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// Copyright 2014 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/quads/draw_polygon.h"
#include <stddef.h>
#include <vector>
#include "cc/output/bsp_compare_result.h"
#include "cc/quads/draw_quad.h"
namespace {
// This threshold controls how "thick" a plane is. If a point's distance is
// <= |compare_threshold|, then it is considered on the plane. Only when this
// boundary is crossed do we consider doing splitting.
static const float compare_threshold = 0.1f;
// |split_threshold| is lower in this case because we want the points created
// during splitting to be well within the range of |compare_threshold| for
// comparison purposes. The splitting operation will produce intersection points
// that fit within a tighter distance to the splitting plane as a result of this
// value. By using a value >= |compare_threshold| we run the risk of creating
// points that SHOULD be intersecting the "thick plane", but actually fail to
// test positively for it because |split_threshold| allowed them to be outside
// this range.
// This is really supposd to be compare_threshold / 2.0f, but that would
// create another static initializer.
static const float split_threshold = 0.05f;
static const float normalized_threshold = 0.001f;
} // namespace
namespace cc {
DrawPolygon::DrawPolygon() {
DrawPolygon::DrawPolygon(const DrawQuad* original,
const std::vector<gfx::Point3F>& in_points,
const gfx::Vector3dF& normal,
int draw_order_index)
: order_index_(draw_order_index), original_ref_(original), is_split_(true) {
for (size_t i = 0; i < in_points.size(); i++) {
normal_ = normal;
// This takes the original DrawQuad that this polygon should be based on,
// a visible content rect to make the 4 corner points from, and a transformation
// to move it and its normal into screen space.
DrawPolygon::DrawPolygon(const DrawQuad* original_ref,
const gfx::RectF& visible_layer_rect,
const gfx::Transform& transform,
int draw_order_index)
: normal_(0.0f, 0.0f, 1.0f),
is_split_(false) {
gfx::Point3F points[8];
int num_vertices_in_clipped_quad;
gfx::QuadF send_quad(visible_layer_rect);
// Doing this mapping here is very important, since we can't just transform
// the points without clipping and not run into strange geometry issues when
// crossing w = 0. At this point, in the constructor, we know that we're
// working with a quad, so we can reuse the MathUtil::MapClippedQuad3d
// function instead of writing a generic polygon version of it.
transform, send_quad, points, &num_vertices_in_clipped_quad);
for (int i = 0; i < num_vertices_in_clipped_quad; i++) {
DrawPolygon::~DrawPolygon() {
scoped_ptr<DrawPolygon> DrawPolygon::CreateCopy() {
scoped_ptr<DrawPolygon> new_polygon(new DrawPolygon());
new_polygon->order_index_ = order_index_;
new_polygon->original_ref_ = original_ref_;
new_polygon->points_ = points_;
return new_polygon;
// If this were to be more generally used and expected to be applicable
// replacing this with Newell's algorithm (or an improvement thereof)
// would be preferable, but usually this is coming in from a rectangle
// that has been transformed to screen space and clipped.
// Averaging a few near diagonal cross products is pretty good in that case.
void DrawPolygon::ConstructNormal() {
int delta = points_.size() / 2;
for (size_t i = 1; i + delta < points_.size(); i++) {
normal_ +=
CrossProduct(points_[i] - points_[0], points_[i + delta] - points_[0]);
float normal_magnitude = normal_.Length();
if (normal_magnitude != 0 && normal_magnitude != 1) {
normal_.Scale(1.0f / normal_magnitude);
#if defined(OS_WIN)
// Allows the unittest to invoke this for the more general constructor.
void DrawPolygon::RecomputeNormalForTesting() {
float DrawPolygon::SignedPointDistance(const gfx::Point3F& point) const {
return gfx::DotProduct(point - points_[0], normal_);
// Checks whether or not shape a lies on the front or back side of b, or
// whether they should be considered coplanar. If on the back side, we
// say A_BEFORE_B because it should be drawn in that order.
// Assumes that layers are split and there are no intersecting planes.
BspCompareResult DrawPolygon::SideCompare(const DrawPolygon& a,
const DrawPolygon& b) {
// Let's make sure that this is normalized. Without this SignedPointDistance
// will not be right, but putting the check in there will validate it
// redundantly for each point.
DCHECK_GE(normalized_threshold, std::abs(b.normal_.LengthSquared() - 1.0f));
int pos_count = 0;
int neg_count = 0;
for (size_t i = 0; i < a.points_.size(); i++) {
float sign = b.SignedPointDistance(a.points_[i]);
if (sign < -compare_threshold) {
} else if (sign > compare_threshold) {
if (pos_count && neg_count) {
return BSP_SPLIT;
if (pos_count) {
return BSP_FRONT;
if (neg_count) {
return BSP_BACK;
double dot = gfx::DotProduct(a.normal_, b.normal_);
if ((dot >= 0.0f && a.order_index_ >= b.order_index_) ||
(dot <= 0.0f && a.order_index_ <= b.order_index_)) {
// The sign of the dot product of the normals along with document order
// determine which side it goes on, the vertices are ambiguous.
static bool LineIntersectPlane(const gfx::Point3F& line_start,
const gfx::Point3F& line_end,
const gfx::Point3F& plane_origin,
const gfx::Vector3dF& plane_normal,
gfx::Point3F* intersection,
float distance_threshold) {
gfx::Vector3dF start_to_origin_vector = plane_origin - line_start;
gfx::Vector3dF end_to_origin_vector = plane_origin - line_end;
double start_distance = gfx::DotProduct(start_to_origin_vector, plane_normal);
double end_distance = gfx::DotProduct(end_to_origin_vector, plane_normal);
// The case where one vertex lies on the thick-plane and the other
// is outside of it.
if (std::abs(start_distance) <= distance_threshold &&
std::abs(end_distance) > distance_threshold) {
intersection->SetPoint(line_start.x(), line_start.y(), line_start.z());
return true;
// This is the case where we clearly cross the thick-plane.
if ((start_distance > distance_threshold &&
end_distance < -distance_threshold) ||
(start_distance < -distance_threshold &&
end_distance > distance_threshold)) {
gfx::Vector3dF v = line_end - line_start;
float total_distance = std::abs(start_distance) + std::abs(end_distance);
float lerp_factor = std::abs(start_distance) / total_distance;
intersection->SetPoint(line_start.x() + (v.x() * lerp_factor),
line_start.y() + (v.y() * lerp_factor),
line_start.z() + (v.z() * lerp_factor));
return true;
return false;
// This function is separate from ApplyTransform because it is often unnecessary
// to transform the normal with the rest of the polygon.
// When drawing these polygons, it is necessary to move them back into layer
// space before sending them to OpenGL, which requires using ApplyTransform,
// but normal information is no longer needed after sorting.
void DrawPolygon::ApplyTransformToNormal(const gfx::Transform& transform) {
// Now we use the inverse transpose of |transform| to transform the normal.
gfx::Transform inverse_transform;
bool inverted = transform.GetInverse(&inverse_transform);
if (!inverted)
gfx::Point3F new_normal(normal_.x(), normal_.y(), normal_.z());
// Make sure our normal is still normalized.
normal_ = gfx::Vector3dF(new_normal.x(), new_normal.y(), new_normal.z());
float normal_magnitude = normal_.Length();
if (normal_magnitude != 0 && normal_magnitude != 1) {
normal_.Scale(1.0f / normal_magnitude);
void DrawPolygon::ApplyTransform(const gfx::Transform& transform) {
for (size_t i = 0; i < points_.size(); i++) {
// TransformToScreenSpace assumes we're moving a layer from its layer space
// into 3D screen space, which for sorting purposes requires the normal to
// be transformed along with the vertices.
void DrawPolygon::TransformToScreenSpace(const gfx::Transform& transform) {
// In the case of TransformToLayerSpace, we assume that we are giving the
// inverse transformation back to the polygon to move it back into layer space
// but we can ignore the costly process of applying the inverse to the normal
// since we know the normal will just reset to its original state.
void DrawPolygon::TransformToLayerSpace(
const gfx::Transform& inverse_transform) {
normal_ = gfx::Vector3dF(0.0f, 0.0f, -1.0f);
bool DrawPolygon::Split(const DrawPolygon& splitter,
scoped_ptr<DrawPolygon>* front,
scoped_ptr<DrawPolygon>* back) {
gfx::Point3F intersections[2];
std::vector<gfx::Point3F> out_points[2];
// vertex_before stores the index of the vertex before its matching
// intersection.
// i.e. vertex_before[0] stores the vertex we saw before we crossed the plane
// which resulted in the line/plane intersection giving us intersections[0].
size_t vertex_before[2];
size_t points_size = points_.size();
size_t current_intersection = 0;
size_t current_vertex = 0;
// We will only have two intersection points because we assume all polygons
// are convex.
while (current_intersection < 2) {
if (LineIntersectPlane(points_[(current_vertex % points_size)],
points_[(current_vertex + 1) % points_size],
split_threshold)) {
vertex_before[current_intersection] = current_vertex % points_size;
// We found both intersection points so we're done already.
if (current_intersection == 2) {
if (current_vertex++ > (points_size)) {
DCHECK_EQ(current_intersection, static_cast<size_t>(2));
// Since we found both the intersection points, we can begin building the
// vertex set for both our new polygons.
size_t start1 = (vertex_before[0] + 1) % points_size;
size_t start2 = (vertex_before[1] + 1) % points_size;
size_t points_remaining = points_size;
// First polygon.
DCHECK_GE(vertex_before[1], start1);
for (size_t i = start1; i <= vertex_before[1]; i++) {
// Second polygon.
size_t index = start2;
for (size_t i = 0; i < points_remaining; i++) {
out_points[1].push_back(points_[index % points_size]);
// Give both polygons the original splitting polygon's ID, so that they'll
// still be sorted properly in co-planar instances.
scoped_ptr<DrawPolygon> poly1(
new DrawPolygon(original_ref_, out_points[0], normal_, order_index_));
scoped_ptr<DrawPolygon> poly2(
new DrawPolygon(original_ref_, out_points[1], normal_, order_index_));
DCHECK_GE(poly1->points().size(), 3u);
DCHECK_GE(poly2->points().size(), 3u);
if (SideCompare(*poly1, splitter) == BSP_FRONT) {
*front = std::move(poly1);
*back = std::move(poly2);
} else {
*front = std::move(poly2);
*back = std::move(poly1);
return true;
// This algorithm takes the first vertex in the polygon and uses that as a
// pivot point to fan out and create quads from the rest of the vertices.
// |offset| starts off as the second vertex, and then |op1| and |op2| indicate
// offset+1 and offset+2 respectively.
// After the first quad is created, the first vertex in the next quad is the
// same as all the rest, the pivot point. The second vertex in the next quad is
// the old |op2|, the last vertex added to the previous quad. This continues
// until all points are exhausted.
// The special case here is where there are only 3 points remaining, in which
// case we use the same values for vertex 3 and 4 to make a degenerate quad
// that represents a triangle.
void DrawPolygon::ToQuads2D(std::vector<gfx::QuadF>* quads) const {
if (points_.size() <= 2)
gfx::PointF first(points_[0].x(), points_[0].y());
size_t offset = 1;
while (offset < points_.size() - 1) {
size_t op1 = offset + 1;
size_t op2 = offset + 2;
if (op2 >= points_.size()) {
// It's going to be a degenerate triangle.
op2 = op1;
gfx::PointF(points_[offset].x(), points_[offset].y()),
gfx::PointF(points_[op1].x(), points_[op1].y()),
gfx::PointF(points_[op2].x(), points_[op2].y())));
offset = op2;
} // namespace cc