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LibGfx: Implement even-odd method for Quad::contains() and add tests

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This commit is contained in:
Aaron Van Doren 2024-08-02 19:21:34 -07:00 • committed by Jelle Raaijmakers
commit 5c4e162ba9
Notes: github-actions[bot] 2024-12-15 22:28:50 +00:00
3 changed files with 112 additions and 11 deletions
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47
Libraries/LibGfx/Quad.h
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@ -6,8 +6,8 @@
#pragma once
#include <AK/Array.h>
#include <LibGfx/Point.h>
#include <LibGfx/Triangle.h>
namespace Gfx {
@ -29,20 +29,45 @@ public:
Rect<T> bounding_rect() const
{
auto top = min(min(m_p1.y(), m_p2.y()), min(m_p3.y(), m_p4.y()));
auto right = max(max(m_p1.x(), m_p2.x()), max(m_p3.x(), m_p4.x()));
auto bottom = max(max(m_p1.y(), m_p2.y()), max(m_p3.y(), m_p4.y()));
auto left = min(min(m_p1.x(), m_p2.x()), min(m_p3.x(), m_p4.x()));
return { left, top, right - left, bottom - top };
T left = min(min(m_p1.x(), m_p2.x()), min(m_p3.x(), m_p4.x()));
T right = max(max(m_p1.x(), m_p2.x()), max(m_p3.x(), m_p4.x()));
T width = right - left;
T top = min(min(m_p1.y(), m_p2.y()), min(m_p3.y(), m_p4.y()));
T bottom = max(max(m_p1.y(), m_p2.y()), max(m_p3.y(), m_p4.y()));
T height = bottom - top;
return { left, top, width, height };
}
bool contains(Point<T> point) const
{
// FIXME: There's probably a smarter way to do this.
return Triangle(m_p1, m_p2, m_p3).contains(point)
|| Triangle(m_p1, m_p3, m_p4).contains(point)
|| Triangle(m_p2, m_p4, m_p1).contains(point)
|| Triangle(m_p2, m_p4, m_p3).contains(point);
// Even-Odd algorithm: https://www.geeksforgeeks.org/even-odd-method-winding-number-method-inside-outside-test-of-a-polygon/
//
// 1. "Constructing a line segment between the point (P) to be examined and a known point outside the polygon"
// - We're using horizontal line from (point.x, point.y) to (bounding_rect().left + bounding_rect().width + 1, point.y)
// (i.e. just +1 to right of furthest-right point in quad)
//
// 2. "The number of times the line segment intersects the polygon boundary is then counted."
// - We count the line's intersections with the quad by checking each quad edge for intersection (1-2, 2-3, 3-4, 4-1)
//
// 3. "The point (P) is an internal point if the number of polygon edges intersected by this line is odd;
// otherwise, the point is an external point."
u8 intersections_with_quad = 0;
auto const quad_points = AK::Array { &m_p1, &m_p2, &m_p3, &m_p4, &m_p1 };
for (size_t i = 0, j = 1; i < 4 && j < 5; i++, j++) {
if ((quad_points[i]->y() > point.y()) == (quad_points[j]->y() > point.y())) {
continue;
}
T x_coord_of_intersection_with_edge = (quad_points[j]->x() - quad_points[i]->x()) * (point.y() - quad_points[i]->y()) / (quad_points[j]->y() - quad_points[i]->y()) + quad_points[i]->x();
if (point.x() < x_coord_of_intersection_with_edge) {
intersections_with_quad++;
}
}
return (intersections_with_quad % 2) == 1;
}
private: