Programmable optical grids can be projected directly on the surface of unpolished test-pieces to realize an absolute measurement method called `coded light approach' to obtain a full depth- image, requiring only a small number of projections. In combination with the phase shifting method subpixel accuracy is achieved. Geometry-checking and controlling the surface quality of objects with mirror-like surfaces is done with indirect illumination. Using a line-projector, stripes are projected to a diffusely reflecting surface, e.g. a wall. The test piece is positioned so that the lines are seen indirectly by the camera, reflected in the surface.
Henning G.E. Wolf,
"Programmable optical grids: new possibilities for 2D and 3D image processing", Proc. SPIE 1989, Computer Vision for Industry, (17 December 1993); doi: 10.1117/12.164880; https://doi.org/10.1117/12.164880