A few years ago I blogged about gamma correction for png files. Here’s the very, very short version:
- PC and Mac monitors are calibrated differently. Dark tones on a PC appear darker than on a Mac. The curve of how colors are mapped to the monitor is the gamma correction curve, typically expressed as a number like 1.8 for Mac and 2.2 for PC. The higher the number, the more Gothic your dark tones.
- A png file can have a gamma value written into the file, which tells X-Plane (and anyone else) what kind of monitor the png was drawn on. This lets X-Plane brighten a png from a Mac when you are on a PC, and darken a png from a PC when you are on a Mac.
- If you leave off the gamma value on your png, we assume 1.8 (Mac) which can be bad if you’re a PC author.
While this is confusing, it was an improvement over the BMP situation (where everything was set up for a Mac and PC users had to simply crank their monitor brightness).
In version 9 we added a gamma correction setting to X-Plane. The setting you enter in the rendering settings is how “dark” your monitor is (bigger number = darker). We then attempt to compensate by lightening the textures more; thus a bigger number results in a lighter looking X-Plane (because you told us your monitor was dark and we tried to “fix it”).
There are two other developments since the original png situation which have unfortunately been a step backward in terms of X-Plane color correction.
DDS and Gamma
The handling of DDS and gamma is, to put it mildly, quite problematic. The problem is two-fold:
- DDS doesn’t actually have gamma information, so we can’t tag DDSes as having originated on Macs and PCs. So we assume a DDS is authored at a gamma of 1.8 (Mac). I think DDSTool correctly does a gamma correction when grinding files at other gammas.
- (If you are a real graphics programmer, please do not read this next sentence.) X-Plane attempts to adjust the color of the DDS in its compressed form. This is a big hack designed to keep framerate high, but it’s really not a very good idea. The result can be color distortion when a DDS is viewed at 2.2 gamma.
So that’s not good, but what happened next made things a lot worse.
Apple Goes Gothic
Apple adopted the sRGB color profile for OS X 10.6, which has a gamma curve of about 2.2. So now the situation with DDS is particularly ugly:
- All DDS are authored at a gamma of 1.8.
- All users are moving toward a display gamma of 2.2.
- X-Plane thus has to always color correct, but its color correction is low quality for performance reasons.
This is…very sad.
There are two things we can do about this:
- In the short term, we can provide post-decompression color correction. This will cost a (hopefully) small amount of framerate and improve color fidelity for users with 2.2 gamma. This is the kind of thing that any user with a modern card would want, but that we might make optional for users with very old hardware.
- In the long term, we can provide a gamma calibration in the text files that wrap DDS files so that authors can mark their DDS as already being 2.2. This will mean that for most users X-Plane won’t have to do any color correction at all.