Methods and apparatus for simulating fluid motion using procedural shape growth. In a vector-based, fluid motion simulation technique, fluid location may be defined by groups of one or more polygons deposited on a digital canvas. Two or more polygons may overlap. The polygons may be semitransparent. To simulate fluid motion, vertices that specify the edges of each deposited polygon are independently moved. By moving the vertices, a polygon may grow, and fluid motion effects may be simulated, including but not limited to directional flow and blending effects. A randomization technique may be applied to the movement at each vertex to simulate the non-uniform spreading of fluids. Overlapped polygons may be blended with overlapping polygons to simulate the mixing of fluids. The technique may be applied, for example, in watercolor painting simulation, where groups of one or more polygons are deposited using brush strokes.