Watercolor Backrun Lab simulates the backrun (also called 'bloom' or 'cauliflower') effect in watercolor painting — the organic, fractal-edged shapes that form when wet paint flows back into a drying wash. These happy accidents are prized by watercolorists for their natural, unpredictable beauty.
The shader creates backrun blooms using FBM domain-warped shapes that expand outward with fractal edges. As the simulated wash dries, pigment accumulates at the advancing edge of the backrun front, creating the characteristic dark rim with lighter interior. Paper granulation texture modulates pigment deposition — paint settles into the paper's valleys while skipping over peaks. Multiple layers are composited using glazing (transparent layering) to build depth.
Animated drying fronts show the temporal progression from wet-on-wet blooming to the final dried state. Interactive controls adjust pigment concentration, paper wetness, granulation intensity, and bloom count.