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_MIN = 0;
24     public static final int GAMMA_SPACE_MAX = 65535;
25 
26     // Hybrid Log Gamma constant values
27     private static final float R = 0.5f;
28     private static final float A = 0.17883277f;
29     private static final float B = 0.28466892f;
30     private static final float C = 0.55991073f;
31 
32     /**
33      * A function for converting from the gamma space that the slider works in to the
34      * linear space that the setting works in.
35      *
36      * The gamma space effectively provides us a way to make linear changes to the slider that
37      * result in linear changes in perception. If we made changes to the slider in the linear space
38      * then we'd see an approximately logarithmic change in perception (c.f. Fechner's Law).
39      *
40      * Internally, this implements the Hybrid Log Gamma electro-optical transfer function, which is
41      * a slight improvement to the typical gamma transfer function for displays whose max
42      * brightness exceeds the 120 nit reference point, but doesn't set a specific reference
43      * brightness like the PQ function does.
44      *
45      * Note that this transfer function is only valid if the display's backlight value is a linear
46      * control. If it's calibrated to be something non-linear, then a different transfer function
47      * should be used.
48      *
49      * @param val The slider value.
50      * @param min The minimum acceptable value for the setting.
51      * @param max The maximum acceptable value for the setting.
52      * @return The corresponding setting value.
53      */
convertGammaToLinear(int val, int min, int max)54     public static final int convertGammaToLinear(int val, int min, int max) {
55         final float normalizedVal = MathUtils.norm(GAMMA_SPACE_MIN, GAMMA_SPACE_MAX, val);
56         final float ret;
57         if (normalizedVal <= R) {
58             ret = MathUtils.sq(normalizedVal / R);
59         } else {
60             ret = MathUtils.exp((normalizedVal - C) / A) + B;
61         }
62 
63         // HLG is normalized to the range [0, 12], so we need to re-normalize to the range [0, 1]
64         // in order to derive the correct setting value.
65         return Math.round(MathUtils.lerp(min, max, ret / 12));
66     }
67 
68     /**
69      * Version of {@link #convertGammaToLinear} that takes and returns float values.
70      * TODO(flc): refactor Android Auto to use float version
71      *
72      * @param val The slider value.
73      * @param min The minimum acceptable value for the setting.
74      * @param max The maximum acceptable value for the setting.
75      * @return The corresponding setting value.
76      */
convertGammaToLinearFloat(int val, float min, float max)77     public static final float convertGammaToLinearFloat(int val, float min, float max) {
78         final float normalizedVal = MathUtils.norm(GAMMA_SPACE_MIN, GAMMA_SPACE_MAX, val);
79         final float ret;
80         if (normalizedVal <= R) {
81             ret = MathUtils.sq(normalizedVal / R);
82         } else {
83             ret = MathUtils.exp((normalizedVal - C) / A) + B;
84         }
85 
86         // HLG is normalized to the range [0, 12], ensure that value is within that range,
87         // it shouldn't be out of bounds.
88         final float normalizedRet = MathUtils.constrain(ret, 0, 12);
89 
90         // Re-normalize to the range [0, 1]
91         // in order to derive the correct setting value.
92         return MathUtils.lerp(min, max, normalizedRet / 12);
93     }
94 
95     /**
96      * A function for converting from the linear space that the setting works in to the
97      * gamma space that the slider works in.
98      *
99      * The gamma space effectively provides us a way to make linear changes to the slider that
100      * result in linear changes in perception. If we made changes to the slider in the linear space
101      * then we'd see an approximately logarithmic change in perception (c.f. Fechner's Law).
102      *
103      * Internally, this implements the Hybrid Log Gamma opto-electronic transfer function, which is
104      * a slight improvement to the typical gamma transfer function for displays whose max
105      * brightness exceeds the 120 nit reference point, but doesn't set a specific reference
106      * brightness like the PQ function does.
107      *
108      * Note that this transfer function is only valid if the display's backlight value is a linear
109      * control. If it's calibrated to be something non-linear, then a different transfer function
110      * should be used.
111      *
112      * @param val The brightness setting value.
113      * @param min The minimum acceptable value for the setting.
114      * @param max The maximum acceptable value for the setting.
115      * @return The corresponding slider value
116      */
convertLinearToGamma(int val, int min, int max)117     public static final int convertLinearToGamma(int val, int min, int max) {
118         return convertLinearToGammaFloat((float) val, (float) min, (float) max);
119     }
120 
121     /**
122      * Version of {@link #convertLinearToGamma} that takes float values.
123      * TODO: brightnessfloat merge with above method(?)
124      * @param val The brightness setting value.
125      * @param min The minimum acceptable value for the setting.
126      * @param max The maximum acceptable value for the setting.
127      * @return The corresponding slider value
128      */
convertLinearToGammaFloat(float val, float min, float max)129     public static final int convertLinearToGammaFloat(float val, float min, float max) {
130         // For some reason, HLG normalizes to the range [0, 12] rather than [0, 1]
131         final float normalizedVal = MathUtils.norm(min, max, val) * 12;
132         final float ret;
133         if (normalizedVal <= 1f) {
134             ret = MathUtils.sqrt(normalizedVal) * R;
135         } else {
136             ret = A * MathUtils.log(normalizedVal - B) + C;
137         }
138 
139         return Math.round(MathUtils.lerp(GAMMA_SPACE_MIN, GAMMA_SPACE_MAX, ret));
140     }
141 }
142