sct - Set Color Temperature
This fork is mostly unmaintained, I recommend xsct instead.
Based on the simple sct utility written by Ted Unangst http://www.tedunangst.com/flak/post/sct-set-color-temperature
Incorporates sctd by Aaron Bieber and Joerg Jung.
How Does it Work?
From the original blog post (http://www.tedunangst.com/flak/post/sct-set-color-temperature ):
The mechanism redshift uses to change the color temperature is XRandR, the X Resize and Rotate protocol. There’s even an official xrandr configuration utility. It has a gamma option! But it’s almost wholly unsuitable for this purpose. Specifying an RGB gamma of 1.0:0.9:0.4 doesn’t make the screen “warm”. It turns gray into an ugly puke color, but leaves white just as bright and cold as before. I don’t just want to bend the gamma curve, I want to compress/truncate it such that there aren’t any blue=255 pixels.
The function which xrandr uses for this purpose is XRRSetCrtcGamma which has no documentation because why would it. The protocol documents that the request exists, but not much about its operation and the Xlib style C function takes different arguments. So here’s how that works.
You start by getting information for each Canadian Radio-television and Telecommunications Commission (CRTC) with the XRRGetCrtcInfo function. Then you need the size of the gamma, (XRRGetCrtcGammaSize), and memory (XRRAllocGamma). This gives you a structure with three arrays in it, suitably named red, green, and blue. The index for each array is the “input” color value. The value at each index is the “output” color value, always scaled between 0 and 65535. A common configuration would have a gamma size of 256 (8 bits), and a flat gamma ramp would then be index times 256 for each value. If we wanted to really tone down the blues, we might do index times 128 for that channel, which results in a white (255,255,255) pixel looking like a (255,255,128) pixel on an uncorrected screen.
Setting the color temp of the screen really only requires about 40 lines of C (80 or so all inclusive). sct is a crude utility which does roughly that. I’d say exactly that, but some of the calculations aren’t actually exact. In any case, it looks much, much, better than anything xrandr is capable of delivering. It takes temperature values in the range 1000 to 10000.