Architecture

Layers

src/
  domain/          pure model and rules: tiles, document, operations, history, brush engine, raster, text
  application/     use cases and ports: runtime, document, view, tool contract, collaboration, export
  infrastructure/  port implementations: WebGPU, Canvas2D text, file codec, exporters, input, worker bridge, transports
  tools/           ITool implementations; they see the engine only through ToolContext
  index.ts, worker.ts, tools.ts, collab.ts, exporting.ts, tiles.ts
                   entry points: peinten, peinten/worker, peinten/tools, peinten/collab, peinten/exporting,
                   peinten/tiles

Dependencies point inward only. Biome's noRestrictedImports enforces this for domain, application, tools and infrastructure; only the entry points may import every layer. domain is also type-checked without DOM or WebGPU types. Where the domain needs the outside world, it defines a port and receives an implementation: FilterExecutor for GPU filters, ColorMixer for pigment mixing, TextRasterizer for text.

Pixels and state

Pixels are premultiplied RGBA in libmypaint's fix15 format (1 << 15 is 1.0), in 64 × 64 tiles of Uint16Array. Selections use single-channel tiles of the same size, so the dab code can paint into either.

Committed tiles are immutable. A stroke copies a tile the first time it touches it and writes into the copy. Comparing tile references is enough to find what changed, which drives GPU uploads and thumbnails. History keeps whole immutable states, so rolling back only swaps references.

The document state is an immutable value updated only by applying operations. While a stroke is in progress, its working tiles live in a separate StrokeSession and shadow the committed tiles on screen; the state changes once, when the stroke commits. Strokes that collaborators are still drawing are replayed into working tiles of their own, which shadow the committed tiles the same way.

Rendering

The layer tree is flattened on the CPU into a small stack program: push a group, blend a layer, pop. A compute shader runs it per dirty tile into an rgba16float composite. Adjustment layers split the program into stages with the filter pass in between. The display pass draws the composite through the viewport transform with a checkerboard, marching ants or the quick mask tint, then the tools' vector overlay.

History and ordering

The timeline keeps batches with the state before and after each one. Confirmed batches form a prefix in sequencer order; pending local batches follow. When something lands in the middle (a peer commit, an undo, a rejected batch), the timeline rolls back to the earliest affected entry and replays the rest. Undo is a set-op-enabled operation, so it travels to collaborators like any other edit.

Worker bridge

peinten/worker uses comlink. Snapshots arrive as whole objects and are merged into the previous ones on the main thread to keep the identity of unchanged parts. Canvases are transferred once and reused by id.