Bug 775451: Reduce diagonal checkerboarding r=cjones

gfx/layers/ipc/AsyncPanZoomController.cpp

@@ -449,11 +449,8 @@ float AsyncPanZoomController::PanDistance() {
   return NS_hypot(mX.PanDistance(), mY.PanDistance());
 }
 
-const nsPoint AsyncPanZoomController::GetVelocityVector() {
-  return nsPoint(
-    mX.GetVelocity(),
-    mY.GetVelocity()
-  );
+const gfx::Point AsyncPanZoomController::GetVelocityVector() {
+  return gfx::Point(mX.GetVelocity(), mY.GetVelocity());
 }
 
 void AsyncPanZoomController::StartPanning(const MultiTouchInput& aEvent) {
@@ -607,13 +604,30 @@ void AsyncPanZoomController::ScaleWithFocus(float aScale, const nsIntPoint& aFoc
   mFrameMetrics = metrics;
 }
 
+bool AsyncPanZoomController::EnlargeDisplayPortAlongAxis(float aViewport,
+                                                         float aVelocity,
+                                                         float* aDisplayPortOffset,
+                                                         float* aDisplayPortLength)
+{
+  const float MIN_SKATE_SIZE_MULTIPLIER = 2.0f;
+  const float MAX_SKATE_SIZE_MULTIPLIER = 4.0f;
+
+  if (fabsf(aVelocity) > MIN_SKATE_SPEED) {
+    *aDisplayPortLength = aViewport * clamped(fabsf(aVelocity),
+      MIN_SKATE_SIZE_MULTIPLIER, MAX_SKATE_SIZE_MULTIPLIER);
+    *aDisplayPortOffset = aVelocity > 0 ? 0 : aViewport - *aDisplayPortLength;
+    return true;
+  }
+  return false;
+}
+
 const nsIntRect AsyncPanZoomController::CalculatePendingDisplayPort() {
   float scale = mFrameMetrics.mResolution.width;
   nsIntRect viewport = mFrameMetrics.mViewport;
   viewport.ScaleRoundIn(1 / scale);
 
   nsIntPoint scrollOffset = mFrameMetrics.mViewportScrollOffset;
-  nsPoint velocity = GetVelocityVector();
+  gfx::Point velocity = GetVelocityVector();
 
   // The displayport is relative to the current scroll offset. Here's a little
   // diagram to make it easier to see:
@@ -640,28 +654,25 @@ const nsIntRect AsyncPanZoomController::CalculatePendingDisplayPort() {
   // and far top, it is clear that this distance is 1/4 of the displayport's
   // height/width dimension.
   const float STATIONARY_SIZE_MULTIPLIER = 2.0f;
-  const float SKATE_SIZE_MULTIPLIER = 3.0f;
   gfx::Rect displayPort(0, 0,
                         viewport.width * STATIONARY_SIZE_MULTIPLIER,
                         viewport.height * STATIONARY_SIZE_MULTIPLIER);
 
-  // Iff there's motion along only one axis of movement, and it's above a
-  // threshold, then we want to paint a larger area in the direction of that
-  // motion so that it's less likely to checkerboard. Also note that the other
-  // axis doesn't need its displayport enlarged beyond the viewport dimension,
-  // since it is impossible for it to checkerboard along that axis until motion
-  // begins on it.
-  if (fabsf(velocity.x) > MIN_SKATE_SPEED && fabsf(velocity.y) < MIN_SKATE_SPEED) {
-    displayPort.height = viewport.height;
-    displayPort.width = viewport.width * SKATE_SIZE_MULTIPLIER;
-    displayPort.x = velocity.x > 0 ? 0 : viewport.width - displayPort.width;
-  } else if (fabsf(velocity.x) < MIN_SKATE_SPEED && fabsf(velocity.y) > MIN_SKATE_SPEED) {
-    displayPort.width = viewport.width;
-    displayPort.height = viewport.height * SKATE_SIZE_MULTIPLIER;
-    displayPort.y = velocity.y > 0 ? 0 : viewport.height - displayPort.height;
-  } else {
+  // If there's motion along an axis of movement, and it's above a threshold,
+  // then we want to paint a larger area in the direction of that motion so that
+  // it's less likely to checkerboard.
+  bool enlargedX = EnlargeDisplayPortAlongAxis(
+    viewport.width, velocity.x, &displayPort.x, &displayPort.width);
+  bool enlargedY = EnlargeDisplayPortAlongAxis(
+    viewport.height, velocity.y, &displayPort.y, &displayPort.height);
+
+  if (!enlargedX && !enlargedY) {
     displayPort.x = -displayPort.width / 4;
     displayPort.y = -displayPort.height / 4;
+  } else if (!enlargedX) {
+    displayPort.width = viewport.width;
+  } else if (!enlargedY) {
+    displayPort.height = viewport.height;
   }
 
   gfx::Rect shiftedDisplayPort = displayPort;

gfx/layers/ipc/AsyncPanZoomController.h

@@ -291,7 +291,7 @@ protected:
   /**
    * Gets a vector of the velocities of each axis.
    */
-  const nsPoint GetVelocityVector();
+  const gfx::Point GetVelocityVector();
 
   /**
    * Gets a reference to the first SingleTouchData from a MultiTouchInput.  This
@@ -326,6 +326,17 @@ protected:
    */
   const nsIntRect CalculatePendingDisplayPort();
 
+  /**
+   * Attempts to enlarge the displayport along a single axis. Returns whether or
+   * not the displayport was enlarged. This will fail in circumstances where the
+   * velocity along that axis is not high enough to need any changes. The
+   * displayport metrics are expected to be passed into |aDisplayPortOffset| and
+   * |aDisplayPortLength|. If enlarged, these will be updated with the new
+   * metrics.
+   */
+  bool EnlargeDisplayPortAlongAxis(float aViewport, float aVelocity,
+                                   float* aDisplayPortOffset, float* aDisplayPortLength);
+
   /**
    * Utility function to send updated FrameMetrics to Gecko so that it can paint
    * the displayport area. Calls into GeckoContentController to do the actual