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Сonsider that LargeHole_DontFill is true if angle is negative

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Otherwise, a wrong triangle can be generated

Signed-off-by: AndriyAndreyev <andriy.a.andreyev@gmail.com>
This commit is contained in:
AndriyAndreyev 2022-04-26 21:46:11 +03:00
parent 3380f5c805
commit 64636de5af
2 changed files with 38 additions and 0 deletions
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37
poly2tri/sweep/sweep.cc
View File

@ -262,6 +262,35 @@ void Sweep::FillAdvancingFront(SweepContext& tcx, Node& n)
}
// True if HoleAngle exceeds 90 degrees.
// LargeHole_DontFill checks if the advancing front has a large hole.
// A "Large hole" is a triangle formed by a sequence of points in the advancing
// front where three neighbor points form a triangle.
// And angle between left-top, bottom, and right-top points is more than 90 degrees.
// The first part of the algorithm reviews only three neighbor points, e.g. named A, B, C.
// Additional part of this logic reviews a sequence of 5 points -
// additionally reviews one point before and one after the sequence of three (A, B, C),
// e.g. named X and Y.
// In this case, angles are XBC and ABY and this if angles are negative or more
// than 90 degrees LargeHole_DontFill returns true.
// But there is a configuration when ABC has a negative angle but XBC or ABY is less
// than 90 degrees and positive.
// Then function LargeHole_DontFill return false and initiates filling.
// This filling creates a triangle ABC and adds it to the advancing front.
// But in the case when angle ABC is negative this triangle goes inside the advancing front
// and can intersect previously created triangles.
// This triangle leads to making wrong advancing front and problems in triangulation in the future.
// Looks like such a triangle should not be created.
// The simplest way to check and fix it is to check an angle ABC.
// If it is negative LargeHole_DontFill should return true and
// not initiate creating the ABC triangle in the advancing front.
// X______A Y
// \ /
// \ /
// \ B /
// | /
// | /
// |/
// C
bool Sweep::LargeHole_DontFill(const Node* node) const {
const Node* nextNode = node->next;
@ -269,6 +298,9 @@ bool Sweep::LargeHole_DontFill(const Node* node) const {
if (!AngleExceeds90Degrees(node->point, nextNode->point, prevNode->point))
return false;
if (AngleIsNegative(node->point, nextNode->point, prevNode->point))
return true;
// Check additional points on front.
const Node* next2Node = nextNode->next;
// "..Plus.." because only want angles on same side as point being added.
@ -283,6 +315,11 @@ bool Sweep::LargeHole_DontFill(const Node* node) const {
return true;
}
bool Sweep::AngleIsNegative(const Point* origin, const Point* pa, const Point* pb) const {
const double angle = Angle(origin, pa, pb);
return angle < 0;
}
bool Sweep::AngleExceeds90Degrees(const Point* origin, const Point* pa, const Point* pb) const {
const double angle = Angle(origin, pa, pb);
return ((angle > PI_div2) || (angle < -PI_div2));

1
poly2tri/sweep/sweep.h
View File

@ -172,6 +172,7 @@ private:
// Decision-making about when to Fill hole.
// Contributed by ToolmakerSteve2
bool LargeHole_DontFill(const Node* node) const;
bool AngleIsNegative(const Point* origin, const Point* pa, const Point* pb) const;
bool AngleExceeds90Degrees(const Point* origin, const Point* pa, const Point* pb) const;
bool AngleExceedsPlus90DegreesOrIsNegative(const Point* origin, const Point* pa, const Point* pb) const;
double Angle(const Point* origin, const Point* pa, const Point* pb) const;