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      */
convertGammaToLinear(int val, int min, int max)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      */
convertLinearToGamma(int val, int min, int max)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