you upload your own). Pass the image and canvas sizes as uniforms and crop the overflow.

3. Density → reveal mask

   mask = smoothstep(lo, hi, density.b) turns the painted fluid into a 0..1 reveal amount; mix(base, reveal, mask) blends the sealed and hidden layers. Raising the lower threshold makes the reveal appear later, only under a denser stroke.

4. Velocity → liquid edge + wet rim

   An edge band — mask·(1−mask), peaking at the boundary — is the trick. Offset the reveal UV by velocity·edge so only the boundary refracts (the interior stays sharp), then add reveal·edge·rim for a bright meniscus that traces the stroke.

5. Heal-back and resize

   densityDissipation fades the painted density over time, so the reveal slowly closes on its own. On resize, keep the solver size, the view-size uniform and the renderer in sync.

6. Minimal scope

   It avoids GUI state and preset switching. Read it first when you need the smallest reliable version of the technique.

import { FluidSimulation, FullscreenPass, FULLSCREEN_VERTEX } from 'three-fluid-fx'

const fluid = new FluidSimulation(renderer, { splatRadius: 0.0175, densityDissipation: 0.985 })
attachPointerSplats(renderer.domElement, fluid)

// density.b is the reveal mask, velocity.rg ripples the edge
const composite = new ShaderMaterial({
  vertexShader: FULLSCREEN_VERTEX,
  fragmentShader: /* glsl */ `
    vec4 fl = texture2D(tFluid, vUv);
    float mask = smoothstep(0.06, 0.32, fl.b);
    float edge = mask * (1.0 - mask) * 4.0;          // thin boundary band
    vec3 base = texture2D(tBase, imgUv).rgb;
    vec3 reveal = texture2D(tReveal, imgUv - fl.rg * uEdge * edge).rgb;
    gl_FragColor = vec4(mix(base, reveal, mask) + reveal * edge * uRim, 1.0);
  `,
  uniforms: { tBase, tReveal, tFluid: { value: fluid.densityTexture }, uEdge, uRim },
})
const pass = new FullscreenPass(composite)

Parameters to tune

Source landmarks

Production notes

• Keep the velocity offset weighted to the edge band, or the whole revealed area smears and becomes unreadable.
• Frame the two layers identically (same camera and size) so the reveal lines up — for video, render the pair from one animation/turntable.
• Tune densityDissipation for the feel: low for a wipe-and-restore scrub, high for a reveal that lingers.