implement the logic to set tile priorities based on current matrix

cc/tile_priority.cc

+// Copyright 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "cc/tile_priority.h"
+
+namespace {
+
+const double kMaxTimeToVisibleInSeconds = 1000.0;
+
+// TODO(qinmin): modify ui/range/Range.h to support template so that we
+// don't need to define this.
+struct Range {
+  Range(double start, double end) : start_(start), end_(end) {}
+  Range Intersects(const Range& other);
+  bool IsEmpty();
+  double start_;
+  double end_;
+};
+
+Range Range::Intersects(const Range& other) {
+  start_ = std::max(start_, other.start_);
+  end_ = std::min(end_, other.end_);
+  return Range(start_, end_);
+}
+
+bool Range::IsEmpty() {
+  return start_ >= end_;
+}
+
+// Calculate a time range that |value| will be larger than |threshold|
+// given the velocity of its change.
+Range TimeRangeValueLargerThanThreshold(
+    int value, int threshold, double velocity) {
+  double minimum_time = 0;
+  double maximum_time = kMaxTimeToVisibleInSeconds;
+
+  if (velocity > 0) {
+    if (value < threshold)
+      minimum_time = std::min(kMaxTimeToVisibleInSeconds,
+                              (threshold - value) / velocity);
+  } else if (velocity <= 0) {
+    if (value < threshold)
+      minimum_time = kMaxTimeToVisibleInSeconds;
+    else if (velocity != 0)
+      maximum_time = std::min(maximum_time, (threshold - value) / velocity);
+  }
+
+  return Range(minimum_time, maximum_time);
+}
+
+}  // namespace
+
+namespace cc {
+
+int TilePriority::manhattanDistance(const gfx::RectF& a, const gfx::RectF& b) {

[enne (OOO), 2012/12/03 19:57:28]

Can you write a quick unit test for this math?

[qinmin, 2012/12/04 00:37:30]

Done.

+  gfx::RectF c = gfx::UnionRects(a, b);
+  // Rects touching the edge of the screen should not be considered visible.
+  // So we add 1 pixel here to avoid that situation.
+  int x = static_cast<int>(
+      std::max(0.0f, c.width() - a.width() - b.width() + 1));
+  int y = static_cast<int>(
+      std::max(0.0f, c.height() - a.height() - b.height() + 1));
+  return (x + y);
+}
+
+double TilePriority::TimeForBoundsToIntersect(gfx::RectF previous_bounds,
+                                              gfx::RectF current_bounds,
+                                              double time_delta,
+                                              gfx::RectF target_bounds) {
+  if (current_bounds.Intersects(target_bounds))
+    return 0;
+
+  if (previous_bounds.Intersects(target_bounds) || time_delta == 0)
+    return kMaxTimeToVisibleInSeconds;
+
+  // As we are trying to solve the case of both scaling and scrolling, using
+  // a single coordinate with velocity is not enough. The logic here is to
+  // calculate the velocity for each edge. Then we calculate the time range that
+  // each edge will stay on the same side of the target bounds. If there is an
+  // overlap between these time ranges, the bounds must have intersect with
+  // each other during that period of time.
+  double velocity =
+      (current_bounds.right() - previous_bounds.right()) / time_delta;
+  Range range = TimeRangeValueLargerThanThreshold(
+      current_bounds.right(), target_bounds.x(), velocity);
+
+  velocity = (current_bounds.x() - previous_bounds.x()) / time_delta;
+  range = range.Intersects(TimeRangeValueLargerThanThreshold(
+      -current_bounds.x(), -target_bounds.right(), -velocity));
+
+
+  velocity = (current_bounds.y() - previous_bounds.y()) / time_delta;
+  range = range.Intersects(TimeRangeValueLargerThanThreshold(
+      -current_bounds.y(), -target_bounds.bottom(), -velocity));
+
+  velocity = (current_bounds.bottom() - previous_bounds.bottom()) / time_delta;
+  range = range.Intersects(TimeRangeValueLargerThanThreshold(
+      current_bounds.bottom(), target_bounds.y(), velocity));
+
+  return range.IsEmpty() ? kMaxTimeToVisibleInSeconds : range.start_;
+}
+
+}  // namespace cc