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/*
* Copyright (C) 2018 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.android.settingslib.display;
import android.util.MathUtils;
public class BrightnessUtils {
public static final int GAMMA_SPACE_MIN = 0;
public static final int GAMMA_SPACE_MAX = 65535;
// Hybrid Log Gamma constant values
private static final float R = 0.5f;
private static final float A = 0.17883277f;
private static final float B = 0.28466892f;
private static final float C = 0.55991073f;
/**
* A function for converting from the gamma space that the slider works in to the
* linear space that the setting works in.
*
* The gamma space effectively provides us a way to make linear changes to the slider that
* result in linear changes in perception. If we made changes to the slider in the linear space
* then we'd see an approximately logarithmic change in perception (c.f. Fechner's Law).
*
* Internally, this implements the Hybrid Log Gamma electro-optical transfer function, which is
* a slight improvement to the typical gamma transfer function for displays whose max
* brightness exceeds the 120 nit reference point, but doesn't set a specific reference
* brightness like the PQ function does.
*
* Note that this transfer function is only valid if the display's backlight value is a linear
* control. If it's calibrated to be something non-linear, then a different transfer function
* should be used.
*
* @param val The slider value.
* @param min The minimum acceptable value for the setting.
* @param max The maximum acceptable value for the setting.
* @return The corresponding setting value.
*/
public static final int convertGammaToLinear(int val, int min, int max) {
final float normalizedVal = MathUtils.norm(GAMMA_SPACE_MIN, GAMMA_SPACE_MAX, val);
final float ret;
if (normalizedVal <= R) {
ret = MathUtils.sq(normalizedVal / R);
} else {
ret = MathUtils.exp((normalizedVal - C) / A) + B;
}
// HLG is normalized to the range [0, 12], so we need to re-normalize to the range [0, 1]
// in order to derive the correct setting value.
return Math.round(MathUtils.lerp(min, max, ret / 12));
}
/**
* Version of {@link #convertGammaToLinear} that takes and returns float values.
* TODO(flc): refactor Android Auto to use float version
*
* @param val The slider value.
* @param min The minimum acceptable value for the setting.
* @param max The maximum acceptable value for the setting.
* @return The corresponding setting value.
*/
public static final float convertGammaToLinearFloat(int val, float min, float max) {
final float normalizedVal = MathUtils.norm(GAMMA_SPACE_MIN, GAMMA_SPACE_MAX, val);
final float ret;
if (normalizedVal <= R) {
ret = MathUtils.sq(normalizedVal / R);
} else {
ret = MathUtils.exp((normalizedVal - C) / A) + B;
}
// HLG is normalized to the range [0, 12], ensure that value is within that range,
// it shouldn't be out of bounds.
final float normalizedRet = MathUtils.constrain(ret, 0, 12);
// Re-normalize to the range [0, 1]
// in order to derive the correct setting value.
return MathUtils.lerp(min, max, normalizedRet / 12);
}
/**
* A function for converting from the linear space that the setting works in to the
* gamma space that the slider works in.
*
* The gamma space effectively provides us a way to make linear changes to the slider that
* result in linear changes in perception. If we made changes to the slider in the linear space
* then we'd see an approximately logarithmic change in perception (c.f. Fechner's Law).
*
* Internally, this implements the Hybrid Log Gamma opto-electronic transfer function, which is
* a slight improvement to the typical gamma transfer function for displays whose max
* brightness exceeds the 120 nit reference point, but doesn't set a specific reference
* brightness like the PQ function does.
*
* Note that this transfer function is only valid if the display's backlight value is a linear
* control. If it's calibrated to be something non-linear, then a different transfer function
* should be used.
*
* @param val The brightness setting value.
* @param min The minimum acceptable value for the setting.
* @param max The maximum acceptable value for the setting.
* @return The corresponding slider value
*/
public static final int convertLinearToGamma(int val, int min, int max) {
return convertLinearToGammaFloat((float) val, (float) min, (float) max);
}
/**
* Version of {@link #convertLinearToGamma} that takes float values.
* TODO: brightnessfloat merge with above method(?)
* @param val The brightness setting value.
* @param min The minimum acceptable value for the setting.
* @param max The maximum acceptable value for the setting.
* @return The corresponding slider value
*/
public static final int convertLinearToGammaFloat(float val, float min, float max) {
// For some reason, HLG normalizes to the range [0, 12] rather than [0, 1]
final float normalizedVal = MathUtils.norm(min, max, val) * 12;
final float ret;
if (normalizedVal <= 1f) {
ret = MathUtils.sqrt(normalizedVal) * R;
} else {
ret = A * MathUtils.log(normalizedVal - B) + C;
}
return Math.round(MathUtils.lerp(GAMMA_SPACE_MIN, GAMMA_SPACE_MAX, ret));
}
}
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