[geekmaster]

Originally Posted by hawhill

I don't see that we would have set an upper limit. In fact, in the "+" direction, the configuration value for "gamma" is increased. Doesn't make much sense, because geekmaster is right about that it should be the other way around. However, I think it might be a case of e-ink speciality that the highest pixel value means the darkest shade of gray.

To put things short: on "vol+" key, gamma is multiplied by 1.25, on "vol-" key it is multiplied by 0.8. So just count "+" presses and then replace the value you have already found in unireader.lua by 1.25^n(presses). Or just try some values without bothering for math.

This operation is done before mapping 8bpp grayscale to 4bpp grayscale, so the effect makes at least some sense.

This could be a configurable global setting in some future version of this software.

Doing gamma adjustments on bright areas is a valid operation, and I use it to prevent blooming (wash-outs in bright areas), by inverting an image and doing gamma adjustments on the negative, then inverting it back.

Traditional gamma adjustments primarily affect dark areas though, to compress the large linear brightness range between daytime and night into the fewest bits by exploiting non-linearity in the human eye. When applying an exponential curve to the inverted image, it is technically not "gamma", and this appears to be what you are adjusting because of the inverted color palette (0=white) on the K3 and earlier (and the K4 when booted from the main partition).

In all my new code, I use a non-inverted (0=black) color palette, and for kindles that use an inverted color palette (such as the K3) I use the "inverted update" ioctl() function.

Originally Posted by hawhill

... So just count "+" presses and then replace the value you have already found in unireader.lua by 1.25^n(presses). Or just try some values without bothering for math.

Except our poster probably wants you to change it for him and send the replacement files to him, in a nice and simple update*.bin file, not wanting to be bothered by "technical details" and all...