Reference grating can also be omitted and its effect can be simulated in software. Methods are relatively computationally expensive. Shape from Shading Methods Methods based on shape from shading employ photometric stereo techniques to produce depth measurements. Using a single camera, two or more images are taken of an object in a fixed position but under different lighting conditions. By studying the changes in brightness over a surface and employing constraints in the orientation of surfaces, certain depth information may be calculated.
Methods only work well on objects with uniform surface texture.
It is difficult to infer absolute depth, and only surface orientation is easily inferred. Methods are mostly used when it is desired to extract surface shape information. The primary computational problem of stereoscopy is to find the correspondence of various points in the two images. Both of these tasks are non-trivial and computationally complex. Passive stereo may not produce depth maps within a reasonable time.