details.md

How it works (detailed)

Current OLED screens have a separate brightness control input to control brightness at the highest input signal. If we want to display white at a brightness of 20 cd/m², we can do either of the following:

1. Adjust the brightness control so that the brightness at the highest input signal is 20 cd/m², then use the highest signal ("white") as input;
2. With a higher brightness control input, say 160 cd/m², use a lower signal ("gray") as input to produce 12.5% of maximum brightness (20 cd/m² m²) output.

Some OLED screens use a higher PWM frequency and duty cycle in the latter case, which some users feel helps reduce discomfort when viewing the screen for a long time or in dark environments.

The relationship between signal level and output brightness is not linear, so the second method is not simply multiplying the signal by a factor. However, given the following premises, we can achieve precise control:

1. The Android system assumes that the screen has an sRGB response curve;
2. Based on the sRGB response curve, the system framework and HAL provide RGB matrix transformation in linear space;
3. Starting from Android 12, the system framework and HAL interface support absolute brightness control in cd/m² (or nits).