Surface texture: canvas weave, watercolour paper, stucco, brushed metal and eleven more. The picture is not distorted at all, only lit, as though it had been printed onto that surface and a light were raking across it.
A height pattern is generated, the slope is measured by comparing four neighbouring points, and that slope decides which parts catch the light and which fall into shadow.
The fifteen patterns are built quite differently from one another. Canvas multiplies two waves crossing at right angles to weave a grid. Crosshatch does the same on the diagonal. Paper stacks a few rounds of creased noise. The material surfaces use noise with deliberately uneven cell shapes, stretched long in one direction, so fibres emerge from the structure of the pattern rather than from smearing it afterwards.