Gamma Correction And The 10-Bit Dac; Gamma Correction - Planar DX/PCI User Manual

Gamma Correction and the 10-Bit DAC | 5

Gamma Correction and the 10-Bit DAC
All DOME display controllers use a 10-bit DAC in their
video output circuitry. Nearly all competitive display
boards use 8-bit DACs.
The DAC converts the digital image data stored in the
board's video memory to an analog signal that drives the
display. An 8-bit DAC can send no more than 256 different
analog values to the display, while a 10-bit DAC can provide
up to 1024 values. An 8-bit DAC can therefore result in as
much as a 30% reduction in gray shades available to display
an image. With such a reduction, critical image details can
be lost.
Displays do not produce a linear response across the full
range of pixel values. If there are 256 possible analog values,
the change in luminance from value 0 (zero) to value 1 (one)
differs from the change of value 127 to 128. Likewise,
doubling the pixel value sent to the display does not
precisely double the brightness emitted.

Gamma correction

This nonlinearity can be corrected using a method known as
gamma correction. With this technique, the DAC's lookup
table compensates for the nonlinearity by adjusting the
values sent to the display.
An 8-bit DAC can produce only 256 such values, and a
nonlinear translation must sacrifice some of these values.
For this reason, gamma correction performed with an 8-bit
DAC sends substantially fewer than 256 distinct values to
the display.