Revert of Splitting of layers for correct intersections

cc/trees/layer_sorter.cc

+// Copyright 2011 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/trees/layer_sorter.h"
+
+#include <algorithm>
+#include <deque>
+#include <limits>
+#include <vector>
+
+#include "base/logging.h"
+#include "cc/base/math_util.h"
+#include "cc/layers/render_surface_impl.h"
+#include "ui/gfx/transform.h"
+
+namespace cc {
+
+// This epsilon is used to determine if two layers are too close to each other
+// to be able to tell which is in front of the other.  It's a relative epsilon
+// so it is robust to changes in scene scale.  This value was chosen by picking
+// a value near machine epsilon and then increasing it until the flickering on
+// the test scene went away.
+const float k_layer_epsilon = 1e-4f;
+
+inline static float PerpProduct(const gfx::Vector2dF& u,
+                                const gfx::Vector2dF& v) {
+  return u.x() * v.y() - u.y() * v.x();
+}
+
+// Tests if two edges defined by their endpoints (a,b) and (c,d) intersect.
+// Returns true and the point of intersection if they do and false otherwise.
+static bool EdgeEdgeTest(const gfx::PointF& a,
+                         const gfx::PointF& b,
+                         const gfx::PointF& c,
+                         const gfx::PointF& d,
+                         gfx::PointF* r) {
+  gfx::Vector2dF u = b - a;
+  gfx::Vector2dF v = d - c;
+  gfx::Vector2dF w = a - c;
+
+  float denom = PerpProduct(u, v);
+
+  // If denom == 0 then the edges are parallel. While they could be overlapping
+  // we don't bother to check here as the we'll find their intersections from
+  // the corner to quad tests.
+  if (!denom)
+    return false;
+
+  float s = PerpProduct(v, w) / denom;
+  if (s < 0.f || s > 1.f)
+    return false;
+
+  float t = PerpProduct(u, w) / denom;
+  if (t < 0.f || t > 1.f)
+    return false;
+
+  u.Scale(s);
+  *r = a + u;
+  return true;
+}
+
+GraphNode::GraphNode(LayerImpl* layer_impl)
+    : layer(layer_impl),
+      incoming_edge_weight(0.f) {}
+
+GraphNode::~GraphNode() {}
+
+LayerSorter::LayerSorter()
+    : z_range_(0.f) {}
+
+LayerSorter::~LayerSorter() {}
+
+static float CheckFloatingPointNumericAccuracy(float a, float b) {
+  float abs_dif = std::abs(b - a);
+  float abs_max = std::max(std::abs(b), std::abs(a));
+  // Check to see if we've got a result with a reasonable amount of error.
+  return abs_dif / abs_max;
+}
+
+// Checks whether layer "a" draws on top of layer "b". The weight value returned
+// is an indication of the maximum z-depth difference between the layers or zero
+// if the layers are found to be intesecting (some features are in front and
+// some are behind).
+LayerSorter::ABCompareResult LayerSorter::CheckOverlap(LayerShape* a,
+                                                       LayerShape* b,
+                                                       float z_threshold,
+                                                       float* weight) {
+  *weight = 0.f;
+
+  // Early out if the projected bounds don't overlap.
+  if (!a->projected_bounds.Intersects(b->projected_bounds))
+    return NONE;
+
+  gfx::PointF aPoints[4] = { a->projected_quad.p1(),
+                             a->projected_quad.p2(),
+                             a->projected_quad.p3(),
+                             a->projected_quad.p4() };
+  gfx::PointF bPoints[4] = { b->projected_quad.p1(),
+                             b->projected_quad.p2(),
+                             b->projected_quad.p3(),
+                             b->projected_quad.p4() };
+
+  // Make a list of points that inside both layer quad projections.
+  std::vector<gfx::PointF> overlap_points;
+
+  // Check all four corners of one layer against the other layer's quad.
+  for (int i = 0; i < 4; ++i) {
+    if (a->projected_quad.Contains(bPoints[i]))
+      overlap_points.push_back(bPoints[i]);
+    if (b->projected_quad.Contains(aPoints[i]))
+      overlap_points.push_back(aPoints[i]);
+  }
+
+  // Check all the edges of one layer for intersection with the other layer's
+  // edges.
+  gfx::PointF r;
+  for (int ea = 0; ea < 4; ++ea)
+    for (int eb = 0; eb < 4; ++eb)
+      if (EdgeEdgeTest(aPoints[ea], aPoints[(ea + 1) % 4],
+                       bPoints[eb], bPoints[(eb + 1) % 4],
+                       &r))
+        overlap_points.push_back(r);
+
+  if (overlap_points.empty())
+    return NONE;
+
+  // Check the corresponding layer depth value for all overlap points to
+  // determine which layer is in front.
+  float max_positive = 0.f;
+  float max_negative = 0.f;
+
+  // This flag tracks the existance of a numerically accurate seperation
+  // between two layers.  If there is no accurate seperation, the layers
+  // cannot be effectively sorted.
+  bool accurate = false;
+
+  for (size_t o = 0; o < overlap_points.size(); o++) {
+    float za = a->LayerZFromProjectedPoint(overlap_points[o]);
+    float zb = b->LayerZFromProjectedPoint(overlap_points[o]);
+
+    // Here we attempt to avoid numeric issues with layers that are too
+    // close together.  If we have 2-sided quads that are very close
+    // together then we will draw them in document order to avoid
+    // flickering.  The correct solution is for the content maker to turn
+    // on back-face culling or move the quads apart (if they're not two
+    // sides of one object).
+    if (CheckFloatingPointNumericAccuracy(za, zb) > k_layer_epsilon)
+      accurate = true;
+
+    float diff = za - zb;
+    if (diff > max_positive)
+      max_positive = diff;
+    if (diff < max_negative)
+      max_negative = diff;
+  }
+
+  // If we can't tell which should come first, we use document order.
+  if (!accurate)
+    return A_BEFORE_B;
+
+  float max_diff =
+      std::abs(max_positive) > std::abs(max_negative) ?
+          max_positive : max_negative;
+
+  // If the results are inconsistent (and the z difference substantial to rule
+  // out numerical errors) then the layers are intersecting. We will still
+  // return an order based on the maximum depth difference but with an edge
+  // weight of zero these layers will get priority if a graph cycle is present
+  // and needs to be broken.
+  if (max_positive > z_threshold && max_negative < -z_threshold)
+    *weight = 0.f;
+  else
+    *weight = std::abs(max_diff);
+
+  // Maintain relative order if the layers have the same depth at all
+  // intersection points.
+  if (max_diff <= 0.f)
+    return A_BEFORE_B;
+
+  return B_BEFORE_A;
+}
+
+LayerShape::LayerShape() {}
+
+LayerShape::LayerShape(float width,
+                       float height,
+                       const gfx::Transform& draw_transform) {
+  gfx::QuadF layer_quad(gfx::RectF(0.f, 0.f, width, height));
+
+  // Compute the projection of the layer quad onto the z = 0 plane.
+
+  gfx::PointF clipped_quad[8];
+  int num_vertices_in_clipped_quad;
+  MathUtil::MapClippedQuad(draw_transform,
+                           layer_quad,
+                           clipped_quad,
+                           &num_vertices_in_clipped_quad);
+
+  if (num_vertices_in_clipped_quad < 3) {
+    projected_bounds = gfx::RectF();
+    return;
+  }
+
+  projected_bounds =
+      MathUtil::ComputeEnclosingRectOfVertices(clipped_quad,
+                                               num_vertices_in_clipped_quad);
+
+  // NOTE: it will require very significant refactoring and overhead to deal
+  // with generalized polygons or multiple quads per layer here. For the sake of
+  // layer sorting it is equally correct to take a subsection of the polygon
+  // that can be made into a quad. This will only be incorrect in the case of
+  // intersecting layers, which are not supported yet anyway.
+  projected_quad.set_p1(clipped_quad[0]);
+  projected_quad.set_p2(clipped_quad[1]);
+  projected_quad.set_p3(clipped_quad[2]);
+  if (num_vertices_in_clipped_quad >= 4) {
+    projected_quad.set_p4(clipped_quad[3]);
+  } else {
+    // This will be a degenerate quad that is actually a triangle.
+    projected_quad.set_p4(clipped_quad[2]);
+  }
+
+  // Compute the normal of the layer's plane.
+  bool clipped = false;
+  gfx::Point3F c1 =
+      MathUtil::MapPoint(draw_transform, gfx::Point3F(0.f, 0.f, 0.f), &clipped);
+  gfx::Point3F c2 =
+      MathUtil::MapPoint(draw_transform, gfx::Point3F(0.f, 1.f, 0.f), &clipped);
+  gfx::Point3F c3 =
+      MathUtil::MapPoint(draw_transform, gfx::Point3F(1.f, 0.f, 0.f), &clipped);
+  // TODO(shawnsingh): Deal with clipping.
+  gfx::Vector3dF c12 = c2 - c1;
+  gfx::Vector3dF c13 = c3 - c1;
+  layer_normal = gfx::CrossProduct(c13, c12);
+
+  transform_origin = c1;
+}
+
+LayerShape::~LayerShape() {}
+
+// Returns the Z coordinate of a point on the layer that projects
+// to point p which lies on the z = 0 plane. It does it by computing the
+// intersection of a line starting from p along the Z axis and the plane
+// of the layer.
+float LayerShape::LayerZFromProjectedPoint(const gfx::PointF& p) const {
+  gfx::Vector3dF z_axis(0.f, 0.f, 1.f);
+  gfx::Vector3dF w = gfx::Point3F(p) - transform_origin;
+
+  float d = gfx::DotProduct(layer_normal, z_axis);
+  float n = -gfx::DotProduct(layer_normal, w);
+
+  // Check if layer is parallel to the z = 0 axis which will make it
+  // invisible and hence returning zero is fine.
+  if (!d)
+    return 0.f;
+
+  // The intersection point would be given by:
+  // p + (n / d) * u  but since we are only interested in the
+  // z coordinate and p's z coord is zero, all we need is the value of n/d.
+  return n / d;
+}
+
+void LayerSorter::CreateGraphNodes(LayerImplList::iterator first,
+                                   LayerImplList::iterator last) {
+  DVLOG(2) << "Creating graph nodes:";
+  float min_z = FLT_MAX;
+  float max_z = -FLT_MAX;
+  for (LayerImplList::const_iterator it = first; it < last; it++) {
+    nodes_.push_back(GraphNode(*it));
+    GraphNode& node = nodes_.at(nodes_.size() - 1);
+    RenderSurfaceImpl* render_surface = node.layer->render_surface();
+    if (!node.layer->DrawsContent() && !render_surface)
+      continue;
+
+    DVLOG(2) << "Layer " << node.layer->id() <<
+        " (" << node.layer->bounds().width() <<
+        " x " << node.layer->bounds().height() << ")";
+
+    gfx::Transform draw_transform;
+    float layer_width, layer_height;
+    if (render_surface) {
+      draw_transform = render_surface->draw_transform();
+      layer_width = render_surface->content_rect().width();
+      layer_height = render_surface->content_rect().height();
+    } else {
+      draw_transform = node.layer->draw_transform();
+      layer_width = node.layer->content_bounds().width();
+      layer_height = node.layer->content_bounds().height();
+    }
+
+    node.shape = LayerShape(layer_width, layer_height, draw_transform);
+
+    max_z = std::max(max_z, node.shape.transform_origin.z());
+    min_z = std::min(min_z, node.shape.transform_origin.z());
+  }
+
+  z_range_ = std::abs(max_z - min_z);
+}
+
+void LayerSorter::CreateGraphEdges() {
+  DVLOG(2) << "Edges:";
+  // Fraction of the total z_range below which z differences
+  // are not considered reliable.
+  const float z_threshold_factor = 0.01f;
+  float z_threshold = z_range_ * z_threshold_factor;
+
+  for (size_t na = 0; na < nodes_.size(); na++) {
+    GraphNode& node_a = nodes_[na];
+    if (!node_a.layer->DrawsContent() && !node_a.layer->render_surface())
+      continue;
+    for (size_t nb = na + 1; nb < nodes_.size(); nb++) {
+      GraphNode& node_b = nodes_[nb];
+      if (!node_b.layer->DrawsContent() && !node_b.layer->render_surface())
+        continue;
+      float weight = 0.f;
+      ABCompareResult overlap_result = CheckOverlap(&node_a.shape,
+                                                    &node_b.shape,
+                                                    z_threshold,
+                                                    &weight);
+      GraphNode* start_node = NULL;
+      GraphNode* end_node = NULL;
+      if (overlap_result == A_BEFORE_B) {
+        start_node = &node_a;
+        end_node = &node_b;
+      } else if (overlap_result == B_BEFORE_A) {
+        start_node = &node_b;
+        end_node = &node_a;
+      }
+
+      if (start_node) {
+        DVLOG(2) << start_node->layer->id() << " -> " << end_node->layer->id();
+        edges_.push_back(GraphEdge(start_node, end_node, weight));
+      }
+    }
+  }
+
+  for (size_t i = 0; i < edges_.size(); i++) {
+    GraphEdge& edge = edges_[i];
+    active_edges_[&edge] = &edge;
+    edge.from->outgoing.push_back(&edge);
+    edge.to->incoming.push_back(&edge);
+    edge.to->incoming_edge_weight += edge.weight;
+  }
+}
+
+// Finds and removes an edge from the list by doing a swap with the
+// last element of the list.
+void LayerSorter::RemoveEdgeFromList(GraphEdge* edge,
+                                     std::vector<GraphEdge*>* list) {
+  std::vector<GraphEdge*>::iterator iter =
+      std::find(list->begin(), list->end(), edge);
+  DCHECK(iter != list->end());
+  list->erase(iter);
+}
+
+// Sorts the given list of layers such that they can be painted in a
+// back-to-front order. Sorting produces correct results for non-intersecting
+// layers that don't have cyclical order dependencies. Cycles and intersections
+// are broken (somewhat) aribtrarily. Sorting of layers is done via a
+// topological sort of a directed graph whose nodes are the layers themselves.
+// An edge from node A to node B signifies that layer A needs to be drawn before
+// layer B. If A and B have no dependency between each other, then we preserve
+// the ordering of those layers as they were in the original list.
+//
+// The draw order between two layers is determined by projecting the two
+// triangles making up each layer quad to the Z = 0 plane, finding points of
+// intersection between the triangles and backprojecting those points to the
+// plane of the layer to determine the corresponding Z coordinate. The layer
+// with the lower Z coordinate (farther from the eye) needs to be rendered
+// first.
+//
+// If the layer projections don't intersect, then no edges (dependencies) are
+// created between them in the graph. HOWEVER, in this case we still need to
+// preserve the ordering of the original list of layers, since that list should
+// already have proper z-index ordering of layers.
+//
+void LayerSorter::Sort(LayerImplList::iterator first,
+                       LayerImplList::iterator last) {
+  DVLOG(2) << "Sorting start ----";
+  CreateGraphNodes(first, last);
+
+  CreateGraphEdges();
+
+  std::vector<GraphNode*> sorted_list;
+  std::deque<GraphNode*> no_incoming_edge_node_list;
+
+  // Find all the nodes that don't have incoming edges.
+  for (NodeList::iterator la = nodes_.begin(); la < nodes_.end(); la++) {
+    if (!la->incoming.size())
+      no_incoming_edge_node_list.push_back(&(*la));
+  }
+
+  DVLOG(2) << "Sorted list: ";
+  while (active_edges_.size() || no_incoming_edge_node_list.size()) {
+    while (no_incoming_edge_node_list.size()) {
+      // It is necessary to preserve the existing ordering of layers, when there
+      // are no explicit dependencies (because this existing ordering has
+      // correct z-index/layout ordering). To preserve this ordering, we process
+      // Nodes in the same order that they were added to the list.
+      GraphNode* from_node = no_incoming_edge_node_list.front();
+      no_incoming_edge_node_list.pop_front();
+
+      // Add it to the final list.
+      sorted_list.push_back(from_node);
+
+      DVLOG(2) << from_node->layer->id() << ", ";
+
+      // Remove all its outgoing edges from the graph.
+      for (size_t i = 0; i < from_node->outgoing.size(); i++) {
+        GraphEdge* outgoing_edge = from_node->outgoing[i];
+
+        active_edges_.erase(outgoing_edge);
+        RemoveEdgeFromList(outgoing_edge, &outgoing_edge->to->incoming);
+        outgoing_edge->to->incoming_edge_weight -= outgoing_edge->weight;
+
+        if (!outgoing_edge->to->incoming.size())
+          no_incoming_edge_node_list.push_back(outgoing_edge->to);
+      }
+      from_node->outgoing.clear();
+    }
+
+    if (!active_edges_.size())
+      break;
+
+    // If there are still active edges but the list of nodes without incoming
+    // edges is empty then we have run into a cycle. Break the cycle by finding
+    // the node with the smallest overall incoming edge weight and use it. This
+    // will favor nodes that have zero-weight incoming edges i.e. layers that
+    // are being occluded by a layer that intersects them.
+    float min_incoming_edge_weight = FLT_MAX;
+    GraphNode* next_node = NULL;
+    for (size_t i = 0; i < nodes_.size(); i++) {
+      if (nodes_[i].incoming.size() &&
+          nodes_[i].incoming_edge_weight < min_incoming_edge_weight) {
+        min_incoming_edge_weight = nodes_[i].incoming_edge_weight;
+        next_node = &nodes_[i];
+      }
+    }
+    DCHECK(next_node);
+    // Remove all its incoming edges.
+    for (size_t e = 0; e < next_node->incoming.size(); e++) {
+      GraphEdge* incoming_edge = next_node->incoming[e];
+
+      active_edges_.erase(incoming_edge);
+      RemoveEdgeFromList(incoming_edge, &incoming_edge->from->outgoing);
+    }
+    next_node->incoming.clear();
+    next_node->incoming_edge_weight = 0.f;
+    no_incoming_edge_node_list.push_back(next_node);
+    DVLOG(2) << "Breaking cycle by cleaning up incoming edges from " <<
+        next_node->layer->id() <<
+        " (weight = " << min_incoming_edge_weight << ")";
+  }
+
+  // Note: The original elements of the list are in no danger of having their
+  // ref count go to zero here as they are all nodes of the layer hierarchy and
+  // are kept alive by their parent nodes.
+  int count = 0;
+  for (LayerImplList::iterator it = first; it < last; it++)
+    *it = sorted_list[count++]->layer;
+
+  DVLOG(2) << "Sorting end ----";
+
+  nodes_.clear();
+  edges_.clear();
+  active_edges_.clear();
+}
+
+}  // namespace cc