Lazy transformations for CrossSection (#355)

Add transformation composition with lazy application when needed, similar to the arrangement used in the CSG tree backing Manifold.

src/cross_section/include/cross_section.h

@@ -16,8 +16,11 @@
 
 #include <clipper2/clipper.h>
 
+#include <memory>
+
 #include "clipper2/clipper.core.h"
 #include "clipper2/clipper.offset.h"
+#include "glm/ext/matrix_float3x2.hpp"
 #include "glm/ext/vector_float2.hpp"
 #include "public.h"
 
@@ -96,8 +99,10 @@ class CrossSection {
   ///@}
 
  private:
-  C2::PathsD paths_;
+  mutable std::shared_ptr<const C2::PathsD> paths_;
+  mutable glm::mat3x2 transform_ = glm::mat3x2(1.0f);
   CrossSection(C2::PathsD paths);
+  C2::PathsD GetPaths() const;
 };
 /** @} */
 

src/cross_section/src/cross_section.cpp

@@ -16,11 +16,13 @@
 
 #include <clipper2/clipper.h>
 
+#include <memory>
 #include <vector>
 
 #include "clipper2/clipper.core.h"
 #include "clipper2/clipper.engine.h"
 #include "clipper2/clipper.offset.h"
+#include "glm/ext/matrix_float3x2.hpp"
 #include "glm/ext/vector_float2.hpp"
 #include "glm/geometric.hpp"
 #include "glm/glm.hpp"
@@ -91,21 +93,42 @@ C2::PathD pathd_of_contour(const SimplePolygon& ctr) {
   }
   return p;
 }
+
+C2::PathsD transform(const C2::PathsD ps, const glm::mat3x2 m) {
+  const bool invert = glm::determinant(glm::mat2(m)) < 0;
+  auto transformed = C2::PathsD();
+  transformed.reserve(ps.size());
+  for (auto path : ps) {
+    auto sz = path.size();
+    auto s = C2::PathD(sz);
+    for (int i = 0; i < sz; ++i) {
+      auto idx = invert ? sz - 1 - i : i;
+      s[idx] = v2_to_pd(m * glm::vec3(path[i].x, path[i].y, 1));
+    }
+    transformed.push_back(s);
+  }
+  return transformed;
+}
+
+std::shared_ptr<const C2::PathsD> shared_paths(const C2::PathsD& ps) {
+  return std::make_shared<const C2::PathsD>(ps);
+}
 }  // namespace
 
 namespace manifold {
-CrossSection::CrossSection() { paths_ = C2::PathsD(); }
+CrossSection::CrossSection() { paths_ = shared_paths(C2::PathsD()); }
 CrossSection::~CrossSection() = default;
 CrossSection::CrossSection(CrossSection&&) noexcept = default;
 CrossSection& CrossSection::operator=(CrossSection&&) noexcept = default;
 CrossSection::CrossSection(const CrossSection& other) {
-  paths_ = C2::PathsD(other.paths_);
+  paths_ = other.paths_;
+  transform_ = other.transform_;
 }
-CrossSection::CrossSection(C2::PathsD ps) { paths_ = ps; }
+CrossSection::CrossSection(C2::PathsD ps) { paths_ = shared_paths(ps); }
 
 CrossSection::CrossSection(const SimplePolygon& contour, FillRule fillrule) {
   auto ps = C2::PathsD{(pathd_of_contour(contour))};
-  paths_ = C2::Union(ps, fr(fillrule), precision_);
+  paths_ = shared_paths(C2::Union(ps, fr(fillrule), precision_));
 }
 
 CrossSection::CrossSection(const Polygons& contours, FillRule fillrule) {
@@ -114,7 +137,18 @@ CrossSection::CrossSection(const Polygons& contours, FillRule fillrule) {
   for (auto ctr : contours) {
     ps.push_back(pathd_of_contour(ctr));
   }
-  paths_ = C2::Union(ps, fr(fillrule), precision_);
+  paths_ = shared_paths(C2::Union(ps, fr(fillrule), precision_));
+}
+
+// All access to paths_ should be done through the GetPaths() method, which
+// applies the accumulated transform_
+C2::PathsD CrossSection::GetPaths() const {
+  if (transform_ == glm::mat3x2(1.0f)) {
+    return *paths_;
+  }
+  paths_ = shared_paths(transform(*paths_, transform_));
+  transform_ = glm::mat3x2(1.0f);
+  return *paths_;
 }
 
 CrossSection CrossSection::Square(const glm::vec2 dims, bool center) {
@@ -151,8 +185,8 @@ CrossSection CrossSection::Circle(float radius, int circularSegments) {
 CrossSection CrossSection::Boolean(const CrossSection& second,
                                    OpType op) const {
   auto ct = cliptype_of_op(op);
-  auto res = C2::BooleanOp(ct, C2::FillRule::Positive, paths_, second.paths_,
-                           precision_);
+  auto res = C2::BooleanOp(ct, C2::FillRule::Positive, GetPaths(),
+                           second.GetPaths(), precision_);
   return CrossSection(res);
 }
 
@@ -163,15 +197,15 @@ CrossSection CrossSection::BatchBoolean(
   else if (crossSections.size() == 1)
     return crossSections[0];
 
-  auto subjs = crossSections[0].paths_;
+  auto subjs = crossSections[0].GetPaths();
   int n_clips = 0;
   for (int i = 1; i < crossSections.size(); ++i) {
-    n_clips += crossSections[i].paths_.size();
+    n_clips += crossSections[i].GetPaths().size();
   }
   auto clips = C2::PathsD();
   clips.reserve(n_clips);
   for (int i = 1; i < crossSections.size(); ++i) {
-    auto ps = crossSections[i].paths_;
+    auto ps = crossSections[i].GetPaths();
     clips.insert(clips.end(), ps.begin(), ps.end());
   }
 
@@ -228,105 +262,77 @@ CrossSection& CrossSection::operator^=(const CrossSection& Q) {
 
 CrossSection CrossSection::RectClip(const Rect& rect) const {
   auto r = C2::RectD(rect.min.x, rect.min.y, rect.max.x, rect.max.y);
-  auto ps = C2::RectClip(r, paths_, false, precision_);
+  auto ps = C2::RectClip(r, GetPaths(), false, precision_);
   return CrossSection(ps);
 }
 
 CrossSection CrossSection::Translate(const glm::vec2 v) const {
-  auto ps = C2::TranslatePaths(paths_, v.x, v.y);
-  return CrossSection(ps);
+  glm::mat3x2 m(1.0f, 0.0f,  //
+                0.0f, 1.0f,  //
+                v.x, v.y);
+  return Transform(m);
 }
 
 CrossSection CrossSection::Rotate(float degrees) const {
-  auto rotated = C2::PathsD();
-  rotated.reserve(paths_.size());
   auto s = sind(degrees);
   auto c = cosd(degrees);
-  for (auto path : paths_) {
-    auto r = C2::PathD();
-    r.reserve(path.size());
-    for (auto p : path) {
-      auto rx = (p.x * c) - (p.y * s);
-      auto ry = (p.y * c) + (p.x * s);
-      r.push_back(C2::PointD(rx, ry));
-    }
-    rotated.push_back(r);
-  }
-  return CrossSection(rotated);
+  glm::mat3x2 m(c, s,   //
+                -s, c,  //
+                0.0f, 0.0f);
+  return Transform(m);
 }
 
 CrossSection CrossSection::Scale(const glm::vec2 scale) const {
-  auto scaled = C2::PathsD();
-  scaled.reserve(paths_.size());
-  for (auto path : paths_) {
-    auto s = C2::PathD();
-    s.reserve(path.size());
-    for (auto p : path) {
-      s.push_back(C2::PointD(p.x * scale.x, p.y * scale.y));
-    }
-    scaled.push_back(s);
-  }
-  return CrossSection(scaled);
+  glm::mat3x2 m(scale.x, 0.0f,  //
+                0.0f, scale.y,  //
+                0.0f, 0.0f);
+  return Transform(m);
 }
 
 CrossSection CrossSection::Mirror(const glm::vec2 ax) const {
   if (glm::length(ax) == 0.) {
     return CrossSection();
   }
-  auto mirrored = C2::PathsD();
-  mirrored.reserve(paths_.size());
-  for (auto path : paths_) {
-    auto sz = path.size();
-    auto m = C2::PathD(sz);
-    for (int i = 0; i < sz; ++i) {
-      auto v = v2_of_pd(path[sz - 1 - i]);
-      m[i] = v2_to_pd(ax * (2 * glm::dot(v, ax) / glm::dot(ax, ax)) - v);
-    }
-    mirrored.push_back(m);
-  }
-  return CrossSection(mirrored);
+  auto n = glm::normalize(glm::abs(ax));
+  auto m = glm::mat3x2(glm::mat2(1.0f) - 2.0f * glm::outerProduct(n, n));
+  return Transform(m);
 }
 
 CrossSection CrossSection::Transform(const glm::mat3x2& m) const {
-  auto transformed = C2::PathsD();
-  transformed.reserve(paths_.size());
-  for (auto path : paths_) {
-    auto s = C2::PathD();
-    s.reserve(path.size());
-    for (auto p : path) {
-      s.push_back(v2_to_pd(m * glm::vec3(p.x, p.y, 1)));
-    }
-    transformed.push_back(s);
-  }
-  return CrossSection(transformed);
+  auto transformed = CrossSection();
+  transformed.transform_ = m * glm::mat3(transform_);
+  transformed.paths_ = paths_;
+  return transformed;
 }
 
 CrossSection CrossSection::Simplify(double epsilon) const {
-  auto ps = SimplifyPaths(paths_, epsilon, false);
+  auto ps = SimplifyPaths(GetPaths(), epsilon, false);
   return CrossSection(ps);
 }
 
 CrossSection CrossSection::Offset(double delta, JoinType jointype,
                                   double miter_limit,
                                   double arc_tolerance) const {
-  auto ps = C2::InflatePaths(paths_, delta, jt(jointype), C2::EndType::Polygon,
-                             miter_limit, precision_, arc_tolerance);
+  auto ps =
+      C2::InflatePaths(GetPaths(), delta, jt(jointype), C2::EndType::Polygon,
+                       miter_limit, precision_, arc_tolerance);
   return CrossSection(ps);
 }
 
-double CrossSection::Area() const { return C2::Area(paths_); }
+double CrossSection::Area() const { return C2::Area(GetPaths()); }
 Rect CrossSection::Bounds() const {
-  auto r = C2::GetBounds(paths_);
+  auto r = C2::GetBounds(GetPaths());
   return Rect({r.left, r.bottom}, {r.right, r.top});
 }
-bool CrossSection::IsEmpty() const { return paths_.empty(); }
+bool CrossSection::IsEmpty() const { return GetPaths().empty(); }
 
 Polygons CrossSection::ToPolygons() const {
   auto polys = Polygons();
-  polys.reserve(paths_.size());
-  for (auto p : paths_) {
+  auto paths = GetPaths();
+  polys.reserve(paths.size());
+  for (auto p : paths) {
     auto sp = SimplePolygon();
-    sp.reserve(paths_.size());
+    sp.reserve(p.size());
     for (auto v : p) {
       sp.push_back({v.x, v.y});
     }

test/cross_section_test.cpp

@@ -16,6 +16,7 @@
 
 #include <gtest/gtest.h>
 
+#include "glm/geometric.hpp"
 #include "manifold.h"
 #include "polygon.h"
 #include "public.h"
@@ -30,16 +31,15 @@ using namespace manifold;
 TEST(CrossSection, MirrorUnion) {
   auto a = CrossSection::Square({5., 5.}, true);
   auto b = a.Translate({2.5, 2.5});
-  auto cross = a + b + b.Mirror({-1, 1});
+  auto cross = a + b + b.Mirror({1, 1});
   auto result = Manifold::Extrude(cross, 5.);
 
 #ifdef MANIFOLD_EXPORT
   if (options.exportModels)
     ExportMesh("cross_section_mirror_union.glb", result.GetMesh(), {});
 #endif
 
-  auto area_a = a.Area();
-  EXPECT_EQ(area_a + 1.5 * area_a, cross.Area());
+  EXPECT_FLOAT_EQ(2.5 * a.Area(), cross.Area());
   EXPECT_TRUE(a.Mirror(glm::vec2(0)).IsEmpty());
 }
 
@@ -89,7 +89,11 @@ TEST(CrossSection, Transform) {
                     0.0f, 0.0f, 1.0f);
 
   auto b = sq.Transform(trans * scale * rot);
+  auto b_copy = CrossSection(b);
 
-  Identical(Manifold::Extrude(a, 1.).GetMesh(),
-            Manifold::Extrude(b, 1.).GetMesh());
+  auto ex_b = Manifold::Extrude(b, 1.).GetMesh();
+  Identical(Manifold::Extrude(a, 1.).GetMesh(), ex_b);
+
+  // same transformations are applied in b_copy (giving same result)
+  Identical(ex_b, Manifold::Extrude(b_copy, 1.).GetMesh());
 }