CGSD - Gamma Correction Explained

Note: This diagram will not look very good in 8-bit color and your browser must support tables for the correct layout.

This diagram shows the cumulative effect of each of the gamma steps
Input Signal Software Correction Required Built In Hardware Correction Output Signal
Sun & PC
no
Correction
. . . .
Sun & PC
full
Software
Correction
. .
Macintosh
Hardware
Correction
Only
. .
Macintosh
Hardware
and
Software
Correction
SGI
Hardware
Correction
Only
. .
SGI
Hardware
and
Software
Correction

The table above shows the effects of various gamma corrections for a sample input. The sample input is a linear greyscale which was first made in Lab color space in Adobe Photoshop. The input is changed by software gamma correction, hardware gamma correction and, at the far right, by the the CRT display. This document assumes that the CRT has a luminance to input voltage response curve which matches a 2.5 power function.

The Software Correction Required column shows what level of gamma correction should be applied in software on the various platforms to achieve the correct output signal. In this case, the correct output means that it exactly matches the input signal - the linear greyscale.

The Built-In Hardware Correction column refers to the gamma correction done by the hardware on each platform. It is stated that Suns and PC's (Intel Standard) have no built-in hardware correction. This is the standard for these platforms, but certain graphics cards on each platform may perform some gamma correction.

The Output Signal column refers to the luminance to input voltage response curve as described above.

Note: The greyscale images shown above could themselves require gamma correction for display on your monitor. They were design to be viewed on a system that does no gamma correction in its hardware (such as a Sun or a PC) If you are viewing these images on a Macintosh or an SGI or have some other system (including Sun and PC) configured differently, they will appear a little too light (bleached out). Their purpose is mostly to illustrate what happens in the various steps relative to each other, and not as absolute representations of what you will on each system when viewed from your system. The only solution to this would be for us to put up many different copies corrected for different gamma settings and ask you which one you want to view (as we do with our Texture Library)

Again, we would like to thank Charles Poynton. Please see references to him at the Gamma Correction Home Page.