diff --git a/services/core/java/com/android/server/wm/CircularDisplayMask.java b/services/core/java/com/android/server/wm/CircularDisplayMask.java
index ae41541310799..0a9b33404b37a 100644
--- a/services/core/java/com/android/server/wm/CircularDisplayMask.java
+++ b/services/core/java/com/android/server/wm/CircularDisplayMask.java
@@ -21,6 +21,7 @@ import static com.android.server.wm.WindowManagerDebugConfig.DEBUG_SURFACE_TRACE
 import static com.android.server.wm.WindowManagerDebugConfig.TAG_WITH_CLASS_NAME;
 import static com.android.server.wm.WindowManagerDebugConfig.TAG_WM;
 
+import android.graphics.Bitmap;
 import android.graphics.Canvas;
 import android.graphics.Color;
 import android.graphics.Paint;
@@ -85,12 +86,115 @@ class CircularDisplayMask {
         mSurfaceControl = ctrl;
         mDrawNeeded = true;
         mPaint = new Paint();
-        mPaint.setAntiAlias(true);
-        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
+        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
         mScreenOffset = screenOffset;
         mMaskThickness = maskThickness;
     }
 
+    static private double distanceFromCenterSquared(double x, double y) {
+        return x*x + y*y;
+    }
+
+    static private double distanceFromCenter(double x, double y) {
+        return Math.sqrt(distanceFromCenterSquared(x, y));
+    }
+
+    static private double verticalLineIntersectsCircle(double x, double radius) {
+        return Math.sqrt(radius*radius - x*x);
+    }
+
+    static private double  horizontalLineIntersectsCircle(double y, double radius) {
+        return Math.sqrt(radius*radius - y*y);
+    }
+
+    static private double triangleArea(double width, double height) {
+        return width * height / 2.0;
+    }
+
+    static private double trapezoidArea(double width, double height1, double height2) {
+        return width * (height1 + height2) / 2.0;
+    }
+
+    static private double areaUnderChord(double radius, double chordLength) {
+        double isocelesHeight = Math.sqrt(radius*radius - chordLength * chordLength / 4.0);
+        double areaUnderIsoceles = isocelesHeight * chordLength / 2.0;
+        double halfAngle = Math.asin(chordLength / (2.0 * radius));
+        double areaUnderArc = halfAngle * radius * radius;
+
+        return areaUnderArc - triangleArea(chordLength, isocelesHeight);
+    }
+
+    // Returns the fraction of the pixel at (px, py) covered by
+    // the circle with center (cx, cy) and radius 'radius'
+    static private double calcPixelShading(double cx, double cy, double px,
+            double py, double radius) {
+        // Translate so the center is at the origin
+        px -= cx;
+        py -= cy;
+
+        // Reflect across the axis so the point is in the first quadrant
+        px = Math.abs(px);
+        py = Math.abs(py);
+
+        // One more transformation which simplifies the logic later
+        if (py > px) {
+            double temp;
+
+            temp = px;
+            px = py;
+            py = temp;
+        }
+
+        double left = px - 0.5;
+        double right = px + 0.5;
+        double bottom = py - 0.5;
+        double top = py + 0.5;
+
+        if (distanceFromCenterSquared(left, bottom) > radius*radius) {
+            return 0.0;
+        }
+
+        if (distanceFromCenterSquared(right, top) < radius*radius) {
+            return 1.0;
+        }
+
+        // Check if only the bottom-left corner of the pixel is inside the circle
+        if (distanceFromCenterSquared(left, top) > radius*radius) {
+            double triangleWidth = horizontalLineIntersectsCircle(bottom, radius) - left;
+            double triangleHeight = verticalLineIntersectsCircle(left, radius) - bottom;
+            double chordLength = distanceFromCenter(triangleWidth, triangleHeight);
+
+            return triangleArea(triangleWidth, triangleHeight)
+                   + areaUnderChord(radius, chordLength);
+
+        }
+
+        // Check if only the top-right corner of the pixel is outside the circle
+        if (distanceFromCenterSquared(right, bottom) < radius*radius) {
+            double triangleWidth = right - horizontalLineIntersectsCircle(top, radius);
+            double triangleHeight = top - verticalLineIntersectsCircle(right, radius);
+            double chordLength = distanceFromCenter(triangleWidth, triangleHeight);
+
+            return 1 - triangleArea(triangleWidth, triangleHeight)
+                   + areaUnderChord(radius, chordLength);
+        }
+
+        // It must be that the top-left and bottom-left corners are inside the circle
+        double trapezoidWidth1 = horizontalLineIntersectsCircle(top, radius) - left;
+        double trapezoidWidth2 = horizontalLineIntersectsCircle(bottom, radius) - left;
+        double chordLength = distanceFromCenter(1, trapezoidWidth2 - trapezoidWidth1);
+        double shading = trapezoidArea(1.0, trapezoidWidth1, trapezoidWidth2)
+                         + areaUnderChord(radius, chordLength);
+
+        // When top >= 0 and bottom <= 0 it's possible for the circle to intersect the pixel 4 times.
+        // If so, remove the area of the section which crosses the right-hand edge.
+        if (top >= 0 && bottom <= 0 && radius > right) {
+            shading -= areaUnderChord(radius, 2 * verticalLineIntersectsCircle(right, radius));
+        }
+
+        return shading;
+    }
+
     private void drawIfNeeded() {
         if (!mDrawNeeded || !mVisible || mDimensionsUnequal) {
             return;
@@ -123,11 +227,41 @@ class CircularDisplayMask {
             break;
         }
 
-        int circleRadius = mScreenSize.x / 2;
         c.drawColor(Color.BLACK);
 
-        // The radius is reduced by mMaskThickness to provide an anti aliasing effect on the display edges.
-        c.drawCircle(circleRadius, circleRadius, circleRadius - mMaskThickness, mPaint);
+        int maskWidth = mScreenSize.x - 2*mMaskThickness;
+        int maskHeight;
+
+        // Don't render the whole mask if it is partly offscreen.
+        if (maskWidth > mScreenSize.y) {
+            maskHeight = mScreenSize.y;
+        } else {
+            // To ensure the mask can be properly centered on the canvas the
+            // bitmap dimensions must have the same parity as those of the canvas.
+            maskHeight = mScreenSize.y - ((mScreenSize.y - maskWidth) & ~1);
+        }
+
+        double cx = (maskWidth - 1.0) / 2.0;
+        double cy = (maskHeight - 1.0) / 2.0;
+        double radius = maskWidth / 2.0;
+        int[] pixels = new int[maskWidth * maskHeight];
+
+        for (int py=0; py<maskHeight; py++) {
+            for (int px=0; px<maskWidth; px++) {
+                double shading = calcPixelShading(cx, cy, px, py, radius);
+                pixels[maskWidth*py + px] =
+                    Color.argb(255 - (int)Math.round(255.0*shading), 0, 0, 0);
+            }
+        }
+
+        Bitmap transparency = Bitmap.createBitmap(pixels, maskWidth, maskHeight,
+            Bitmap.Config.ARGB_8888);
+
+        c.drawBitmap(transparency,
+                     (float)mMaskThickness,
+                     (float)((mScreenSize.y - maskHeight) / 2),
+                     mPaint);
+
         mSurface.unlockCanvasAndPost(c);
     }