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

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

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20 changes: 18 additions & 2 deletions Lights.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,7 @@
#include <android-base/logging.h>
#include <dirent.h>
#include <fstream>
#include <math.h>

static const std::string BACKLIGHT_DIR = "/sys/class/backlight";
static const std::string LEDS_DIR = "/sys/class/leds";
Expand Down Expand Up @@ -59,11 +60,26 @@ static int32_t rgbToBrightness(int32_t color)
return (77 * r + 150 * g + 29 * b) >> 8;
}

static float convertLinearToGammaFloat(float val)
{
const float r = 0.5;
const float a = 0.17883277;
const float b = 0.28466892;
const float c = 0.55991073;
// HLG normalizes to the range [0, 12] rather than [0, 1]
float normalizedVal = val * 12;

if (normalizedVal <= 1.0)
return sqrt(normalizedVal) * r;
else
return a * log(normalizedVal - b) + c;
}

ndk::ScopedAStatus Backlight::setLightState(const HwLightState &state) const
{
auto brightness = rgbToBrightness(state.color);
// Adding half of the max (255/2=127) provides proper rounding while staying in integer mode:
brightness = (brightness * maxBrightness + 127) / 255;
brightness = convertLinearToGammaFloat((float)brightness / 255) * maxBrightness;

if (state.brightnessMode == BrightnessMode::LOW_PERSISTENCE)
LOG(ERROR) << "TODO: Implement Low Persistence brightness mode";
LOG(DEBUG) << "Changing backlight to level " << brightness << "/" << maxBrightness;
Expand Down