Three-Fenestra: an interior mapping shader for Three.js

Status: v0.2.0, pre-1.0.

A MeshStandardMaterial subclass that adds parallax-based fake-3D rooms inside flat window planes, plus an optional PBR front layer for curtains, blinds, mullions, and glass dirt. Works in vanilla Three.js and React Three Fiber.

Live demo: three-fenestra.codedgar.com

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Why Three-Fenestra

The interior mapping technique was proposed by Joost van Dongen in 2008 to fake the look of furnished rooms behind window planes without modelling or lighting them. The original is one shader, one texture, no front layer.

Modern building renders need more than that: curtains that catch sun, glass that gets dirty, mullions that cast shadow, windows that switch from "lit" to "dark" by time of day. Three-Fenestra keeps the cheap ray-march at the core and stacks the modern bits on top:

• A back atlas of interior rooms (the original technique)
• A PBR front overlay with optional normal / roughness / metalness atlases
• A transmission term so curtains can still bleed warm light at night
• Uniforms wired for a day/night controller (you supply the controller)

It is one material per window mesh. Drop it into any existing scene that already uses Three's standard lighting and it composites correctly.

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Install

npm install three-fenestra three

three >= 0.150 is a peer dependency.

Starter atlases

The package ships two ready-to-use 4×4 atlases under three-fenestra/starter/ so you don't have to author your own to see something render on day one:

Path	What it is
three-fenestra/starter/rooms.webp	Back atlas — 4×4 grid of interior rooms.
three-fenestra/starter/overlay.webp	Front atlas — 4×4 grid of curtain / blind variants with alpha.

Any modern bundler (Vite, webpack, Parcel, esbuild) imports them as URLs. Once you outgrow the starters, follow Creating your own atlases.

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Quick start (vanilla Three.js)

import * as THREE from 'three';
import { InteriorMappingMaterial } from 'three-fenestra';
import roomsUrl   from 'three-fenestra/starter/rooms.webp';
import overlayUrl from 'three-fenestra/starter/overlay.webp';

const atlas = new THREE.TextureLoader().load(roomsUrl);
atlas.colorSpace = THREE.SRGBColorSpace;
atlas.wrapS = atlas.wrapT = THREE.ClampToEdgeWrapping;

const material = new InteriorMappingMaterial({
  backAtlas: atlas,
  backAtlasCols: 4,
  backAtlasRows: 4,
  depth: 1.0,
  backScale: 0.66,
  planeSize: new THREE.Vector2(width, height),
  windowId: new THREE.Vector3(x, y, z), // per-window seed for cell picking
  roughness: 0.15,
  metalness: 0.0,
});

const mesh = new THREE.Mesh(new THREE.PlaneGeometry(width, height), material);

The interior renders with no front textures. Add the PBR front layer at any time — the starter overlay.png is a 4×4 curtain atlas you can drop in:

const overlay = new THREE.TextureLoader().load(overlayUrl);
overlay.colorSpace = THREE.SRGBColorSpace;
material.setFrontAtlas(overlay, 4, 4);

// Optional companion PBR maps if you have your own:
material.setFrontNormalAtlas(curtainNormal, 1);   // samples .xy
material.setFrontRoughnessAtlas(curtainRough);    // samples .g
material.setFrontMetalnessAtlas(curtainMetal);    // samples .b

Where front alpha is 0, you see the interior through the "glass." Where it is 1, you see the front layer lit by scene lights via standard PBR, fresnel included.

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React Three Fiber

R3F passes constructor arguments via args as a single-element array (the material takes one options object):

import { extend, type ThreeElement } from '@react-three/fiber';
import { InteriorMappingMaterial } from 'three-fenestra';

extend({ InteriorMappingMaterial });

declare module '@react-three/fiber' {
  interface ThreeElements {
    interiorMappingMaterial: ThreeElement<typeof InteriorMappingMaterial>;
  }
}

<mesh>
  <planeGeometry args={[width, height]} />
  <interiorMappingMaterial
    args={[{
      backAtlas: atlas,
      planeSize: new THREE.Vector2(width, height),
      windowId: new THREE.Vector3(x, y, z),
    }]}
  />
</mesh>

Setting individual props on <interiorMappingMaterial /> after construction works for the runtime knobs (depth, backScale, interiorEmissive, frontTransmission, frontAlphaBoost) because those are wired to setters. Texture swaps should go through setFrontAtlas(...) / setBackAtlas(...) via a ref.

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API

Constructor parameters

All MeshStandardMaterialParameters are accepted, plus:

Parameter	Type	Default	Description
backAtlas	Texture	required	The interior (rooms) atlas.
backAtlasCols, backAtlasRows	number	4, 4	Grid dimensions of the back atlas.
depth	number	1.0	Apparent room depth, in plane-local units.
backScale	number	0.66	Back-wall fill factor (0.05–0.999).
planeSize	Vector2	required	Must match the geometry's width × height.
windowId	Vector3	required	Per-window seed (typically the window center) for atlas cell picking.
interiorEmissive	Color	(1, 1, 1)	Multiplier on the interior contribution before adding to the lit output. Use to tint warm and scale up for "lights on" night mode (e.g. new Color(2, 1.5, 1)).
frontAtlas	Texture?	—	Front overlay atlas (RGBA: color + alpha).
frontAtlasCols, frontAtlasRows	number	1, 1	Front atlas grid.
frontNormalAtlas	Texture?	—	Tangent-space normal map atlas.
frontNormalScale	number	1.0	Multiplier on .xy of the normal sample.
frontRoughnessAtlas	Texture?	—	Roughness atlas (samples .g).
frontMetalnessAtlas	Texture?	—	Metalness atlas (samples .b).
frontTransmission	number	0.25	Fraction of interior light that bleeds through the opaque front layer, tinted by the front color. 0 = front fully blocks interior, 1 = no blocking.
frontAlphaBoost	number	1.0	Raises effective opacity of the front layer (pow(alpha, 1/boost)). > 1 makes semi-transparent pixels read as more opaque without re-authoring the texture.

Glass surface (optional)

These give the glass area (where the front layer is transparent) the look of a real pane: dirt, refraction, fresnel sheen. All default to zero / off; turn on the ones you want.

Parameter	Type	Default	Description
glassThickness	number	0	Apparent glass thickness in plane-local units. Parallax-shifts the front-overlay sample so it appears to sit on the inside face of the pane rather than glued to the outside surface. 0 disables.
refractionStrength	number	0	Magnitude (in cell-UV units) of the interior ray-march perturbation driven by glassDirtMap. Sells the "looking through real glass" effect. Keep tiny — typical range 0.003–0.015. 0 disables.
glassDirtMap	Texture?	—	Grayscale noise texture used as the dirt/specular modulator over the glass area, and as the source of the refraction perturbation. Centered around 0.5; values > 0.5 roughen the glass, < 0.5 polish it.
glassDirtStrength	number	0.35	How strongly the dirt map modulates roughness on the glass area.
glassFresnelStrength	number	0	Schlick fresnel sheen added to the glass at grazing angles. Primary "this is a pane of glass" cue. Demo uses ~0.5.
glassFresnelColor	Color	(0.85, 0.92, 1.0)	Tint of the fresnel sheen. Cool white reads as sky reflection.
glassSmudgeStrength	number	0	Additive brightness of dirt visible as smudges on the glass surface. Different from glassDirtStrength (roughness modulation).

Runtime setters

// Core knobs
material.depth              = 0.8;
material.backScale          = 0.6;
material.interiorEmissive   = new THREE.Color(2.0, 1.5, 1.0);  // copies into uniform
material.frontTransmission  = 0.10;
material.frontAlphaBoost    = 1.0;

// Glass-surface knobs
material.glassThickness       = 0.04;
material.refractionStrength   = 0.005;
material.glassDirtStrength    = 0.35;
material.glassFresnelStrength = 0.5;
material.glassFresnelColor    = new THREE.Color(0.85, 0.92, 1.0);  // copies into uniform
material.glassSmudgeStrength  = 0.1;

// Texture swaps (pass null to disable)
material.setBackAtlas(newAtlas);
material.setFrontAtlas(tex, cols, rows);
material.setFrontNormalAtlas(tex, scale);
material.setFrontRoughnessAtlas(tex);
material.setFrontMetalnessAtlas(tex);
material.setGlassDirtMap(tex);

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Day / night

The library does not ship a day/night controller. It gives you the uniforms to build one. Typical recipe:

const day   = { emissive: new THREE.Color(1, 1, 1),         transmission: 0.15 };
const night = { emissive: new THREE.Color(1.7, 1.35, 0.95), transmission: 0.10 };

function setMode(p: typeof day) {
  for (const m of materials) {
    m.interiorEmissive  = p.emissive;
    m.frontTransmission = p.transmission;
  }
}

For full "lights on at night," pair this with:

• Reduced scene ambient and sun, but not to zero. Building exteriors at night still receive skyglow, streetlights, and reflections. Curtain colours need ambient to read as fabric, not as backlit cutouts.
• A cool-tinted ambient with a warm interior emissive for the classic night-city contrast.
• UnrealBloomPass on the composer (low strength, around 0.4) to spill the lit-window contribution onto neighbouring pixels.

The bundled examples/asia-building demo wires all three; check main.ts for a working reference.

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Creating your own atlases

Two textures drive the look: the back atlas (interior rooms) and the optional front atlas (curtains, blinds, glass overlays). Both are grids of square cells; the shader picks a cell per window using a deterministic hash of windowId, so the same window always gets the same room across re-renders.

Back atlas (interior rooms)

A grid of square room photos. Each cell is one "room" the ray-march will land you inside.

Spec	Recommendation
Grid	4×4 (16 variants) is the sweet spot for masking repetition across hundreds of windows. 2×2 is fine for small scenes.
Cell aspect	Square (1:1). The ray-march assumes a unit cube per cell.
Image size	Power of two (1024×1024, 2048×2048). Lets Three generate mipmaps.
Color space	sRGB. Set texture.colorSpace = SRGBColorSpace so sampling converts to linear for PBR.
Edge bleed	The shader insets each cell by 0.001 to prevent bleed; keep ~2 px gutter inside each cell as insurance.
Wrap	ClampToEdgeWrapping on both axes.
Filter	LinearMipmapLinearFilter (min) + LinearFilter (mag), anisotropy: 8+.
Content	Frame each cell as if looking through a window from outside. Centre the composition; the back wall should fill ~60–70% of the cell (matches default backScale). Already-lit photography works best; the shader treats interior pixels as pre-lit.

Front atlas (curtains, blinds, overlays)

A grid of window dressings. Each cell sits on top of one window using the same cell-picking logic.

Spec	Recommendation
Grid	Match the variety you want. 4×4 = 16 variants.
Cell aspect	Square. Real windows are not square; the shader stretches the cell to the window's actual aspect, so pick curtain compositions that survive a moderate stretch.
Format	PNG with alpha (RGBA).
Color space	sRGB.
Trim to edge	Each curtain should fill its cell edge-to-edge with no transparent gutter. If your source has padding, trim it: magick in.png -alpha set -fuzz 10% -bordercolor none -border 1 -trim +repage -resize 256x256^ -gravity center -extent 256x256 out.png
Alpha encoding	The single biggest authoring decision. Opaque (alpha = 1) is curtain fabric. Transparent (alpha = 0) is the glass area you want the interior to show through. Anywhere between is "semi-sheer" and the shader reads it as fractional transmission.

Sheer and lace curtains

Do not author the fabric itself at low alpha unless you genuinely want light to pour through it. Anti-aliased edges are fine; intentional partial transparency on every pixel of the curtain is what causes the "windows evaporate at night" problem.

If you already have a texture with semi-transparent fabric and want it to behave more solidly at night, raise frontAlphaBoost (try 2.0–2.5). It is a render-time knob; no re-export needed.

Front PBR maps (optional)

If you want the curtain fabric to receive proper PBR lighting (fresnel, scene light response):

• Normal atlas: tangent-space, same grid as the albedo atlas. RGB channels = XYZ, encoded [0..1] mapping to [-1..1].
• Roughness atlas: single-channel; the shader samples .g. White = rough, black = mirror.
• Metalness atlas: single-channel; the shader samples .b. Curtains and glass are non-metallic, so almost always 0.

All three must share the front albedo atlas's grid dimensions.

Window mesh setup

Each window is a PlaneGeometry sized to the real window dimensions. Three things every material needs:

1. planeSize: the geometry's (width, height) as a Vector2. The shader uses it to normalise object-space position into local UV.
2. windowId: a Vector3 unique per window. The window's centre in world space is a natural choice.
3. The plane's local +Z must be the outward-facing normal. If your geometry comes from a model with arbitrary orientation, build a basis from (right, up, normal) and apply it via mesh.quaternion.setFromRotationMatrix(makeBasis(right, up, normal)).

See examples/asia-building/main.ts for a complete example pulling per-window data from a JSON descriptor.

Helper scripts

The examples/asia-building/tools/ folder has small Python scripts used to build the demo's atlases:

• detect_windows.py, extract_windows.py: pull window crops from a facade photo
• analyze_atlas.py: sanity-check cell layout and channel content
• glass_dirt.svg: source for the glass-dirt overlay used in the demo

They are unsupported, not packaged, and exist as references. Adapt or ignore.

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Limitations and roadmap

• No envmap / cubemap reflections on the glass area. Would require fresnel-modulated env sampling.
• No refraction distortion. A planned opt-in refractionStrength (default 0) would perturb the ray direction using the front normal map.
• One material per window mesh. Each window carries its own windowId / planeSize uniforms. For very high window counts, an instanced-attribute variant (single material, per-instance attributes) is on the radar.
• Pre-lit interior. The atlas is treated as already-shaded photography; scene lights do not relight the interior. This is by design; relighting fake rooms would defeat the cost saving the technique exists for.

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Development

npm install
npm run dev          # serves examples/asia-building on :5173
npm run build        # produces dist/ (the publishable package)
npm run build:demo   # produces dist-demo/ (static export of the demo)
npm run typecheck

Examples

• examples/asia-building/ — the full demo: 160 windows on a real building, cinematic camera, day/night palette, glass dirt, PBR curtains. What npm run dev serves and what powers the live demo.
• examples/minimal/ — single window plane, ~60 lines. The shortest runnable example for understanding the API surface.

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Credits

• Joost van Dongen, Interior Mapping: A new technique for rendering realistic buildings (2008).
• The Three.js team for MeshStandardMaterial and the onBeforeCompile hook this material extends.

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License

MIT.