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
17package com.android.settingslib.display;
18
19import android.util.MathUtils;
20
21public 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