For warmth and/or brightness = 0, we need to make sure that the
red/green/white LEDs are actually turned off, regardless of the values
for gain/offset.
By default, the ioctl interface is used for accessing the
frontlight. However, the ioctl interface has two problems: the overall
brightness seems to be lower (issue #2321), and it does not support
the natural light support from the Kobo Aura One (issue #2285). This
interface is the first step to fix both these issues.
From observing Nickel's behavior, it appears the white/green/red
intensities are roughly linear dependent on the 4th root of brightness
and warmth values. This leads to a very quick increase of the
intensities for lower brightness/warmth values, which then saturate
for larger ones. The values for gain/offset were determined by a
simple linear regression, but the plan is to create a widget so that
users may adapt these values to their liking.
