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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#include <list>
#include <utility>
#include "mozilla/ArenaAllocator.h"
#include "mozilla/UniquePtr.h"
#include "mozilla/gfx/Polygon.h"
#include "nsTArray.h"
namespace mozilla {
namespace layers {
* Represents a layer that might have a non-rectangular geometry.
template <typename T>
struct BSPPolygon {
explicit BSPPolygon(T* aData) : data(aData) {}
BSPPolygon(T* aData, gfx::Polygon&& aGeometry)
: data(aData), geometry(Some(std::move(aGeometry))) {}
BSPPolygon(T* aData, nsTArray<gfx::Point4D>&& aPoints,
const gfx::Point4D& aNormal)
: data(aData) {
geometry.emplace(std::move(aPoints), aNormal);
T* data;
Maybe<gfx::Polygon> geometry;
* Allocate BSPTreeNodes from a memory arena to improve performance with
* complex scenes.
* The arena size of 4096 bytes was selected as an arbitrary power of two.
* Depending on the platform, this size accommodates roughly 100 BSPTreeNodes.
typedef mozilla::ArenaAllocator<4096, 8> BSPTreeArena;
* Aliases the container type used to store layers within BSPTreeNodes.
template <typename T>
using PolygonList = std::list<BSPPolygon<T>>;
// For tests. Needs to be defined here rather than in TestBSPTree.cpp because we
// need to explicitly instantiate the out-of-line BSPTree methods for it in
// BSPTree.cpp.
class BSPTestData {};
using TestPolygon = BSPPolygon<BSPTestData>;
* Represents a node in a BSP tree. The node contains at least one layer with
* associated geometry that is used as a splitting plane, and at most two child
* nodes that represent the splitting planes that further subdivide the space.
template <typename T>
struct BSPTreeNode {
explicit BSPTreeNode(nsTArray<PolygonList<T>*>& aListPointers)
: front(nullptr), back(nullptr) {
// Store the layer list pointer to free memory when BSPTree is destroyed.
const gfx::Polygon& First() const {
return *layers.front().geometry;
static void* operator new(size_t aSize, BSPTreeArena& mPool) {
return mPool.Allocate(aSize);
BSPTreeNode* front;
BSPTreeNode* back;
PolygonList<T> layers;
* BSPTree class takes a list of layers as an input and uses binary space
* partitioning algorithm to create a tree structure that can be used for
* depth sorting.
* Sources for more information:
template <typename T>
class BSPTree final {
* The constructor modifies layers in the given list.
explicit BSPTree(std::list<BSPPolygon<T>>& aLayers) {
mRoot = new (mPool) BSPTreeNode(mListPointers);
BuildTree(mRoot, aLayers);
~BSPTree() {
for (PolygonList<T>* listPtr : mListPointers) {
* Builds and returns the back-to-front draw order for the created BSP tree.
nsTArray<BSPPolygon<T>> GetDrawOrder() const {
nsTArray<BSPPolygon<T>> layers;
BuildDrawOrder(mRoot, layers);
return layers;
BSPTreeArena mPool;
BSPTreeNode<T>* mRoot;
nsTArray<PolygonList<T>*> mListPointers;
* BuildDrawOrder and BuildTree are called recursively. The depth of the
* recursion depends on the amount of polygons and their intersections.
void BuildDrawOrder(BSPTreeNode<T>* aNode,
nsTArray<BSPPolygon<T>>& aLayers) const;
void BuildTree(BSPTreeNode<T>* aRoot, PolygonList<T>& aLayers);
} // namespace layers
} // namespace mozilla