About Rosemaling

What is this?

Rosemaling is a physically based painting simulator. Ordinary painting apps store display colors (RGB) and blend them with arithmetic; that is why mixing blue and yellow in them gives grey. Rosemaling instead stores which real pigments, and how much of each, sit at every point of the canvas, and computes what that physical film of paint looks like — so blue + yellow makes green, thin paint glazes, thick paint hides, and a painting can be re-lit or examined like a real object.

Everything runs on your device, in the browser. After one visit it works fully offline and can be installed to a home screen; no image or document ever leaves your machine.

How it works

Every texel of the canvas holds pigment amounts — quantities of up to eight real paints plus a clear-medium channel — not a color. Appearance is computed from that state each frame:

• Measured pigments. Each paint in the libraries (Golden Matte Fluid and Heavy Body acrylics, and 40 classical oil pigments) carries measured absorption and scattering spectra — K(λ) and S(λ) — from published spectroscopy, sampled on a 31-band grid across the visible spectrum (extended into UV and near-IR for the technical views).
• Spectral Kubelka–Munk optics. Mixtures combine by concentration-weighted K and S; the renderer solves the finite-thickness Kubelka–Munk equations for the reflectance of the wet film over whatever lies beneath it — the ground, or dried paint layers. Thin, medium-rich films fall into a Beer–Lambert glaze regime; thick films hide. Subtractive mixing, hiding power, and glazing are consequences of the physics, not painted-on features.
• Two engines, one truth. A WebGPU compute engine runs the product; a pure-TypeScript CPU engine is the reference implementation the GPU kernels are validated against, texel for texel.
• Brush transport. Strokes exchange paint in both directions — the brush deposits and picks up — so brushes get dirty, wet paint smears into wet paint, and the palette surface mixes like a real palette. The brush's reservoir is spatial: an unevenly loaded brush paints unevenly.
• Relief and gloss. Paint has thickness; a height field shades impasto under a movable directional light, canvas weave catches dry-brush strokes, and surface gloss follows the medium content.
• Light × sensor rendering. The final color is the film's reflectance integrated against an illuminant and a set of sensor sensitivities. Swapping those matrices gives daylight, tungsten, UV-365 examination (with real fluorescence, including optical-brightener papers), IR reflectography, and a measured digital-camera rendering — one renderer, many instruments.

Using the Paint page

Getting paint onto the brush

1. Pick a palette (classical oils or a Golden acrylic line) — or press Edit palette… to assign any paint from any library to the eight wells.
2. Click a well to arm a squirt, then tap the palette (or canvas) to drop a glob of paint.
3. Stroke through the glob to load the brush — like a real palette. Strokes on the palette surface mix paint exactly as on the canvas.

Painting

• Size sets the brush radius (down to 1 px); Size ← pressure makes pressure drive the radius. With a pen, pressure and tilt are live; with a mouse, set pressure with the slider or scroll wheel.
• Round and flat brushes; the flat's blade can follow the stroke, hold a fixed angle, or follow pen tilt.
• The brush picks up what it touches — drag through wet paint and the next stroke carries it. Wipe partially unloads the brush; Rinse empties it.
• Glazing: Dip in medium charges the brush with clear medium; heavily diluted paint goes down as a transparent glaze. Glaze over a dried layer to deepen color the way real glazing does.

Layers and drying

• Dry layer commits the wet paint to the dried stack — it stops smearing and becomes a substrate you can glaze over.
• Dried layers can be undried (made editable again), deleted, or reordered; nothing is ever flattened to pixels.
• The ground select swaps the substrate — white gesso, black, cream paper, umber-toned, or an optically brightened paper (which glows under UV).

Looking at the painting

• Light azimuth/elevation move the raking light; impasto and canvas texture respond. Toggle Relief and Texture off for a flat view.
• Spectral view re-renders the same paint under D65 daylight, tungsten, D50, UV-365 examination, IR reflectography, or a real camera's sensitivities. A provenance note warns when a view depends on assumed (rather than measured) data.
• Thickness mode shows the film-height map.

Documents

• Undo/redo: buttons or Ctrl/Cmd+Z, Ctrl/Cmd+Shift+Z.
• Save/Load writes a .rosem file — the full pigment state of every layer, reopenable and re-lightable, not a flat image. Export PNG renders the current view.
• Canvas size adapts to your hardware by default; larger sizes cost real GPU memory.

The other pages

The Mixing lab is the physics workbench: pick two or three real paints, sweep the mixing ratio, and compare spectral Kubelka–Munk mixing against Mixbox and naive RGB interpolation, with the underlying spectra and a virtual drawdown card. The Feel bench is a development tool that replays a deterministic stroke set under different feel parameters so brush tuning is comparative rather than vibes.

Accuracy & provenance

Rosemaling prefers measured data and says so when it doesn't have it. Each pigment's spectra are labeled by provenance (measured / derived / assumed, per spectral range); technical views display a reliability warning when the materials on screen lean on assumed data outside their measured range. The simulation is a model, not a metrological instrument: Kubelka–Munk is an approximation, some scattering coefficients are derived under stated assumptions, and feel parameters are tuned by hand. Where spectral data could not be found, plausibility wins over false precision — and the UI tells you.

Credits, copyright & licensing

Rosemaling is a personal, non-commercial project. The source code is © its authors, all rights reserved; no open-source license is granted at this time. The simulator is built on generously shared measurement data — full provenance, exact sources, and terms for every dataset are recorded in the repository (data/*/SOURCE.md).

• Golden acrylic paint spectra — two-constant K/S for Golden Matte Fluid (Okumura 2005, RIT MS thesis) and Golden Heavy Body (Berns 2016, CIC24), courtesy of Golden Artist Colors and R. S. Berns / RIT Munsell Color Science Laboratory (Mellon "Art Spectral Imaging" project). Measured reflectance from the Berns 2022 Golden acrylics release.
• Classical oil pigments — IFAC-CNR Fiber Optics Reflectance Spectra of Pictorial Materials database (Aldrovandi et al.), 270–1700 nm; digitized from the published plots. Modern watercolours — IFAC-CNR MoWCReS database. Both cited with thanks to IFAC-CNR, Florence.
• Pigments Checker reference spectra — Cultural Heritage Science Open Source (CHSOS, A. Cosentino).
• Colorimetry — CIE 1931 color-matching functions and standard illuminants from the official CIE datatables (CIE 018:2019 / ISO/CIE 11664).
• Camera sensitivities — Jiang, Liu, Gu & Süsstrunk, "What is the Space of Spectral Sensitivity Functions for Digital Color Cameras?" (WACV 2013), via Zenodo — CC BY-NC-SA 4.0.
• Optical constants (n, k) — refractiveindex.info, CC0 1.0 (public domain).
• Fluorescence data — PhotochemCAD and the Oregon Medical Laser Center (OMLC) spectra collections; FPbase (CC BY-SA 4.0); optical-brightener curves digitized from AAT Bioquest; UV-fluorescence literature by Pelagotti et al. 2005, Pronti et al. 2017, and Caccia et al. 2023.
• CMYK ink characterization — FOGRA51 (Fogra / ICC).
• Mixbox © Secret Weapons — used strictly as a non-commercial calibration reference in the mixing lab under CC BY-NC 4.0.

"Golden" and other paint names are trademarks of their respective owners and are used here to identify the measured materials; no affiliation or endorsement is implied. Simulated renderings are approximations of the named paints, not color-accurate specifications. If you are a rights holder of any dataset above and want a correction, please open an issue on the repository.