“A tutorial on compensation tables” by Catmull

The generation of computer-shaded pictures requires that a program calculate intensities for each pixel. These intensities are then viewed on a monitor or transferred to film. It is desirable that the intensity values on the monitor or the reflectance values on the film have a linear relationship with the original calculated intensities both to give the user fine control over the appearance of the object and to do anti-aliasing properly. However, after the calculated intensity values are sent to the DAC’c some nonlinear distortions occur both in the CRT and on film. We can compensate for this by adjusting the intensity number just before digital-to-analog conversion. This compensation can be accomplished by a table lookup using the intensity as an index into the table and the corresponding value as the new intensity. Fortunately, most hardware manufacturers have made table lookup features part of their equipment. However, the entries in the table must still be generated. The methods for doing this are not well known in the computer graphics community. This paper presents an algorithm for generating a compensation table.

1. Heyl, Jack, A Digital Correction for Cross-absorption Photographic Dyes, master thesis, Dept. of Comp. Sci., Univ. of Utah, 1974
2. Frei, Werner, Visual and Density Correction for CRT Film Recording, U.S.C. Image Processing Institute Report 620, Sept. 1975, p. 100-108.