Setting color brightness on Linux/Xorg

Question

Is there any command (or API) to set X.Org/Linux color brightness?

In other words, I need something as handy as the xgamma command but for changing RGB brightness real-time.

Is this possibile?

Answer 1

Use the XF86VidMode* family of functions.

#include <X11/Xlib.h>
#include <X11/extensions/xf86vmode.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    Display *display;
    int screen;
    int major, minor;
    int i;
    XF86VidModeGamma orig;

    display = XOpenDisplay(NULL);
    if (!display) return -1;
    screen = DefaultScreen(display);
    if (!XF86VidModeQueryVersion(display, &major, &minor)
            || major < 2 || major == 2 && minor < 0
            || !XF86VidModeGetGamma(display, screen, &orig)) {
        XCloseDisplay(display);
        return -1;
    }

    for (i = 0; i <= 32; i++) {
        XF86VidModeGamma gamma;
        gamma.red = exp2f(2 - fabs(i - 16) / 4);
        gamma.green = gamma.red;
        gamma.blue = gamma.red;
        if (!XF86VidModeSetGamma(display, screen, &gamma)) break;
        printf("gamma: %f %f %f", gamma.red, gamma.green, gamma.blue);
        if (!XF86VidModeGetGamma(display, screen, &gamma)) break;
        printf(" -> %f %f %f\n", gamma.red, gamma.green, gamma.blue);
        sleep(1);
    }
    XF86VidModeSetGamma(display, screen, &orig);
    XF86VidModeGetGamma(display, screen, &orig);

    XCloseDisplay(display);
    return 0;
}

This brings the gamma from 0.25 to 4.0 and back, and then restores the original gamma.

Or you could just repeatedly call system("xgamma -gamma %f"), with pretty much the same results.