[Nexus 5 Themes and Apps] mag01:
So after playing with a colorimeter I think I've got much better idea of how this thing works.

First, the series really start from highlights and end in shadows.

And now you can split the series into two parts.

1-12 - white to ~ 50% - these are "inverted" in regards to their values as higher value means darker color etc.

13-23 - ~50% to black

Now the scale isn't linear. In the middle of the range there are biggest steps while at both ends they're extremely small. There are about 6 values at both ends that cover the near white or near black values, while the remaining 5-6 values cover the majority of the range. I believe this could due to fine grain control over the tint of some mixed colors because that's exactly what they affect most.

When you want to directly modify the 0% and 100% levels you'll get immediate results with numbers 17 and 7, however you should always change these remaining 6 accordingly as well.

In any case, numbers at positions 7 - 17 will modify the curve the most. The remaining numbers don't deliver that much directly visible results, but they still affect other colors.

As for positives/negatives stuff - they both do the same thing, just normally you should change them both at the same time. But in the case you want to do smaller change than you can achieve by modifying any value by 1, you can try to modify only positive, or negative which will result in smaller (approx. 50% ?) change. But I wouldn't keep these values too far from each other, Better to keep them the same, or different by just 1.

See attached GN5-gamma-range.png where I tried to cover this all (more or less successfully :D ) The dashed lines in the right half show the original values 1-12 while the solid lines show them corrected in regards to their real effect.

It seems that the effect of changing any value in the series increases towards the end of the range. To do any significant change at the highlights side you have to increase/decrease each value more than when you want to do the same at the shadows side.

Another attachment covers the white point thing. This seems to be some sort of gamma + gain adjustment.

When you decrease it, the gamma increases as the curve is more and more bent downwards. But at the same time the maximum brightness slightly decreases which results on less bright highlights (especially white) and lower dynamic range.

When you increase it, the gamma decreases ad the curve straightens (and when you continues it can bend the other way). The maximum brightness increases until the technological maximum of the display is reached for each color, then clipping will start.

Few additional points

- default WP value 32 seems to be the best one. It provides the maximum (color-balanced) contrast ratio as it's already at the technological maximum for the red color. If you increase the WP further, the red color will start clipping in highlights and you only get green and blue scaled properly.

- some values for different settings of WP for my display (remember that your display may be different):

WP | gamma | contrast (not color-balanced) | color temperature

16 | 3.52 | 682:1 | 7800-5300

24 | 2.68 | 867:1 | 6500-6000

29 | 2.23 | 933:1 | 6300

32 | 1.98 | 963:1 | 6400

40 | 1.45 | 1053:1 | 6200-7200

Note the variations in color temperature. This is because the red color can't go any higher (and it even doesn't go lower much) while green and especially blue curves vary a lot. When you increase the WP value, the blue will start at lower level but will be reaching higher and higher levels towards the white (exceeding the red). When you decrease the WP value, the blue will start at higher level but will be reaching much lower levels (than red) towards the white. This causes that the average color temperature changes with WP, and when you go too far from the default it will also won't stay constant (or near-constant). While this can be compensated by modifying the curve directly (via those 23 numbers) you need to keep in mind that the red is already at its limit and you can't go any higher with it (at the highlights side)

However as a simple change of gamma, the often mentioned value 29 is still quite decent.

But if you want to do it properly, you should change the curve for each color. Then you may use the above description as a hint for how to do it properly.

Definitely changing just few numbers out of all 23 isn't the right approach as it will result in various partial color shifts, casts.. whatever.. also banding may appear etc.

And one more note - as the red is at its max. limit at the highlights side, the blue is already at its min. limit at the shadows side.