DrawPolygon class with Unit Tests

cc/quads/draw_polygon.cc

+// 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 <vector>
+
+#include "cc/output/bsp_compare_result.h"
+
+namespace {
+// This allows for some imperfection in the normal comparison when checking if
+// two pieces of geometry are coplanar.
+static const float coplanar_dot_epsilon = 0.01f;
+// This threshold controls how "thick" a plane is. If a point's distance is

[enne (OOO), 2014/07/28 23:11:34]

What are these units in? (both compare and split thresholds?) Distance
in...device pixels?

[troyhildebrandt, 2014/07/28 23:48:45]

I believe they're in device pixels, if that's what the 2D layers we receive
before transforming into 3D are.

+// <= |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 = 1.0f;
+// |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.
+static const float split_threshold = 0.5f;
+}  // namespace
+
+namespace cc {
+
+gfx::Vector3dF DrawPolygon::default_normal = gfx::Vector3dF(0.0f, 0.0f, -1.0f);
+
+DrawPolygon::DrawPolygon() {
+}
+
+DrawPolygon::DrawPolygon(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) {
+  for (size_t i = 0; i < in_points.size(); i++) {
+    points_.push_back(in_points[i]);
+  }
+  normal_ = normal;
+}
+
+DrawPolygon::~DrawPolygon() {
+}
+
+scoped_ptr<DrawPolygon> DrawPolygon::CreateCopy() {
+  DrawPolygon* new_polygon = new DrawPolygon();
+  new_polygon->order_index_ = order_index_;
+  new_polygon->original_ref_ = original_ref_;
+  new_polygon->points_.reserve(points_.size());
+  new_polygon->points_ = points_;
+  new_polygon->normal_.set_x(normal_.x());
+  new_polygon->normal_.set_y(normal_.y());
+  new_polygon->normal_.set_z(normal_.z());
+  return scoped_ptr<DrawPolygon>(new_polygon);
+}
+
+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 ABeforeB 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) {
+  // Right away let's check if they're coplanar
+  double dot = gfx::DotProduct(a.normal_, b.normal_);
+  float sign;
+  bool normal_match = false;
+  // This check assumes that the normals are normalized.
+  if (std::abs(dot) >= 1.0f - coplanar_dot_epsilon) {
+    normal_match = true;
+    // The normals are matching enough that we only have to test one point.
+    sign = gfx::DotProduct(a.points_[0] - b.points_[0], b.normal_);
+    // Is it on either side of the splitter?
+    if (sign < -compare_threshold) {
+      return BSP_BACK;
+    }
+
+    if (sign > compare_threshold) {
+      return BSP_FRONT;
+    }
+
+    // No it wasn't, so the sign of the dot product of the normals
+    // along with document order determines which side it goes on.
+    if (dot >= 0.0f) {
+      if (a.order_index_ < b.order_index_) {
+        return BSP_COPLANAR_FRONT;
+      }
+      return BSP_COPLANAR_BACK;
+    }
+
+    if (a.order_index_ < b.order_index_) {
+      return BSP_COPLANAR_BACK;
+    }
+    return BSP_COPLANAR_FRONT;
+  }
+
+  unsigned int pos_count = 0;

[enne (OOO), 2014/07/28 23:11:34]

unsigned int => int

[troyhildebrandt, 2014/07/28 23:48:45]

Done.

+  unsigned int neg_count = 0;
+  for (size_t i = 0; i < a.points_.size(); i++) {
+    if (!normal_match || (normal_match && i > 0)) {
+      sign = gfx::DotProduct(a.points_[i] - b.points_[0], b.normal_);
+    }
+
+    if (sign < -compare_threshold) {
+      ++neg_count;
+    } else if (sign > compare_threshold) {
+      ++pos_count;
+    }
+
+    if (pos_count && neg_count) {
+      return BSP_SPLIT;
+    }
+  }
+
+  if (pos_count) {
+    return BSP_FRONT;
+  }
+  return BSP_BACK;
+}
+
+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);
+  DCHECK(inverted);
+  if (!inverted)
+    return;
+  inverse_transform.Transpose();
+
+  gfx::Point3F new_normal(normal_.x(), normal_.y(), normal_.z());
+  inverse_transform.TransformPoint(&new_normal);
+  // 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++) {
+    transform.TransformPoint(&points_[i]);
+  }
+}
+
+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],
+                           splitter.points_[0],
+                           splitter.normal_,
+                           &intersections[current_intersection],
+                           split_threshold)) {
+      vertex_before[current_intersection] = current_vertex % points_size;
+      current_intersection++;
+      // We found both intersection points so we're done already.
+      if (current_intersection == 2) {
+        break;
+      }
+    }
+    if (current_vertex++ > points_size) {
+      break;
+    }
+  }
+  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.
+  out_points[0].push_back(intersections[0]);
+  for (size_t i = start1; i <= vertex_before[1]; i++) {
+    out_points[0].push_back(points_[i]);
+    --points_remaining;
+  }
+  out_points[0].push_back(intersections[1]);
+
+  // Second polygon.
+  out_points[1].push_back(intersections[1]);
+  size_t index = start2;
+  for (unsigned int i = 0; i < points_remaining; i++) {

[enne (OOO), 2014/07/28 23:11:34]

size_t

[troyhildebrandt, 2014/07/28 23:48:45]

Done.

+    out_points[1].push_back(points_[index % points_size]);
+    ++index;
+  }
+  out_points[1].push_back(intersections[0]);
+
+  // 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_));
+
+  if (SideCompare(*poly1, splitter) == BSP_FRONT) {
+    *front = poly1.Pass();
+    *back = poly2.Pass();
+  } else {
+    *front = poly2.Pass();
+    *back = poly1.Pass();
+  }
+  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)
+    return;
+
+  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;
+    }
+    quads->push_back(
+        gfx::QuadF(first,
+                   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