Watercolour text
Type a title and watch drops of water land on the painted letters. The water re-wets the paint and pushes it outward. It wicks along the paper's fibres and frays the edges, and as it dries it leaves a dark rim where the pigment piled up. Click to drop water yourself; drag to trail it.
What the water does
- Re-wetting. Dry watercolour is not glued down. Water dissolves part of it back into suspension, so a drop lifts paint and carries it.
- The dark rim. A wet patch evaporates fastest at its edge. Water flows outward to replace what is lost there, and brings pigment with it. This is the same flow that makes a coffee ring, and it is why watercolour washes dry with a darker outline.
- Fraying. Paper is a mat of fibres. Water creeps along them by capillary action, faster along some fibres than others, so the wet front breaks into fingers and carries a little pigment into the cracks.
- Runs. A drop that slides drags paint along its path and leaves a smear.
How it works
It runs on the GPU as a simplified version of the watercolour model of Curtis, Anderson, Seims, Fleischer and Salesin ("Computer-Generated Watercolor", SIGGRAPH 1997). A thin layer of water flows downhill over the paper's bumps, with conservative fluxes, so water and floating pigment are neither created nor lost. A second store of water soaks into the paper and wicks between neighbouring cells, but only out of cells wet enough to give. Pigment moves between two layers: floating in the water, and settled on the paper. It lifts off wherever the paper is wet and settles again as the water thins. Evaporation is faster at the rim of each wet patch, and that drives the outward flow. The paper itself is generated noise: a height for its grain, a fibre map for wicking, and a capacity for how much water it holds.
It is a look-alike, not a laboratory model. The rates were tuned by eye, and the water is shown as a lens with a glint so you can see where it is.