Radeon 10-bit Gamma


What it is

An update to the Radeon driver allows one to make a cscSetGamma control call and pass a 256-entry gamma table that contains two bytes (16 bits) per entry rather than the usual one byte. Only the upper 10 bits of the 16 bit entries are significant. More details are in the files you can download below.

What can I do with it?

This will probably be of primary interest to vision scientists, but could be useful for anyone who wants to provide very fine intensity changes on their displays, either in space or time. In vision science experimenters often want to measure the smallest change, or "threshold" in contrast that the subject can detect, and unfortunately in an 8-bit per channel system, there's no way to make changes along a single axis in color space fine enough to accurately measure the threshold. There are research-grade setups which provide up to 12 bits per channel color resolution, but they are often expensive or obsolete. The Radeon is a mainstream solution, is inexpensive and available now (costing about $150 in October 2001).


I learned that there were 10 bit DACs on the card in February 2001. Home theater enthusiasts and ATI PC driver engineers were extolling their virtues with regard to DVD playback quality in this thread on the AVS Forum. The 10-bit DACs are mentioned in passing on page 19 of the Radeon Digital Video white paper. I communicated with ATI's Mac driver people and requested 10-bit DAC support. After some back and forth on the best way to do so, they came up with a driver update and were very helpful and responsive. With the release of MacOS 9.2.1 (8/20/01) ATI and Apple made the extra gamma support available to everyone, but for OEM Radeons only, not retail versions. Having a retail version myself, I didn't think 9.2.1 contained the driver update. On 10/1/01 ATI released the September ROM Update which brought 10-bit gamma support to the retail cards. Wee ha!

The second generation Radeons (7000,7500,8500) have drivers that support 10-bit DACs out of the box, so 10-bit DAC support at ATI seems to be the default now (at least in the Mac drivers)!


Last updated 04/2003