Backlight Dimmer

This is a small utility i use to linearize my backlight.

Generally PWM based backlights have the problem of strongly non-linear perceived brightness, unless this is already being comprensated for by your operating system. This utility assumes that the eye will respond logarithmically to the true light intensity, and "predistorts" the output signal using an exponential to achieve linear brightness scaling.

For this, we fit an exponential between 0 and 1 (later mapped to 0 and max_brightness respectively) with a scaling factor alpha, where x is the perceived brightness between 0 and 1 (0 to 100%):

y = f(x) = (exp(alpha * x) - 1) / (exp(alpha) - 1)

For the resulting f(x), f(0) = 0 and f(1) = 1 for all alpha, where alpha is a factor determining "how exponential" the curve appears. Values of around 3-4 for alpha usually work out well.

Because this is a monotonic function, we can easily calculate an inverse and use that to determine the current perceived brightness from the current PWM percentage:

x = g(y) = log(y * (exp(alpha) - 1) + 1) / alpha

Finally because we're only interested in updating the y value and don't actually need to know x, we can simplify y' = f(g(y) + delta) to:

y' = y * exp(alpha*delta) + (exp(alpha*delta) - 1) / (exp(alpha) - 1)

Finally this results in the following simple algorithm implemented in this tool:

• Read current and max backlight PWM values (brightness and max_brightness for amdgpu backlights respectively)
• Update y to y' as described above.
• Scale real brightness \in [0;1] to [0;`max_brightness] and update backlight.

Installation / Usage

Adjust the compile-time constants to suit your system. Then run the Makefile and store the binary in a location that's referenced by your path.

To increase the brightness of your backlight by "1%", run:

$ ./set_brightness +1

Conversely, to decrease the brightness by "5%":

$ set_brightness -5