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      1 /*
      2  * Copyright (C) 2018 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.android.settingslib.display;
     18 
     19 import android.util.MathUtils;
     20 
     21 public class BrightnessUtils {
     22 
     23     public static final int GAMMA_SPACE_MAX = 1023;
     24 
     25     // Hybrid Log Gamma constant values
     26     private static final float R = 0.5f;
     27     private static final float A = 0.17883277f;
     28     private static final float B = 0.28466892f;
     29     private static final float C = 0.55991073f;
     30 
     31     /**
     32      * A function for converting from the gamma space that the slider works in to the
     33      * linear space that the setting works in.
     34      *
     35      * The gamma space effectively provides us a way to make linear changes to the slider that
     36      * result in linear changes in perception. If we made changes to the slider in the linear space
     37      * then we'd see an approximately logarithmic change in perception (c.f. Fechner's Law).
     38      *
     39      * Internally, this implements the Hybrid Log Gamma electro-optical transfer function, which is
     40      * a slight improvement to the typical gamma transfer function for displays whose max
     41      * brightness exceeds the 120 nit reference point, but doesn't set a specific reference
     42      * brightness like the PQ function does.
     43      *
     44      * Note that this transfer function is only valid if the display's backlight value is a linear
     45      * control. If it's calibrated to be something non-linear, then a different transfer function
     46      * should be used.
     47      *
     48      * @param val The slider value.
     49      * @param min The minimum acceptable value for the setting.
     50      * @param max The maximum acceptable value for the setting.
     51      * @return The corresponding setting value.
     52      */
     53     public static final int convertGammaToLinear(int val, int min, int max) {
     54         final float normalizedVal = MathUtils.norm(0, GAMMA_SPACE_MAX, val);
     55         final float ret;
     56         if (normalizedVal <= R) {
     57             ret = MathUtils.sq(normalizedVal / R);
     58         } else {
     59             ret = MathUtils.exp((normalizedVal - C) / A) + B;
     60         }
     61 
     62         // HLG is normalized to the range [0, 12], so we need to re-normalize to the range [0, 1]
     63         // in order to derive the correct setting value.
     64         return Math.round(MathUtils.lerp(min, max, ret / 12));
     65     }
     66 
     67     /**
     68      * A function for converting from the linear space that the setting works in to the
     69      * gamma space that the slider works in.
     70      *
     71      * The gamma space effectively provides us a way to make linear changes to the slider that
     72      * result in linear changes in perception. If we made changes to the slider in the linear space
     73      * then we'd see an approximately logarithmic change in perception (c.f. Fechner's Law).
     74      *
     75      * Internally, this implements the Hybrid Log Gamma opto-electronic transfer function, which is
     76      * a slight improvement to the typical gamma transfer function for displays whose max
     77      * brightness exceeds the 120 nit reference point, but doesn't set a specific reference
     78      * brightness like the PQ function does.
     79      *
     80      * Note that this transfer function is only valid if the display's backlight value is a linear
     81      * control. If it's calibrated to be something non-linear, then a different transfer function
     82      * should be used.
     83      *
     84      * @param val The brightness setting value.
     85      * @param min The minimum acceptable value for the setting.
     86      * @param max The maximum acceptable value for the setting.
     87      * @return The corresponding slider value
     88      */
     89     public static final int convertLinearToGamma(int val, int min, int max) {
     90         // For some reason, HLG normalizes to the range [0, 12] rather than [0, 1]
     91         final float normalizedVal = MathUtils.norm(min, max, val) * 12;
     92         final float ret;
     93         if (normalizedVal <= 1f) {
     94             ret = MathUtils.sqrt(normalizedVal) * R;
     95         } else {
     96             ret = A * MathUtils.log(normalizedVal - B) + C;
     97         }
     98 
     99         return Math.round(MathUtils.lerp(0, GAMMA_SPACE_MAX, ret));
    100     }
    101 }
    102