blob: 6729b197ce0944ea7719bddd8cd6256257d71453 [file] [log] [blame]
// 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(gfx::Vector2dF u, 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(gfx::PointF a,
gfx::PointF b,
gfx::PointF c,
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;
*r = a + u;
return true;
GraphNode::GraphNode(LayerImpl* layer_impl)
: layer(layer_impl),
incoming_edge_weight(0.f) {}
GraphNode::~GraphNode() {}
: 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.p4() };
gfx::PointF bPoints[4] = { b->projected_quad.p1(),
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]))
if (b->projected_quad.Contains(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],
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 ABeforeB;
float max_diff =
fabsf(max_positive) > fabsf(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;
*weight = fabsf(max_diff);
// Maintain relative order if the layers have the same depth at all
// intersection points.
if (max_diff <= 0.f)
return ABeforeB;
return BBeforeA;
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;
if (num_vertices_in_clipped_quad < 3) {
projected_bounds = gfx::RectF();
projected_bounds =
// 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.
if (num_vertices_in_clipped_quad >= 4) {
} else {
// This will be a degenerate quad that is actually a triangle.
// 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(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++) {
GraphNode& node = - 1);
RenderSurfaceImpl* render_surface = node.layer->render_surface();
if (!node.layer->DrawsContent() && !render_surface)
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_ = fabsf(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())
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())
float weight = 0.f;
ABCompareResult overlap_result = CheckOverlap(&node_a.shape,
GraphNode* start_node = NULL;
GraphNode* end_node = NULL;
if (overlap_result == ABeforeB) {
start_node = &node_a;
end_node = &node_b;
} else if (overlap_result == BBeforeA) {
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);>incoming.push_back(&edge);>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());
// 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);
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())
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();
// Add it to the final list.
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];
RemoveEdgeFromList(outgoing_edge, &outgoing_edge->to->incoming);
outgoing_edge->to->incoming_edge_weight -= outgoing_edge->weight;
if (!outgoing_edge->to->incoming.size())
if (!active_edges_.size())
// 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];
// Remove all its incoming edges.
for (size_t e = 0; e < next_node->incoming.size(); e++) {
GraphEdge* incoming_edge = next_node->incoming[e];
RemoveEdgeFromList(incoming_edge, &incoming_edge->from->outgoing);
next_node->incoming_edge_weight = 0.f;
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 ----";
} // namespace cc