[Nexus 5 Original Android Development] yorici:
Haha, perhaps my thought was DisPlay :)

And I make my own gamma images using Photoshop. Long story short: We need to make our own and the process is difficult to understand.

Long story:

The problem in general is that a gamma image consists of two layers. A background made up out of a grid. The DT-app alternates between RGB(0,0,0) and RGB(255,0,0) (for red gamma image). Then you have the second layer on top of that, consisting of a number of rectangles. The rectangles are each made up of plain colours, so not two colours like the background. Each rectangle represents a gamma value. For 255 gamma 2,2 would be 186 (255*0,5^(1/gamma)). So your gamma image basically consists out of three values: 0, 186 (and for the other gammas also some around 186) and 255. These three aren't controlled by all the 24 values, but by just 4. (186 being controlled by 10 and 11, 255 by value 1 and 0 by value 24)

If you calibrate your screen, you will only have gamma 2,2 on values 10 and 11. And actually not even that is 100% 2,2, because 186 is made up with a portion of value 11 and a portion of value 10.

Now you could easily think that you can calibrate other values, but there are some problems:

Values 10 and lower require an RGB value higher than 255, which is impossible.

Calibrating values 15 to 24 have lower RGB values. Therefore the rectangles have sometimes the same values, making no difference. Also lower RGB means darker and harder to see differences

Calibrating values 11 to 14 is possible, but to center gamma 2,2 around value 14, the formula becomes 110*0,5^(1/gamma). Now the rectangle with gamma 2,2 becomes 80. But the required background is 110. 110 is controlled by value 13 and value 14. Okay, you could calibrate value 13 first, but 13 depends in the same way on 11,12. This goes on till value 11 which depends on value 7 and 8. And we can't calibrate values 7 and 8 (yet).

For the first problem I thought of the following solution: If you would need a theoretical RGB value of 280, you could also use 255 and 35. The background just creates a halftone. And 255,35 creates the same halftone as 280 would. But the calibration is now dependent on another value we can not calibrate (for now). So problems persist.

Then I thought, the gamma of values 1 and 24 (responsible for RGB-255 and 0) are not gamma 2,2 either, instead a much higher and lower gamma. So I thought, let's change the values 7 and 8 (needed for calibration of value 11) to RGB-255 (Thus input is 0). This would create the same effect optically of full red and deep blacks. This actually worked for value 11. I managed to calibrate it. I also made values 10 and 12 RGB-255 thinking that RGB-175 (I made that gamma 2,2) was only affected by value 11. So changing value 10 and 12 would make it much more easier to see when value 11 is calibrated correctly.

But if I try this for other values, I get weird values. I started rethinking every step and I came to the conclusion that I still do not fully understand how this works. I don't think the calibration of value 11 was just dumb luck either, but I think I cannot calibrate value 12 using the same method.

Value 12 has the a background made up of RGB-196 and RGB-0. The difference between full red(255) and 196 is bigger than the 240 and 255. Also it doesn't help that RGB-196 is controlled by values 10&11. Value 11 affects gamma 2,3 and higher of value 12. In theory this shouldn't matter, but I found out that the RGB-values that are most affected by the registervalues are not solely affected by one registervalue. It is though in a ratio of 99% vs 1%.

This problem doesn't require a lot of mathematical knowledge, just some good brains. Thus if you have ideas, tell me. I'll answer your questions too if this story was too short :p. I have posted my findings in the Faux Gamma App thread. There it is probably better explained. So please help:)