Fix ArcMotion path creation in some quadrants.

Bug 27577177

When ArcMotion created paths in some quadrants, the control
points were placed in the wrong place, making a non-circular
arc instead of an arc of a circle.

Change-Id: Ie842524c09385038fb1f991b877b01ee5e740f20

core/java/android/transition/ArcMotion.java

@@ -15,13 +15,13 @@
  */
 package android.transition;
 
-import com.android.internal.R;
-
 import android.content.Context;
 import android.content.res.TypedArray;
 import android.graphics.Path;
 import android.util.AttributeSet;
 
+import com.android.internal.R;
+
 /**
  * A PathMotion that generates a curved path along an arc on an imaginary circle containing
  * the two points. If the horizontal distance between the points is less than the vertical
@@ -207,7 +207,7 @@ public class ArcMotion extends PathMotion {
             ey = (startY + endY) / 2;
         } else {
             float deltaX = endX - startX;
-            float deltaY = startY - endY; // Y is inverted compared to diagram above.
+            float deltaY = endY - startY;
             // hypotenuse squared.
             float h2 = deltaX * deltaX + deltaY * deltaY;
 
@@ -219,24 +219,35 @@ public class ArcMotion extends PathMotion {
             float midDist2 = h2 * 0.25f;
 
             float minimumArcDist2 = 0;
+            boolean isQuadrant1Or3 = (deltaX * deltaY) > 0;
 
-            if (Math.abs(deltaX) < Math.abs(deltaY)) {
+            if ((Math.abs(deltaX) < Math.abs(deltaY))) {
                 // Similar triangles bfa and bde mean that (ab/fb = eb/bd)
                 // Therefore, eb = ab * bd / fb
                 // ab = hypotenuse
                 // bd = hypotenuse/2
                 // fb = deltaY
                 float eDistY = h2 / (2 * deltaY);
-                ey = endY + eDistY;
-                ex = endX;
+                if (isQuadrant1Or3) {
+                    ey = startY + eDistY;
+                    ex = startX;
+                } else {
+                    ey = endY - eDistY;
+                    ex = endX;
+                }
 
                 minimumArcDist2 = midDist2 * mMinimumVerticalTangent
                         * mMinimumVerticalTangent;
             } else {
                 // Same as above, but flip X & Y
                 float eDistX = h2 / (2 * deltaX);
-                ex = endX + eDistX;
-                ey = endY;
+                if (isQuadrant1Or3) {
+                    ex = endX - eDistX;
+                    ey = endY;
+                } else {
+                    ex = startX + eDistX;
+                    ey = startY;
+                }
 
                 minimumArcDist2 = midDist2 * mMinimumHorizontalTangent
                         * mMinimumHorizontalTangent;