From 04c78a99f0b0dc68c6140f5afffe016e42a3a519 Mon Sep 17 00:00:00 2001
From: Anthony Han <modan@google.com>
Date: Fri, 3 May 2019 13:34:57 -0700
Subject: Interpolate low light white balance

Instead of setting color temperature to a default in low lux conditions,
add a spline to interpolate from color temperature that reflects the
ambient to a default.

Bug: 131259669
Test: atest AmbientLuxTest

Change-Id: I623190b796c67bab88e2eea3b4cf5a5b9ba14ae2
---
 core/res/res/values/config.xml                     |  25 +-
 core/res/res/values/symbols.xml                    |   3 +-
 .../server/display/whitebalance/AmbientFilter.java |   4 +-
 .../DisplayWhiteBalanceController.java             |  61 +++--
 .../whitebalance/DisplayWhiteBalanceFactory.java   |  12 +-
 .../display/whitebalance/AmbientLuxTest.java       | 299 +++++++++++++++++++++
 6 files changed, 376 insertions(+), 28 deletions(-)
 create mode 100644 services/tests/servicestests/src/com/android/server/display/whitebalance/AmbientLuxTest.java

diff --git a/core/res/res/values/config.xml b/core/res/res/values/config.xml
index a467f40949dd..768eb5a1db2a 100644
--- a/core/res/res/values/config.xml
+++ b/core/res/res/values/config.xml
@@ -4017,9 +4017,28 @@
     </array>
 
     <!-- See DisplayWhiteBalanceController.
-         The ambient brightness threshold (in lux) beneath which we fall back to a fixed ambient
-         color temperature. -->
-    <item name="config_displayWhiteBalanceLowLightAmbientBrightnessThreshold" format="float" type="dimen">10.0</item>
+         A float array containing a list of ambient brightnesses, in Lux. This array,
+         together with config_displayWhiteBalanceLowLightAmbientBiases, is used to generate a
+         lookup table used in DisplayWhiteBalanceController. This lookup table is used to map
+         ambient brightness readings to a bias, where the bias is used to linearly interpolate
+         between ambient color temperature and
+         config_displayWhiteBalanceLowLightAmbientColorTemperature.
+         This table is optional. If used, this array must,
+         1) Contain at least two entries
+         2) Be the same length as config_displayWhiteBalanceLowLightAmbientBiases. -->
+    <array name ="config_displayWhiteBalanceLowLightAmbientBrightnesses">
+        <item>10.0</item>
+        <item>10.0</item>
+    </array>
+
+    <!-- See DisplayWhiteBalanceController.
+         An array containing a list of biases. See
+         config_displayWhiteBalanceLowLightAmbientBrightnesses for additional details.
+         This array must be in the range of [0.0, 1.0]. -->
+    <array name ="config_displayWhiteBalanceLowLightAmbientBiases">
+        <item>0.0</item>
+        <item>1.0</item>
+    </array>
 
     <!-- See DisplayWhiteBalanceController.
          The ambient color temperature (in cct) to which we fall back when the ambient brightness
diff --git a/core/res/res/values/symbols.xml b/core/res/res/values/symbols.xml
index 8b14da5e948a..1c60ac55f0b2 100644
--- a/core/res/res/values/symbols.xml
+++ b/core/res/res/values/symbols.xml
@@ -3745,7 +3745,8 @@
   <java-symbol type="array" name="config_displayWhiteBalanceBaseThresholds" />
   <java-symbol type="array" name="config_displayWhiteBalanceIncreaseThresholds" />
   <java-symbol type="array" name="config_displayWhiteBalanceDecreaseThresholds" />
-  <java-symbol type="dimen" name="config_displayWhiteBalanceLowLightAmbientBrightnessThreshold" />
+  <java-symbol type="array" name="config_displayWhiteBalanceLowLightAmbientBrightnesses" />
+  <java-symbol type="array" name="config_displayWhiteBalanceLowLightAmbientBiases" />
   <java-symbol type="dimen" name="config_displayWhiteBalanceLowLightAmbientColorTemperature" />
   <java-symbol type="array" name="config_displayWhiteBalanceAmbientColorTemperatures" />
   <java-symbol type="array" name="config_displayWhiteBalanceDisplayColorTemperatures" />
diff --git a/services/core/java/com/android/server/display/whitebalance/AmbientFilter.java b/services/core/java/com/android/server/display/whitebalance/AmbientFilter.java
index 532bbed9588e..123cd73a3ff3 100644
--- a/services/core/java/com/android/server/display/whitebalance/AmbientFilter.java
+++ b/services/core/java/com/android/server/display/whitebalance/AmbientFilter.java
@@ -18,6 +18,7 @@ package com.android.server.display.whitebalance;
 
 import android.util.Slog;
 
+import com.android.internal.annotations.VisibleForTesting;
 import com.android.server.display.utils.RollingBuffer;
 
 import java.io.PrintWriter;
@@ -155,7 +156,8 @@ abstract class AmbientFilter {
     /**
      * A weighted average prioritising recent changes.
      */
-    static class WeightedMovingAverageAmbientFilter extends AmbientFilter {
+    @VisibleForTesting
+    public static class WeightedMovingAverageAmbientFilter extends AmbientFilter {
 
         // How long the latest ambient value change is predicted to last.
         private static final int PREDICTION_TIME = 100; // Milliseconds
diff --git a/services/core/java/com/android/server/display/whitebalance/DisplayWhiteBalanceController.java b/services/core/java/com/android/server/display/whitebalance/DisplayWhiteBalanceController.java
index 0f86b478468f..395319d897ad 100644
--- a/services/core/java/com/android/server/display/whitebalance/DisplayWhiteBalanceController.java
+++ b/services/core/java/com/android/server/display/whitebalance/DisplayWhiteBalanceController.java
@@ -20,6 +20,7 @@ import android.annotation.NonNull;
 import android.util.Slog;
 import android.util.Spline;
 
+import com.android.internal.annotations.VisibleForTesting;
 import com.android.internal.util.Preconditions;
 import com.android.server.LocalServices;
 import com.android.server.display.color.ColorDisplayService.ColorDisplayServiceInternal;
@@ -53,18 +54,21 @@ public class DisplayWhiteBalanceController implements
     private Callbacks mCallbacks;
 
     private AmbientSensor.AmbientBrightnessSensor mBrightnessSensor;
-    private AmbientFilter mBrightnessFilter;
+
+    @VisibleForTesting
+    AmbientFilter mBrightnessFilter;
     private AmbientSensor.AmbientColorTemperatureSensor mColorTemperatureSensor;
-    private AmbientFilter mColorTemperatureFilter;
+
+    @VisibleForTesting
+    AmbientFilter mColorTemperatureFilter;
     private DisplayWhiteBalanceThrottler mThrottler;
 
-    // When the brightness drops below a certain threshold, it affects the color temperature
-    // accuracy, so we fall back to a fixed ambient color temperature.
-    private final float mLowLightAmbientBrightnessThreshold;
     private final float mLowLightAmbientColorTemperature;
 
     private float mAmbientColorTemperature;
-    private float mPendingAmbientColorTemperature;
+
+    @VisibleForTesting
+    float mPendingAmbientColorTemperature;
     private float mLastAmbientColorTemperature;
 
     private ColorDisplayServiceInternal mColorDisplayServiceInternal;
@@ -79,6 +83,9 @@ public class DisplayWhiteBalanceController implements
     // A piecewise linear relationship between ambient and display color temperatures.
     private Spline.LinearSpline mAmbientToDisplayColorTemperatureSpline;
 
+    // A piecewise linear relationship between low light brightness and low light bias.
+    private Spline.LinearSpline mLowLightAmbientBrightnessToBiasSpline;
+
     /**
      * @param brightnessSensor
      *      The sensor used to detect changes in the ambient brightness.
@@ -119,7 +126,8 @@ public class DisplayWhiteBalanceController implements
             @NonNull AmbientSensor.AmbientColorTemperatureSensor colorTemperatureSensor,
             @NonNull AmbientFilter colorTemperatureFilter,
             @NonNull DisplayWhiteBalanceThrottler throttler,
-            float lowLightAmbientBrightnessThreshold, float lowLightAmbientColorTemperature,
+            float[] lowLightAmbientBrightnesses, float[] lowLightAmbientBiases,
+            float lowLightAmbientColorTemperature,
             float[] ambientColorTemperatures, float[] displayColorTemperatures) {
         validateArguments(brightnessSensor, brightnessFilter, colorTemperatureSensor,
                 colorTemperatureFilter, throttler);
@@ -131,7 +139,6 @@ public class DisplayWhiteBalanceController implements
         mColorTemperatureSensor = colorTemperatureSensor;
         mColorTemperatureFilter = colorTemperatureFilter;
         mThrottler = throttler;
-        mLowLightAmbientBrightnessThreshold = lowLightAmbientBrightnessThreshold;
         mLowLightAmbientColorTemperature = lowLightAmbientColorTemperature;
         mAmbientColorTemperature = -1.0f;
         mPendingAmbientColorTemperature = -1.0f;
@@ -139,10 +146,28 @@ public class DisplayWhiteBalanceController implements
         mAmbientColorTemperatureHistory = new History(HISTORY_SIZE);
         mAmbientColorTemperatureOverride = -1.0f;
 
+        try {
+            mLowLightAmbientBrightnessToBiasSpline = new Spline.LinearSpline(
+                    lowLightAmbientBrightnesses, lowLightAmbientBiases);
+        } catch (Exception e) {
+            Slog.e(TAG, "failed to create low light ambient brightness to bias spline.", e);
+            mLowLightAmbientBrightnessToBiasSpline = null;
+        }
+        if (mLowLightAmbientBrightnessToBiasSpline != null) {
+            if (mLowLightAmbientBrightnessToBiasSpline.interpolate(0.0f) != 0.0f ||
+                    mLowLightAmbientBrightnessToBiasSpline.interpolate(Float.POSITIVE_INFINITY)
+                    != 1.0f) {
+                Slog.d(TAG, "invalid low light ambient brightness to bias spline, "
+                        + "bias must begin at 0.0 and end at 1.0");
+                mLowLightAmbientBrightnessToBiasSpline = null;
+            }
+        }
+
         try {
             mAmbientToDisplayColorTemperatureSpline = new Spline.LinearSpline(
                     ambientColorTemperatures, displayColorTemperatures);
         } catch (Exception e) {
+            Slog.e(TAG, "failed to create ambient to display color temperature spline.", e);
             mAmbientToDisplayColorTemperatureSpline = null;
         }
 
@@ -238,8 +263,6 @@ public class DisplayWhiteBalanceController implements
         mColorTemperatureSensor.dump(writer);
         mColorTemperatureFilter.dump(writer);
         mThrottler.dump(writer);
-        writer.println("  mLowLightAmbientBrightnessThreshold="
-                + mLowLightAmbientBrightnessThreshold);
         writer.println("  mLowLightAmbientColorTemperature=" + mLowLightAmbientColorTemperature);
         writer.println("  mAmbientColorTemperature=" + mAmbientColorTemperature);
         writer.println("  mPendingAmbientColorTemperature=" + mPendingAmbientColorTemperature);
@@ -248,6 +271,8 @@ public class DisplayWhiteBalanceController implements
         writer.println("  mAmbientColorTemperatureOverride=" + mAmbientColorTemperatureOverride);
         writer.println("  mAmbientToDisplayColorTemperatureSpline="
                 + mAmbientToDisplayColorTemperatureSpline);
+        writer.println("  mLowLightAmbientBrightnessToBiasSpline="
+                + mLowLightAmbientBrightnessToBiasSpline);
     }
 
     @Override // AmbientSensor.AmbientBrightnessSensor.Callbacks
@@ -276,15 +301,13 @@ public class DisplayWhiteBalanceController implements
                 mAmbientToDisplayColorTemperatureSpline.interpolate(ambientColorTemperature);
         }
 
-        final float ambientBrightness = mBrightnessFilter.getEstimate(time);
-        if (ambientBrightness < mLowLightAmbientBrightnessThreshold) {
-            if (mLoggingEnabled) {
-                Slog.d(TAG, "low light ambient brightness: " + ambientBrightness + " < "
-                        + mLowLightAmbientBrightnessThreshold
-                        + ", falling back to fixed ambient color temperature: "
-                        + ambientColorTemperature + " => " + mLowLightAmbientColorTemperature);
-            }
-            ambientColorTemperature = mLowLightAmbientColorTemperature;
+        float ambientBrightness = mBrightnessFilter.getEstimate(time);
+
+        if (mLowLightAmbientBrightnessToBiasSpline != null) {
+            float bias = mLowLightAmbientBrightnessToBiasSpline.interpolate(ambientBrightness);
+            ambientColorTemperature =
+                    bias * ambientColorTemperature + (1.0f - bias)
+                    * mLowLightAmbientColorTemperature;
         }
 
         if (mAmbientColorTemperatureOverride != -1.0f) {
diff --git a/services/core/java/com/android/server/display/whitebalance/DisplayWhiteBalanceFactory.java b/services/core/java/com/android/server/display/whitebalance/DisplayWhiteBalanceFactory.java
index 6ff2b09988e6..b1b465e3b359 100644
--- a/services/core/java/com/android/server/display/whitebalance/DisplayWhiteBalanceFactory.java
+++ b/services/core/java/com/android/server/display/whitebalance/DisplayWhiteBalanceFactory.java
@@ -63,9 +63,12 @@ public class DisplayWhiteBalanceFactory {
                 createColorTemperatureSensor(handler, sensorManager, resources);
         final AmbientFilter colorTemperatureFilter = createColorTemperatureFilter(resources);
         final DisplayWhiteBalanceThrottler throttler = createThrottler(resources);
-        final float lowLightAmbientBrightnessThreshold = getFloat(resources,
-                com.android.internal.R.dimen
-                .config_displayWhiteBalanceLowLightAmbientBrightnessThreshold);
+        final float[] displayWhiteBalanceLowLightAmbientBrightnesses = getFloatArray(resources,
+                com.android.internal.R.array
+                .config_displayWhiteBalanceLowLightAmbientBrightnesses);
+        final float[] displayWhiteBalanceLowLightAmbientBiases = getFloatArray(resources,
+                com.android.internal.R.array
+                .config_displayWhiteBalanceLowLightAmbientBiases);
         final float lowLightAmbientColorTemperature = getFloat(resources,
                 com.android.internal.R.dimen
                 .config_displayWhiteBalanceLowLightAmbientColorTemperature);
@@ -75,7 +78,8 @@ public class DisplayWhiteBalanceFactory {
                 com.android.internal.R.array.config_displayWhiteBalanceDisplayColorTemperatures);
         final DisplayWhiteBalanceController controller = new DisplayWhiteBalanceController(
                 brightnessSensor, brightnessFilter, colorTemperatureSensor, colorTemperatureFilter,
-                throttler, lowLightAmbientBrightnessThreshold, lowLightAmbientColorTemperature,
+                throttler, displayWhiteBalanceLowLightAmbientBrightnesses,
+                displayWhiteBalanceLowLightAmbientBiases, lowLightAmbientColorTemperature,
                 ambientColorTemperatures, displayColorTempeartures);
         brightnessSensor.setCallbacks(controller);
         colorTemperatureSensor.setCallbacks(controller);
diff --git a/services/tests/servicestests/src/com/android/server/display/whitebalance/AmbientLuxTest.java b/services/tests/servicestests/src/com/android/server/display/whitebalance/AmbientLuxTest.java
new file mode 100644
index 000000000000..de68036d2d4c
--- /dev/null
+++ b/services/tests/servicestests/src/com/android/server/display/whitebalance/AmbientLuxTest.java
@@ -0,0 +1,299 @@
+/*
+ * Copyright (C) 2019 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.whitebalance;
+
+import com.android.internal.R;
+import com.google.common.collect.ImmutableList;
+
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.when;
+import static org.mockito.Matchers.anyLong;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+import org.mockito.Spy;
+
+import android.content.ContextWrapper;
+import android.content.res.Resources;
+import android.content.res.TypedArray;
+import android.hardware.Sensor;
+import android.hardware.SensorEvent;
+import android.hardware.SensorEventListener;
+import android.hardware.SensorManager;
+import android.os.Handler;
+import android.os.Looper;
+import android.util.TypedValue;
+
+import androidx.test.InstrumentationRegistry;
+
+import org.junit.Before;
+import org.junit.After;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Field;
+import java.lang.reflect.Method;
+import java.util.List;
+
+@RunWith(JUnit4.class)
+public final class AmbientLuxTest {
+    private static final int AMBIENT_COLOR_TYPE = 20705;
+    private static final String AMBIENT_COLOR_TYPE_STR = "colorSensoryDensoryDoc";
+    private static final float LOW_LIGHT_AMBIENT_COLOR_TEMPERATURE = 5700;
+
+    private Handler mHandler = new Handler(Looper.getMainLooper());
+    private Sensor mLightSensor;
+    private Sensor mAmbientColorSensor;
+    private ContextWrapper mContextSpy;
+    private Resources mResourcesSpy;
+
+    @Mock private SensorManager mSensorManagerMock;
+
+    @Mock private TypedArray mBrightnesses;
+    @Mock private TypedArray mBiases;
+
+    @Before
+    public void setUp() throws Exception {
+        MockitoAnnotations.initMocks(this);
+        mLightSensor = createSensor(Sensor.TYPE_LIGHT, null);
+        mAmbientColorSensor = createSensor(AMBIENT_COLOR_TYPE, AMBIENT_COLOR_TYPE_STR);
+        mContextSpy = spy(new ContextWrapper(InstrumentationRegistry.getContext()));
+        mResourcesSpy = spy(mContextSpy.getResources());
+        when(mContextSpy.getResources()).thenReturn(mResourcesSpy);
+        when(mSensorManagerMock.getDefaultSensor(Sensor.TYPE_LIGHT)).thenReturn(mLightSensor);
+        final List<Sensor> sensorList = ImmutableList.of(mLightSensor, mAmbientColorSensor);
+        when(mSensorManagerMock.getSensorList(Sensor.TYPE_ALL)).thenReturn(sensorList);
+        when(mResourcesSpy.getString(
+                R.string.config_displayWhiteBalanceColorTemperatureSensorName))
+                .thenReturn(AMBIENT_COLOR_TYPE_STR);
+        when(mResourcesSpy.getInteger(
+                R.integer.config_displayWhiteBalanceDecreaseDebounce))
+                .thenReturn(0);
+        when(mResourcesSpy.getInteger(
+                R.integer.config_displayWhiteBalanceIncreaseDebounce))
+                .thenReturn(0);
+        when(mResourcesSpy.getFloat(
+                R.dimen.config_displayWhiteBalanceLowLightAmbientColorTemperature))
+                .thenReturn(LOW_LIGHT_AMBIENT_COLOR_TEMPERATURE);
+        when(mResourcesSpy.obtainTypedArray(
+                R.array.config_displayWhiteBalanceAmbientColorTemperatures))
+                .thenReturn(createTypedArray());
+        when(mResourcesSpy.obtainTypedArray(
+                R.array.config_displayWhiteBalanceDisplayColorTemperatures))
+                .thenReturn(createTypedArray());
+
+        when(mResourcesSpy.obtainTypedArray(
+                R.array.config_displayWhiteBalanceLowLightAmbientBrightnesses))
+                .thenReturn(mBrightnesses);
+        when(mResourcesSpy.obtainTypedArray(
+                R.array.config_displayWhiteBalanceLowLightAmbientBiases))
+                .thenReturn(mBiases);
+    }
+
+    @Test
+    public void testNoSpline() throws Exception {
+        setBrightnesses();
+        setBiases();
+
+        DisplayWhiteBalanceController controller =
+                DisplayWhiteBalanceFactory.create(mHandler, mSensorManagerMock, mResourcesSpy);
+        final float ambientColorTemperature = 8000.0f;
+        setEstimatedColorTemperature(controller, ambientColorTemperature);
+        controller.mBrightnessFilter = spy(controller.mBrightnessFilter);
+
+        for (float luxOverride = 0.1f; luxOverride <= 10000; luxOverride *= 10) {
+            setEstimatedBrightnessAndUpdate(controller, luxOverride);
+            assertEquals(controller.mPendingAmbientColorTemperature,
+                    ambientColorTemperature, 0.001);
+        }
+    }
+
+    @Test
+    public void testSpline_OneSegment() throws Exception {
+        final float lowerBrightness = 10.0f;
+        final float upperBrightness = 50.0f;
+        setBrightnesses(lowerBrightness, upperBrightness);
+        setBiases(0.0f, 1.0f);
+
+        DisplayWhiteBalanceController controller =
+                DisplayWhiteBalanceFactory.create(mHandler, mSensorManagerMock, mResourcesSpy);
+        final float ambientColorTemperature = 8000.0f;
+        setEstimatedColorTemperature(controller, ambientColorTemperature);
+        controller.mBrightnessFilter = spy(controller.mBrightnessFilter);
+
+        for (float t = 0.0f; t <= 1.0f; t += 0.1f) {
+            setEstimatedBrightnessAndUpdate(controller,
+                    mix(lowerBrightness, upperBrightness, t));
+            assertEquals(controller.mPendingAmbientColorTemperature,
+                    mix(LOW_LIGHT_AMBIENT_COLOR_TEMPERATURE, ambientColorTemperature, t), 0.001);
+        }
+
+        setEstimatedBrightnessAndUpdate(controller, 0.0f);
+        assertEquals(controller.mPendingAmbientColorTemperature,
+                LOW_LIGHT_AMBIENT_COLOR_TEMPERATURE, 0.001);
+
+        setEstimatedBrightnessAndUpdate(controller, upperBrightness + 1.0f);
+        assertEquals(controller.mPendingAmbientColorTemperature, ambientColorTemperature, 0.001);
+    }
+
+    @Test
+    public void testSpline_TwoSegments() throws Exception {
+        final float brightness0 = 10.0f;
+        final float brightness1 = 50.0f;
+        final float brightness2 = 60.0f;
+        setBrightnesses(brightness0, brightness1, brightness2);
+        final float bias0 = 0.0f;
+        final float bias1 = 0.25f;
+        final float bias2 = 1.0f;
+        setBiases(bias0, bias1, bias2);
+
+        DisplayWhiteBalanceController controller =
+                DisplayWhiteBalanceFactory.create(mHandler, mSensorManagerMock, mResourcesSpy);
+        final float ambientColorTemperature = 8000.0f;
+        setEstimatedColorTemperature(controller, ambientColorTemperature);
+        controller.mBrightnessFilter = spy(controller.mBrightnessFilter);
+
+        for (float t = 0.0f; t <= 1.0f; t += 0.1f) {
+            float luxOverride = mix(brightness0, brightness1, t);
+            setEstimatedBrightnessAndUpdate(controller, luxOverride);
+            float bias = mix(bias0, bias1, t);
+            assertEquals(controller.mPendingAmbientColorTemperature,
+                    mix(LOW_LIGHT_AMBIENT_COLOR_TEMPERATURE, ambientColorTemperature, bias), 0.001);
+        }
+
+        for (float t = 0.0f; t <= 1.0f; t += 0.1f) {
+            float luxOverride = mix(brightness1, brightness2, t);
+            setEstimatedBrightnessAndUpdate(controller, luxOverride);
+            float bias = mix(bias1, bias2, t);
+            assertEquals(controller.mPendingAmbientColorTemperature,
+                    mix(LOW_LIGHT_AMBIENT_COLOR_TEMPERATURE, ambientColorTemperature, bias), 0.001);
+        }
+
+        setEstimatedBrightnessAndUpdate(controller, 0.0f);
+        assertEquals(controller.mPendingAmbientColorTemperature,
+                LOW_LIGHT_AMBIENT_COLOR_TEMPERATURE, 0.001);
+
+        setEstimatedBrightnessAndUpdate(controller, brightness2 + 1.0f);
+        assertEquals(controller.mPendingAmbientColorTemperature, ambientColorTemperature, 0.001);
+    }
+
+    @Test
+    public void testSpline_VerticalSegment() throws Exception {
+        final float lowerBrightness = 10.0f;
+        final float upperBrightness = 10.0f;
+        setBrightnesses(lowerBrightness, upperBrightness);
+        setBiases(0.0f, 1.0f);
+
+        DisplayWhiteBalanceController controller =
+                DisplayWhiteBalanceFactory.create(mHandler, mSensorManagerMock, mResourcesSpy);
+        final float ambientColorTemperature = 8000.0f;
+        setEstimatedColorTemperature(controller, ambientColorTemperature);
+        controller.mBrightnessFilter = spy(controller.mBrightnessFilter);
+
+        setEstimatedBrightnessAndUpdate(controller, 0.0f);
+        assertEquals(controller.mPendingAmbientColorTemperature,
+                LOW_LIGHT_AMBIENT_COLOR_TEMPERATURE, 0.001);
+
+        setEstimatedBrightnessAndUpdate(controller, upperBrightness + 1.0f);
+        assertEquals(controller.mPendingAmbientColorTemperature, ambientColorTemperature, 0.001);
+    }
+
+    @Test
+    public void testSpline_InvalidBiases() throws Exception {
+        float[][] invalidBrightnesses =
+                {{10.0f, 1000.0f}, {10.0f, 1000.0f}, {10.0f, 1000.0f}, {10.0f, 1000.0f},
+                 {10.0f, 1000.0f}, {-1.0f, 1.0f}, {-1.0f, 1.0f}};
+        float[][] invalidBiases =
+                {{0.0f, 2.0f}, {0.0f, 0.9f}, {0.1f, 1.0f}, {-0.1f, 1.0f},
+                 {0.1f, 1.1f}, {0.0f, 1.0f}, {-2.0f, 1.0f}};
+        for (int i = 0; i < invalidBrightnesses.length; ++i) {
+            setBrightnesses(invalidBrightnesses[i][0], invalidBrightnesses[i][1]);
+            setBiases(invalidBiases[i][0], invalidBiases[i][1]);
+
+            DisplayWhiteBalanceController controller =
+                    DisplayWhiteBalanceFactory.create(mHandler, mSensorManagerMock, mResourcesSpy);
+            final float ambientColorTemperature = 8000.0f;
+            setEstimatedColorTemperature(controller, ambientColorTemperature);
+            controller.mBrightnessFilter = spy(controller.mBrightnessFilter);
+
+            for (float luxOverride = 0.1f; luxOverride <= 10000; luxOverride *= 10) {
+                setEstimatedBrightnessAndUpdate(controller, luxOverride);
+                assertEquals(controller.mPendingAmbientColorTemperature,
+                        ambientColorTemperature, 0.001);
+            }
+        }
+    }
+
+    private void setEstimatedColorTemperature(DisplayWhiteBalanceController controller,
+                                              float ambientColorTemperature) {
+        AmbientFilter colorTemperatureFilter = spy(controller.mColorTemperatureFilter);
+        controller.mColorTemperatureFilter = colorTemperatureFilter;
+        when(colorTemperatureFilter.getEstimate(anyLong())).thenReturn(ambientColorTemperature);
+    }
+
+    private void setEstimatedBrightnessAndUpdate(DisplayWhiteBalanceController controller,
+                                                 float brightness) {
+        when(controller.mBrightnessFilter.getEstimate(anyLong())).thenReturn(brightness);
+        controller.updateAmbientColorTemperature();
+    }
+
+    private void setBrightnesses(float... vals) {
+        setFloatArrayResource(mBrightnesses, vals);
+    }
+
+    private void setBiases(float... vals) {
+        setFloatArrayResource(mBiases, vals);
+    }
+
+    private void setFloatArrayResource(TypedArray array, float[] vals) {
+        when(array.length()).thenReturn(vals.length);
+        for (int i = 0; i < vals.length; i++) {
+            when(array.getFloat(i, Float.NaN)).thenReturn(vals[i]);
+        }
+    }
+
+    private void setSensorType(Sensor sensor, int type, String strType) throws Exception {
+        Method setter = Sensor.class.getDeclaredMethod("setType", Integer.TYPE);
+        setter.setAccessible(true);
+        setter.invoke(sensor, type);
+        if (strType != null) {
+            Field f = sensor.getClass().getDeclaredField("mStringType");
+            f.setAccessible(true);
+            f.set(sensor, strType);
+        }
+    }
+
+    private Sensor createSensor(int type, String strType) throws Exception {
+        Constructor<Sensor> constr = Sensor.class.getDeclaredConstructor();
+        constr.setAccessible(true);
+        Sensor sensor = constr.newInstance();
+        setSensorType(sensor, type, strType);
+        return sensor;
+    }
+
+    private TypedArray createTypedArray() throws Exception {
+        TypedArray mockArray = mock(TypedArray.class);
+        return mockArray;
+    }
+
+    private static float mix(float a, float b, float t) {
+        return (1.0f - t) * a + t * b;
+    }
+}
-- 
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