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Fix shape outline normal computation

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The previous implementation could fail to determine if normals had to be
flipped or not: it assumed the first vertex position was strictly inside
the shape. This position being set to the bounding box's center, it is
possible for it be on an edge of the shape (consider a right triangle).
This commit is contained in:
kimci86 2024-12-26 19:35:45 +01:00
parent dab1800f61
commit ff774057c7
1 changed files with 15 additions and 11 deletions
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26
src/SFML/Graphics/Shape.cpp
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@ -37,12 +37,14 @@
namespace namespace
{ {
// Compute the normal of a segment // Compute the normal of a segment
sf::Vector2f computeNormal(sf::Vector2f p1, sf::Vector2f p2) sf::Vector2f computeNormal(sf::Vector2f p1, sf::Vector2f p2, bool flipped)
{ {
sf::Vector2f normal = (p2 - p1).perpendicular(); sf::Vector2f normal = (p2 - p1).perpendicular();
const float length = normal.length(); const float length = normal.length();
if (length != 0.f) if (length != 0.f)
normal /= length; normal /= length;
if (flipped)
normal = -normal;
return normal; return normal;
} }
} // namespace } // namespace
@ -291,6 +293,15 @@ void Shape::updateOutline()
const std::size_t count = m_vertices.getVertexCount() - 2; const std::size_t count = m_vertices.getVertexCount() - 2;
m_outlineVertices.resize((count + 1) * 2); m_outlineVertices.resize((count + 1) * 2);
// Determine if points are defined clockwise or counterclockwise. This will impact normals computation.
const bool flipNormals = [this, count]()
{
float twiceArea = 0.f;
for (std::size_t i = 0; i < count; ++i)
twiceArea += m_vertices[i + 1].position.cross(m_vertices[i + 2].position);
return twiceArea >= 0.f;
}();
for (std::size_t i = 0; i < count; ++i) for (std::size_t i = 0; i < count; ++i)
{ {
const std::size_t index = i + 1; const std::size_t index = i + 1;
@ -300,16 +311,9 @@ void Shape::updateOutline()
const Vector2f p1 = m_vertices[index].position; const Vector2f p1 = m_vertices[index].position;
const Vector2f p2 = m_vertices[index + 1].position; const Vector2f p2 = m_vertices[index + 1].position;
// Compute their normal // Compute their normal pointing towards the outside of the shape
Vector2f n1 = computeNormal(p0, p1); const Vector2f n1 = computeNormal(p0, p1, flipNormals);
Vector2f n2 = computeNormal(p1, p2); const Vector2f n2 = computeNormal(p1, p2, flipNormals);
// Make sure that the normals point towards the outside of the shape
// (this depends on the order in which the points were defined)
if (n1.dot(m_vertices[0].position - p1) > 0)
n1 = -n1;
if (n2.dot(m_vertices[0].position - p1) > 0)
n2 = -n2;
// Combine them to get the extrusion direction // Combine them to get the extrusion direction
const float factor = 1.f + (n1.x * n2.x + n1.y * n2.y); const float factor = 1.f + (n1.x * n2.x + n1.y * n2.y);