Add Midtone filter

[individ-divided]

Hello,

I've implemented a midtone filter to LaBLEr. I think this is useful for the LaBLEr app because it can compensate for the fact that a 50% dot coverage isn't 50% gray. One explanation is the non-linear effects of thermal exposure (duration and power). As a quick demonstration, I've inserted a picture below that shows the effect of trying to reduce the darkness of a linear gradient.

The left column shows the non-filtered linear gradient. With the new filter, it is possible to have the gradient remain gradual for the whole span while increasing its brightness (middle column). The right-most column is the original Contrast filter, for comparison.

Filter description
Move the highlights and shadows by adjusting the midpoint.

If the midpoint parameter is less than 50, the filter brightens the midtones. A value greater than 50 will darken the midtones. It applies a non-linear power curve (gamma) filter so that the midpoint is shifted to the new position.

This is a common image processing filter. You can do it in Gimp for instance, via the Levels dialog box. (Use the gray slider.)

This is my first Kotlin experience.

Best,
David