Revert "Revert "[RESTRICT AUTOMERGE]DisplayManager: Switch RampAnimator to ramp in HLG space.""

This reverts commit e828f33713a4a9103f10fdc1affe21e144635c16.

Revert reason: Check if this causes AOC regression

Bug: 223124178
Change-Id: If01ec6728c1723d4763dc32b7b206dd2ee73335f
(cherry picked from commit 5fb150a5a1beaebb228eb6d01d69298b646eade2)
Merged-In: If01ec6728c1723d4763dc32b7b206dd2ee73335f

2 files changed, 96 insertions, 7 deletions

diff --git a/services/core/java/com/android/server/display/BrightnessUtils.java b/services/core/java/com/android/server/display/BrightnessUtils.java
new file mode 100644
index 000000000000..84fa0cccbd10
--- /dev/null
+++ b/services/core/java/com/android/server/display/BrightnessUtils.java
@@ -0,0 +1,83 @@
+/*
+ * Copyright (C) 2022 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.server.display;
+
+import android.util.MathUtils;
+
+/**
+ * Utility class providing functions to convert between linear and perceptual gamma space.
+ *
+ * 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.
+ *
+ * Note: This code is based on the same class in the com.android.settingslib.display package.
+ */
+public class BrightnessUtils {
+
+    // 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 into the linear space.
+     *
+     * @param val The value in the gamma space [0 .. 1.0]
+     * @return The corresponding value in the linear space [0 .. 1.0].
+     */
+    public static final float convertGammaToLinear(float val) {
+        final float ret;
+        if (val <= R) {
+            ret = MathUtils.sq(val / R);
+        } else {
+            ret = MathUtils.exp((val - 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 normalizedRet / 12;
+    }
+
+    /**
+     * A function for converting from the linear space into the gamma space.
+     *
+     * @param val The value in linear space [0 .. 1.0]
+     * @return The corresponding value in gamma space [0 .. 1.0]
+     */
+    public static final float convertLinearToGamma(float val) {
+        // For some reason, HLG normalizes to the range [0, 12] rather than [0, 1]
+        final float normalizedVal = val * 12;
+        final float ret;
+        if (normalizedVal <= 1f) {
+            ret = MathUtils.sqrt(normalizedVal) * R;
+        } else {
+            ret = A * MathUtils.log(normalizedVal - B) + C;
+        }
+        return ret;
+    }
+}
diff --git a/services/core/java/com/android/server/display/RampAnimator.java b/services/core/java/com/android/server/display/RampAnimator.java
index ed3b15fb2661..1ebd1f5a535c 100644
--- a/services/core/java/com/android/server/display/RampAnimator.java
+++ b/services/core/java/com/android/server/display/RampAnimator.java
@@ -23,6 +23,8 @@ import android.view.Choreographer;
 /**
  * A custom animator that progressively updates a property value at
  * a given variable rate until it reaches a particular target value.
+ * The ramping at the given rate is done in the perceptual space using
+ * the HLG transfer functions.
  */
 class RampAnimator<T> {
     private final T mObject;
@@ -57,7 +59,9 @@ class RampAnimator<T> {
      * @param rate The convergence rate in units per second, or 0 to set the value immediately.
      * @return True if the target differs from the previous target.
      */
-    public boolean animateTo(float target, float rate) {
+    public boolean animateTo(float targetLinear, float rate) {
+        // Convert the target from the linear into the HLG space.
+        final float target = BrightnessUtils.convertLinearToGamma(targetLinear);
 
         // Immediately jump to the target the first time.
         if (mFirstTime || rate <= 0) {
@@ -156,7 +160,9 @@ class RampAnimator<T> {
             final float oldCurrentValue = mCurrentValue;
             mCurrentValue = mAnimatedValue;
             if (oldCurrentValue != mCurrentValue) {
-                mProperty.setValue(mObject, mCurrentValue);
+                // Convert value from HLG into linear space for the property.
+                final float linearCurrentVal = BrightnessUtils.convertGammaToLinear(mCurrentValue);
+                mProperty.setValue(mObject, linearCurrentVal);
             }
             if (mTargetValue != mCurrentValue) {
                 postAnimationCallback();
@@ -201,14 +207,14 @@ class RampAnimator<T> {
          * If this is the first time the property is being set or if the rate is 0,
          * the value jumps directly to the target.
          *
-         * @param firstTarget The first target value.
-         * @param secondTarget The second target value.
+         * @param linearFirstTarget The first target value in linear space.
+         * @param linearSecondTarget The second target value in linear space.
          * @param rate The convergence rate in units per second, or 0 to set the value immediately.
          * @return True if either target differs from the previous target.
          */
-        public boolean animateTo(float firstTarget, float secondTarget, float rate) {
-            final boolean firstRetval = mFirst.animateTo(firstTarget, rate);
-            final boolean secondRetval = mSecond.animateTo(secondTarget, rate);
+        public boolean animateTo(float linearFirstTarget, float linearSecondTarget, float rate) {
+            final boolean firstRetval = mFirst.animateTo(linearFirstTarget, rate);
+            final boolean secondRetval = mSecond.animateTo(linearSecondTarget, rate);
             return firstRetval && secondRetval;
         }