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[RFC] backlight: convert linear to gamma float brightness #6

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massonju wants to merge 1 commit into SoMainline:aosp-12.0 from massonju:brightness

Commits on Feb 7, 2023

backlight: convert linear to gamma float brightness

The brightness controller in Android frameworks convert initial value
from the slider to a new value not linear [1].
The value is modified by convertGammaToLinearFloat.

The current implementation in light HAL doesn't take account the none
linearity of this incoming value, these values are logarithmic [2].
Thus the brightness of the panel is very low most of the time.

In order to get back a linear value, one solution is to re-convert the
incoming value to a linear form: convertLinearToGammaFloat.

The implementation convertLinearToGammaFloat has been highly inspired
by the one in BrightnessUtils [3].

[1] https://android.googlesource.com/platform/frameworks/base/+/refs/tags/android-12.0.0_r34/packages/SystemUI/src/com/android/systemui/settings/brightness/BrightnessController.java#354

[2] https://tunjid.medium.com/reverse-engineering-android-pies-logarithmic-brightness-curve-ecd41739d7a2

[3] https://android.googlesource.com/platform/frameworks/base/+/refs/tags/android-12.0.0_r34/packages/SettingsLib/src/com/android/settingslib/display/BrightnessUtils.java#129

Signed-off-by: Julien Masson <jmasson@baylibre.com>

massonju committed Feb 7, 2023