diff --git a/api/current.txt b/api/current.txt
index 9b5481407d5ac..860ae0907f3c1 100644
--- a/api/current.txt
+++ b/api/current.txt
@@ -22876,6 +22876,7 @@ package android.util {
     method public static float cos(float);
     method public static float exp(float);
     method public static float floor(float);
+    method public static float hypot(float, float);
     method public static float sin(float);
     method public static float sqrt(float);
   }
diff --git a/core/java/android/util/FloatMath.java b/core/java/android/util/FloatMath.java
index 1d4eda41473c7..e05169ad6ddfb 100644
--- a/core/java/android/util/FloatMath.java
+++ b/core/java/android/util/FloatMath.java
@@ -80,4 +80,14 @@ public class FloatMath {
      * @return the exponential of value
      */
     public static native float exp(float value);
+
+    /**
+     * Returns {@code sqrt(}<i>{@code x}</i><sup>{@code 2}</sup>{@code +} <i>
+     * {@code y}</i><sup>{@code 2}</sup>{@code )}.
+     *
+     * @param x a float number
+     * @param y a float number
+     * @return the hypotenuse
+     */
+    public static native float hypot(float x, float y);
 }
diff --git a/core/java/android/util/Spline.java b/core/java/android/util/Spline.java
new file mode 100644
index 0000000000000..ed027eb18166e
--- /dev/null
+++ b/core/java/android/util/Spline.java
@@ -0,0 +1,156 @@
+/*
+ * Copyright (C) 2012 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 android.util;
+
+/**
+ * Performs spline interpolation given a set of control points.
+ * @hide
+ */
+public final class Spline {
+    private final float[] mX;
+    private final float[] mY;
+    private final float[] mM;
+
+    private Spline(float[] x, float[] y, float[] m) {
+        mX = x;
+        mY = y;
+        mM = m;
+    }
+
+    /**
+     * Creates a monotone cubic spline from a given set of control points.
+     *
+     * The spline is guaranteed to pass through each control point exactly.
+     * Moreover, assuming the control points are monotonic (Y is non-decreasing or
+     * non-increasing) then the interpolated values will also be monotonic.
+     *
+     * This function uses the Fritsch-Carlson method for computing the spline parameters.
+     * http://en.wikipedia.org/wiki/Monotone_cubic_interpolation
+     *
+     * @param x The X component of the control points, strictly increasing.
+     * @param y The Y component of the control points, monotonic.
+     * @return
+     *
+     * @throws IllegalArgumentException if the X or Y arrays are null, have
+     * different lengths or have fewer than 2 values.
+     * @throws IllegalArgumentException if the control points are not monotonic.
+     */
+    public static Spline createMonotoneCubicSpline(float[] x, float[] y) {
+        if (x == null || y == null || x.length != y.length || x.length < 2) {
+            throw new IllegalArgumentException("There must be at least two control "
+                    + "points and the arrays must be of equal length.");
+        }
+
+        final int n = x.length;
+        float[] d = new float[n - 1]; // could optimize this out
+        float[] m = new float[n];
+
+        // Compute slopes of secant lines between successive points.
+        for (int i = 0; i < n - 1; i++) {
+            float h = x[i + 1] - x[i];
+            if (h <= 0f) {
+                throw new IllegalArgumentException("The control points must all "
+                        + "have strictly increasing X values.");
+            }
+            d[i] = (y[i + 1] - y[i]) / h;
+        }
+
+        // Initialize the tangents as the average of the secants.
+        m[0] = d[0];
+        for (int i = 1; i < n - 1; i++) {
+            m[i] = (d[i - 1] + d[i]) * 0.5f;
+        }
+        m[n - 1] = d[n - 2];
+
+        // Update the tangents to preserve monotonicity.
+        for (int i = 0; i < n - 1; i++) {
+            if (d[i] == 0f) { // successive Y values are equal
+                m[i] = 0f;
+                m[i + 1] = 0f;
+            } else {
+                float a = m[i] / d[i];
+                float b = m[i + 1] / d[i];
+                if (a < 0f || b < 0f) {
+                    throw new IllegalArgumentException("The control points must have "
+                            + "monotonic Y values.");
+                }
+                float h = FloatMath.hypot(a, b);
+                if (h > 9f) {
+                    float t = 3f / h;
+                    m[i] = t * a * d[i];
+                    m[i + 1] = t * b * d[i];
+                }
+            }
+        }
+        return new Spline(x, y, m);
+    }
+
+    /**
+     * Interpolates the value of Y = f(X) for given X.
+     * Clamps X to the domain of the spline.
+     *
+     * @param x The X value.
+     * @return The interpolated Y = f(X) value.
+     */
+    public float interpolate(float x) {
+        // Handle the boundary cases.
+        final int n = mX.length;
+        if (Float.isNaN(x)) {
+            return x;
+        }
+        if (x <= mX[0]) {
+            return mY[0];
+        }
+        if (x >= mX[n - 1]) {
+            return mY[n - 1];
+        }
+
+        // Find the index 'i' of the last point with smaller X.
+        // We know this will be within the spline due to the boundary tests.
+        int i = 0;
+        while (x >= mX[i + 1]) {
+            i += 1;
+            if (x == mX[i]) {
+                return mY[i];
+            }
+        }
+
+        // Perform cubic Hermite spline interpolation.
+        float h = mX[i + 1] - mX[i];
+        float t = (x - mX[i]) / h;
+        return (mY[i] * (1 + 2 * t) + h * mM[i] * t) * (1 - t) * (1 - t)
+                + (mY[i + 1] * (3 - 2 * t) + h * mM[i + 1] * (t - 1)) * t * t;
+    }
+
+    // For debugging.
+    @Override
+    public String toString() {
+        StringBuilder str = new StringBuilder();
+        final int n = mX.length;
+        str.append("[");
+        for (int i = 0; i < n; i++) {
+            if (i != 0) {
+                str.append(", ");
+            }
+            str.append("(").append(mX[i]);
+            str.append(", ").append(mY[i]);
+            str.append(": ").append(mM[i]).append(")");
+        }
+        str.append("]");
+        return str.toString();
+    }
+}
diff --git a/core/jni/android_util_FloatMath.cpp b/core/jni/android_util_FloatMath.cpp
index e30756b5dee3b..529fbe9b64a88 100644
--- a/core/jni/android_util_FloatMath.cpp
+++ b/core/jni/android_util_FloatMath.cpp
@@ -29,6 +29,10 @@ public:
     static float ExpF(JNIEnv* env, jobject clazz, float x) {
         return expf(x);
     }
+
+    static float HypotF(JNIEnv* env, jobject clazz, float x, float y) {
+        return hypotf(x, y);
+    }
 };
 
 static JNINativeMethod gMathUtilsMethods[] = {
@@ -38,6 +42,7 @@ static JNINativeMethod gMathUtilsMethods[] = {
     {"cos", "(F)F", (void*) MathUtilsGlue::CosF},
     {"sqrt", "(F)F", (void*) MathUtilsGlue::SqrtF},
     {"exp", "(F)F", (void*) MathUtilsGlue::ExpF},
+    {"hypot", "(FF)F", (void*) MathUtilsGlue::HypotF},
 };
 
 int register_android_util_FloatMath(JNIEnv* env)
diff --git a/core/res/res/values/config.xml b/core/res/res/values/config.xml
index ee0ff8ed04500..e3c957beb7e94 100755
--- a/core/res/res/values/config.xml
+++ b/core/res/res/values/config.xml
@@ -533,13 +533,22 @@
     <integer name="config_longPressOnHomeBehavior">2</integer>
 
     <!-- Array of light sensor LUX values to define our levels for auto backlight brightness support.
-         The N entries of this array define N + 1 zones as follows:
+         The N entries of this array define N + 1 control points as follows:
 
-         Zone 0:        0 <= LUX < array[0]
-         Zone 1:        array[0] <= LUX < array[1]
+         Point 1:        LUX <= 0 (implicit)
+         Point 2:        0 < level[1] == LUX < level[2]
          ...
-         Zone N:        array[N - 1] <= LUX < array[N]
-         Zone N + 1:    array[N] <= LUX < infinity
+         Point N:        level[N - 1] == LUX < level[N]
+         Point N + 1:    level[N] <= LUX < infinity
+
+         The control points must be strictly increasing.  Each control point
+         corresponds to an entry in the brightness backlight values arrays.
+         For example, if LUX == level[1] (first element of the levels array)
+         then the brightness will be determined by value[1] (first element
+         of the brightness values array).
+
+         Spline interpolation is used to determine the auto-brightness
+         backlight values for LUX levels between these control points.
 
          Must be overridden in platform specific overlays -->
     <integer-array name="config_autoBrightnessLevels">
@@ -552,6 +561,7 @@
     <!-- Array of output values for LCD backlight corresponding to the LUX values
          in the config_autoBrightnessLevels array.  This array should have size one greater
          than the size of the config_autoBrightnessLevels array.
+         The brightness values must be between 0 and 255 and be non-decreasing.
          This must be overridden in platform specific overlays -->
     <integer-array name="config_autoBrightnessLcdBacklightValues">
     </integer-array>
@@ -559,6 +569,7 @@
     <!-- Array of output values for button backlight corresponding to the LUX values
          in the config_autoBrightnessLevels array.  This array should have size one greater
          than the size of the config_autoBrightnessLevels array.
+         The brightness values must be between 0 and 255 and be non-decreasing.
          This must be overridden in platform specific overlays -->
     <integer-array name="config_autoBrightnessButtonBacklightValues">
     </integer-array>
@@ -566,6 +577,7 @@
     <!-- Array of output values for keyboard backlight corresponding to the LUX values
          in the config_autoBrightnessLevels array.  This array should have size one greater
          than the size of the config_autoBrightnessLevels array.
+         The brightness values must be between 0 and 255 and be non-decreasing.
          This must be overridden in platform specific overlays -->
     <integer-array name="config_autoBrightnessKeyboardBacklightValues">
     </integer-array>
diff --git a/services/java/com/android/server/power/DisplayPowerController.java b/services/java/com/android/server/power/DisplayPowerController.java
index 54e2b574a59c8..b0c79fa13d95c 100644
--- a/services/java/com/android/server/power/DisplayPowerController.java
+++ b/services/java/com/android/server/power/DisplayPowerController.java
@@ -33,11 +33,11 @@ import android.os.Looper;
 import android.os.Message;
 import android.os.SystemClock;
 import android.util.Slog;
+import android.util.Spline;
 import android.util.TimeUtils;
 
 import java.io.PrintWriter;
 import java.io.StringWriter;
-import java.util.Arrays;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executor;
 
@@ -98,9 +98,9 @@ final class DisplayPowerController {
     // average of light samples.  Different constants are used
     // to calculate the average light level when adapting to brighter or
     // dimmer environments.
-    // This parameter only controls the averaging of light samples.
-    private static final long BRIGHTENING_LIGHT_TIME_CONSTANT = 1500;
-    private static final long DIMMING_LIGHT_TIME_CONSTANT = 3000;
+    // This parameter only controls the filtering of light samples.
+    private static final long BRIGHTENING_LIGHT_TIME_CONSTANT = 500;
+    private static final long DIMMING_LIGHT_TIME_CONSTANT = 2000;
 
     // Stability requirements in milliseconds for accepting a new brightness
     // level.  This is used for debouncing the light sensor.  Different constants
@@ -144,8 +144,7 @@ final class DisplayPowerController {
 
     // Auto-brightness.
     private boolean mUseSoftwareAutoBrightnessConfig;
-    private int[] mAutoBrightnessLevelsConfig;
-    private int[] mAutoBrightnessLcdBacklightValuesConfig;
+    private Spline mScreenAutoBrightnessSpline;
 
     // Amount of time to delay auto-brightness after screen on while waiting for
     // the light sensor to warm-up in milliseconds.
@@ -289,17 +288,18 @@ final class DisplayPowerController {
         mUseSoftwareAutoBrightnessConfig = resources.getBoolean(
                 com.android.internal.R.bool.config_automatic_brightness_available);
         if (mUseSoftwareAutoBrightnessConfig) {
-            mAutoBrightnessLevelsConfig = resources.getIntArray(
+            int[] lux = resources.getIntArray(
                     com.android.internal.R.array.config_autoBrightnessLevels);
-            mAutoBrightnessLcdBacklightValuesConfig = resources.getIntArray(
+            int[] screenBrightness = resources.getIntArray(
                     com.android.internal.R.array.config_autoBrightnessLcdBacklightValues);
-            if (mAutoBrightnessLcdBacklightValuesConfig.length
-                    != mAutoBrightnessLevelsConfig.length + 1) {
+
+            mScreenAutoBrightnessSpline = createAutoBrightnessSpline(lux, screenBrightness);
+            if (mScreenAutoBrightnessSpline == null) {
                 Slog.e(TAG, "Error in config.xml.  config_autoBrightnessLcdBacklightValues "
-                        + "(size " + mAutoBrightnessLcdBacklightValuesConfig.length + ") "
-                        + "should have exactly one more entry than "
-                        + "config_autoBrightnessLevels (size "
-                        + mAutoBrightnessLevelsConfig.length + ").  "
+                        + "(size " + screenBrightness.length + ") "
+                        + "must be monotic and have exactly one more entry than "
+                        + "config_autoBrightnessLevels (size " + lux.length + ") "
+                        + "which must be strictly increasing.  "
                         + "Auto-brightness will be disabled.");
                 mUseSoftwareAutoBrightnessConfig = false;
             }
@@ -322,6 +322,31 @@ final class DisplayPowerController {
         }
     }
 
+    private static Spline createAutoBrightnessSpline(int[] lux, int[] brightness) {
+        try {
+            final int n = brightness.length;
+            float[] x = new float[n];
+            float[] y = new float[n];
+            y[0] = brightness[0];
+            for (int i = 1; i < n; i++) {
+                x[i] = lux[i - 1];
+                y[i] = brightness[i];
+            }
+
+            Spline spline = Spline.createMonotoneCubicSpline(x, y);
+            if (false) {
+                Slog.d(TAG, "Auto-brightness spline: " + spline);
+                for (float v = 1f; v < lux[lux.length - 1] * 1.25f; v *= 1.25f) {
+                    Slog.d(TAG, String.format("  %7.1f: %7.1f", v, spline.interpolate(v)));
+                }
+            }
+            return spline;
+        } catch (IllegalArgumentException ex) {
+            Slog.e(TAG, "Could not create auto-brightness spline.", ex);
+            return null;
+        }
+    }
+
     /**
      * Returns true if the proximity sensor screen-off function is available.
      */
@@ -768,13 +793,13 @@ final class DisplayPowerController {
             return;
         }
 
-        final int newScreenAutoBrightness = mapLuxToBrightness(mLightMeasurement,
-                mAutoBrightnessLevelsConfig,
-                mAutoBrightnessLcdBacklightValuesConfig);
+        final int newScreenAutoBrightness = interpolateBrightness(
+                mScreenAutoBrightnessSpline, mLightMeasurement);
         if (mScreenAutoBrightness != newScreenAutoBrightness) {
             if (DEBUG) {
                 Slog.d(TAG, "updateAutoBrightness: mScreenAutoBrightness="
-                        + mScreenAutoBrightness);
+                        + mScreenAutoBrightness + "newScreenAutoBrightness="
+                        + newScreenAutoBrightness);
             }
 
             mScreenAutoBrightness = newScreenAutoBrightness;
@@ -784,20 +809,8 @@ final class DisplayPowerController {
         }
     }
 
-    /**
-     * Maps a light sensor measurement in lux to a brightness value given
-     * a table of lux breakpoint values and a table of brightnesses that
-     * is one element larger.
-     */
-    private static int mapLuxToBrightness(float lux,
-            int[] fromLux, int[] toBrightness) {
-        // TODO implement interpolation and possibly range expansion
-        int level = 0;
-        final int count = fromLux.length;
-        while (level < count && lux >= fromLux[level]) {
-            level += 1;
-        }
-        return toBrightness[level];
+    private static int interpolateBrightness(Spline spline, float lux) {
+        return Math.min(255, Math.max(0, (int)Math.round(spline.interpolate(lux))));
     }
 
     private void sendOnStateChanged() {
@@ -839,10 +852,7 @@ final class DisplayPowerController {
         pw.println("  mScreenBrightnessDimConfig=" + mScreenBrightnessDimConfig);
         pw.println("  mUseSoftwareAutoBrightnessConfig="
                 + mUseSoftwareAutoBrightnessConfig);
-        pw.println("  mAutoBrightnessLevelsConfig="
-                + Arrays.toString(mAutoBrightnessLevelsConfig));
-        pw.println("  mAutoBrightnessLcdBacklightValuesConfig="
-                + Arrays.toString(mAutoBrightnessLcdBacklightValuesConfig));
+        pw.println("  mScreenAutoBrightnessSpline=" + mScreenAutoBrightnessSpline);
         pw.println("  mLightSensorWarmUpTimeConfig=" + mLightSensorWarmUpTimeConfig);
 
         if (Looper.myLooper() == mHandler.getLooper()) {