Drag the four pins to land the beam on the poster — a
homography, the same projective map a 3D engine computes for a flat surface.
Light is albedo × (ambient + lamp × slide) in linear space.
Hardness is the one that decides whether this reads as light or as ink:
a threshold quantises, and light never does. Focus is lens defocus,
Scatter is light diffusing sideways inside the paper fibres.