/// <summary>
/// Enforce the Delaunay condition at an edge, fanning out recursively from
/// an existing vertex. Pay special attention to stacking inverted triangles.
/// </summary>
/// <param name="fixuptri"></param>
/// <param name="leftside">
/// Indicates whether or not fixuptri is to the left of
/// the segment being inserted. (Imagine that the segment is pointing up from
/// endpoint1 to endpoint2.)
/// </param>
/// <remarks>
/// This is a support routine for inserting segments into a constrained
/// Delaunay triangulation.
/// The origin of fixuptri is treated as if it has just been inserted, and
/// the local Delaunay condition needs to be enforced. It is only enforced
/// in one sector, however, that being the angular range defined by
/// fixuptri.
/// This routine also needs to make decisions regarding the "stacking" of
/// triangles. (Read the description of ConstrainedEdge() below before
/// reading on here, so you understand the algorithm.) If the position of
/// the new vertex (the origin of fixuptri) indicates that the vertex before
/// it on the polygon is a reflex vertex, then "stack" the triangle by
/// doing nothing. (fixuptri is an inverted triangle, which is how stacked
/// triangles are identified.)
/// Otherwise, check whether the vertex before that was a reflex vertex.
/// If so, perform an edge flip, thereby eliminating an inverted triangle
/// (popping it off the stack). The edge flip may result in the creation
/// of a new inverted triangle, depending on whether or not the new vertex
/// is visible to the vertex three edges behind on the polygon.
/// If neither of the two vertices behind the new vertex are reflex
/// vertices, fixuptri and fartri, the triangle opposite it, are not
/// inverted; hence, ensure that the edge between them is locally Delaunay.
/// </remarks>
private void DelaunayFixup(ref Otri fixuptri, bool leftside)
{
Otri neartri = default(Otri);
Otri fartri = default(Otri);
Osub faredge = default(Osub);
Vertex nearvertex, leftvertex, rightvertex, farvertex;
fixuptri.Lnext(ref neartri);
neartri.Sym(ref fartri);
// Check if the edge opposite the origin of fixuptri can be flipped.
if (fartri.tri.Id == Mesh.DUMMY)
{
return;
}
neartri.Pivot(ref faredge);
if (faredge.seg.hash != Mesh.DUMMY)
{
return;
}
// Find all the relevant vertices.
nearvertex = neartri.Apex();
leftvertex = neartri.Org();
rightvertex = neartri.Dest();
farvertex = fartri.Apex();
// Check whether the previous polygon vertex is a reflex vertex.
if (leftside)
{
if (RobustPredicates.CounterClockwise(nearvertex, leftvertex, farvertex) <= 0.0)
{
// leftvertex is a reflex vertex too. Nothing can
// be done until a convex section is found.
return;
}
}
else
{
if (RobustPredicates.CounterClockwise(farvertex, rightvertex, nearvertex) <= 0.0)
{
// rightvertex is a reflex vertex too. Nothing can
// be done until a convex section is found.
return;
}
}
if (RobustPredicates.CounterClockwise(rightvertex, leftvertex, farvertex) > 0.0)
{
// fartri is not an inverted triangle, and farvertex is not a reflex
// vertex. As there are no reflex vertices, fixuptri isn't an
// inverted triangle, either. Hence, test the edge between the
// triangles to ensure it is locally Delaunay.
if (RobustPredicates.InCircle(leftvertex, farvertex, rightvertex, nearvertex) <= 0.0)
{
return;
}
// Not locally Delaunay; go on to an edge flip.
}
// else fartri is inverted; remove it from the stack by flipping.
mesh.Flip(ref neartri);
fixuptri.Lprev(); // Restore the origin of fixuptri after the flip.
// Recursively process the two triangles that result from the flip.
DelaunayFixup(ref fixuptri, leftside);
DelaunayFixup(ref fartri, leftside);
}
/// <summary>
/// Merge two adjacent Delaunay triangulations into a single Delaunay triangulation.
/// </summary>
/// <param name="farleft">Bounding triangles of the left triangulation.</param>
/// <param name="innerleft">Bounding triangles of the left triangulation.</param>
/// <param name="innerright">Bounding triangles of the right triangulation.</param>
/// <param name="farright">Bounding triangles of the right triangulation.</param>
/// <param name="axis"></param>
/// <remarks>
/// This is similar to the algorithm given by Guibas and Stolfi, but uses
/// a triangle-based, rather than edge-based, data structure.
///
/// The algorithm walks up the gap between the two triangulations, knitting
/// them together. As they are merged, some of their bounding triangles
/// are converted into real triangles of the triangulation. The procedure
/// pulls each hull's bounding triangles apart, then knits them together
/// like the teeth of two gears. The Delaunay property determines, at each
/// step, whether the next "tooth" is a bounding triangle of the left hull
/// or the right. When a bounding triangle becomes real, its apex is
/// changed from NULL to a real vertex.
///
/// Only two new triangles need to be allocated. These become new bounding
/// triangles at the top and bottom of the seam. They are used to connect
/// the remaining bounding triangles (those that have not been converted
/// into real triangles) into a single fan.
///
/// On entry, 'farleft' and 'innerleft' are bounding triangles of the left
/// triangulation. The origin of 'farleft' is the leftmost vertex, and
/// the destination of 'innerleft' is the rightmost vertex of the
/// triangulation. Similarly, 'innerright' and 'farright' are bounding
/// triangles of the right triangulation. The origin of 'innerright' and
/// destination of 'farright' are the leftmost and rightmost vertices.
///
/// On completion, the origin of 'farleft' is the leftmost vertex of the
/// merged triangulation, and the destination of 'farright' is the rightmost
/// vertex.
/// </remarks>
void MergeHulls(ref Otri farleft, ref Otri innerleft, ref Otri innerright,
ref Otri farright, int axis)
{
Otri leftcand = default(Otri), rightcand = default(Otri);
Otri nextedge = default(Otri);
Otri sidecasing = default(Otri), topcasing = default(Otri), outercasing = default(Otri);
Otri checkedge = default(Otri);
Otri baseedge = default(Otri);
Vertex innerleftdest;
Vertex innerrightorg;
Vertex innerleftapex, innerrightapex;
Vertex farleftpt, farrightpt;
Vertex farleftapex, farrightapex;
Vertex lowerleft, lowerright;
Vertex upperleft, upperright;
Vertex nextapex;
Vertex checkvertex;
bool changemade;
bool badedge;
bool leftfinished, rightfinished;
innerleftdest = innerleft.Dest();
innerleftapex = innerleft.Apex();
innerrightorg = innerright.Org();
innerrightapex = innerright.Apex();
// Special treatment for horizontal cuts.
if (UseDwyer && (axis == 1))
{
farleftpt = farleft.Org();
farleftapex = farleft.Apex();
farrightpt = farright.Dest();
farrightapex = farright.Apex();
// The pointers to the extremal vertices are shifted to point to the
// topmost and bottommost vertex of each hull, rather than the
// leftmost and rightmost vertices.
while (farleftapex.Y < farleftpt.Y)
{
farleft.Lnext();
farleft.Sym();
farleftpt = farleftapex;
farleftapex = farleft.Apex();
}
innerleft.Sym(ref checkedge);
checkvertex = checkedge.Apex();
while (checkvertex.Y > innerleftdest.Y)
{
checkedge.Lnext(ref innerleft);
innerleftapex = innerleftdest;
innerleftdest = checkvertex;
innerleft.Sym(ref checkedge);
checkvertex = checkedge.Apex();
}
while (innerrightapex.Y < innerrightorg.Y)
{
innerright.Lnext();
innerright.Sym();
innerrightorg = innerrightapex;
innerrightapex = innerright.Apex();
}
farright.Sym(ref checkedge);
checkvertex = checkedge.Apex();
while (checkvertex.Y > farrightpt.Y)
{
checkedge.Lnext(ref farright);
farrightapex = farrightpt;
farrightpt = checkvertex;
farright.Sym(ref checkedge);
checkvertex = checkedge.Apex();
}
}
// Find a line tangent to and below both hulls.
do
{
changemade = false;
// Make innerleftdest the "bottommost" vertex of the left hull.
if (RobustPredicates.CounterClockwise(innerleftdest, innerleftapex, innerrightorg) > 0.0)
{
innerleft.Lprev();
innerleft.Sym();
innerleftdest = innerleftapex;
innerleftapex = innerleft.Apex();
changemade = true;
}
// Make innerrightorg the "bottommost" vertex of the right hull.
if (RobustPredicates.CounterClockwise(innerrightapex, innerrightorg, innerleftdest) > 0.0)
{
innerright.Lnext();
innerright.Sym();
innerrightorg = innerrightapex;
innerrightapex = innerright.Apex();
changemade = true;
}
} while (changemade);
// Find the two candidates to be the next "gear tooth."
innerleft.Sym(ref leftcand);
innerright.Sym(ref rightcand);
// Create the bottom new bounding triangle.
mesh.MakeTriangle(ref baseedge);
// Connect it to the bounding boxes of the left and right triangulations.
baseedge.Bond(ref innerleft);
baseedge.Lnext();
baseedge.Bond(ref innerright);
baseedge.Lnext();
baseedge.SetOrg(innerrightorg);
baseedge.SetDest(innerleftdest);
// Apex is intentionally left NULL.
// Fix the extreme triangles if necessary.
farleftpt = farleft.Org();
if (innerleftdest == farleftpt)
{
baseedge.Lnext(ref farleft);
}
farrightpt = farright.Dest();
if (innerrightorg == farrightpt)
{
baseedge.Lprev(ref farright);
}
// The vertices of the current knitting edge.
lowerleft = innerleftdest;
lowerright = innerrightorg;
// The candidate vertices for knitting.
upperleft = leftcand.Apex();
upperright = rightcand.Apex();
// Walk up the gap between the two triangulations, knitting them together.
while (true)
{
// Have we reached the top? (This isn't quite the right question,
// because even though the left triangulation might seem finished now,
// moving up on the right triangulation might reveal a new vertex of
// the left triangulation. And vice-versa.)
leftfinished = RobustPredicates.CounterClockwise(upperleft, lowerleft, lowerright) <= 0.0;
rightfinished = RobustPredicates.CounterClockwise(upperright, lowerleft, lowerright) <= 0.0;
if (leftfinished && rightfinished)
{
// Create the top new bounding triangle.
mesh.MakeTriangle(ref nextedge);
nextedge.SetOrg(lowerleft);
nextedge.SetDest(lowerright);
// Apex is intentionally left NULL.
// Connect it to the bounding boxes of the two triangulations.
nextedge.Bond(ref baseedge);
nextedge.Lnext();
nextedge.Bond(ref rightcand);
nextedge.Lnext();
nextedge.Bond(ref leftcand);
// Special treatment for horizontal cuts.
if (UseDwyer && (axis == 1))
{
farleftpt = farleft.Org();
farleftapex = farleft.Apex();
farrightpt = farright.Dest();
farrightapex = farright.Apex();
farleft.Sym(ref checkedge);
checkvertex = checkedge.Apex();
// The pointers to the extremal vertices are restored to the
// leftmost and rightmost vertices (rather than topmost and
// bottommost).
while (checkvertex.X < farleftpt.X)
{
checkedge.Lprev(ref farleft);
farleftapex = farleftpt;
farleftpt = checkvertex;
farleft.Sym(ref checkedge);
checkvertex = checkedge.Apex();
}
while (farrightapex.X > farrightpt.X)
{
farright.Lprev();
farright.Sym();
farrightpt = farrightapex;
farrightapex = farright.Apex();
}
}
return;
}
// Consider eliminating edges from the left triangulation.
if (!leftfinished)
{
// What vertex would be exposed if an edge were deleted?
leftcand.Lprev(ref nextedge);
nextedge.Sym();
nextapex = nextedge.Apex();
// If nextapex is NULL, then no vertex would be exposed; the
// triangulation would have been eaten right through.
if (nextapex != null)
{
// Check whether the edge is Delaunay.
badedge = RobustPredicates.InCircle(lowerleft, lowerright, upperleft, nextapex) > 0.0;
while (badedge)
{
// Eliminate the edge with an edge flip. As a result, the
// left triangulation will have one more boundary triangle.
nextedge.Lnext();
nextedge.Sym(ref topcasing);
nextedge.Lnext();
nextedge.Sym(ref sidecasing);
nextedge.Bond(ref topcasing);
leftcand.Bond(ref sidecasing);
leftcand.Lnext();
leftcand.Sym(ref outercasing);
nextedge.Lprev();
nextedge.Bond(ref outercasing);
// Correct the vertices to reflect the edge flip.
leftcand.SetOrg(lowerleft);
leftcand.SetDest(null);
leftcand.SetApex(nextapex);
nextedge.SetOrg(null);
nextedge.SetDest(upperleft);
nextedge.SetApex(nextapex);
// Consider the newly exposed vertex.
upperleft = nextapex;
// What vertex would be exposed if another edge were deleted?
sidecasing.Copy(ref nextedge);
nextapex = nextedge.Apex();
if (nextapex != null)
{
// Check whether the edge is Delaunay.
badedge = RobustPredicates.InCircle(lowerleft, lowerright, upperleft, nextapex) > 0.0;
}
else
{
// Avoid eating right through the triangulation.
badedge = false;
}
}
}
}
// Consider eliminating edges from the right triangulation.
if (!rightfinished)
{
// What vertex would be exposed if an edge were deleted?
rightcand.Lnext(ref nextedge);
nextedge.Sym();
nextapex = nextedge.Apex();
// If nextapex is NULL, then no vertex would be exposed; the
// triangulation would have been eaten right through.
if (nextapex != null)
{
// Check whether the edge is Delaunay.
badedge = RobustPredicates.InCircle(lowerleft, lowerright, upperright, nextapex) > 0.0;
while (badedge)
{
// Eliminate the edge with an edge flip. As a result, the
// right triangulation will have one more boundary triangle.
nextedge.Lprev();
nextedge.Sym(ref topcasing);
nextedge.Lprev();
nextedge.Sym(ref sidecasing);
nextedge.Bond(ref topcasing);
rightcand.Bond(ref sidecasing);
rightcand.Lprev();
rightcand.Sym(ref outercasing);
nextedge.Lnext();
nextedge.Bond(ref outercasing);
// Correct the vertices to reflect the edge flip.
rightcand.SetOrg(null);
rightcand.SetDest(lowerright);
rightcand.SetApex(nextapex);
nextedge.SetOrg(upperright);
nextedge.SetDest(null);
nextedge.SetApex(nextapex);
// Consider the newly exposed vertex.
upperright = nextapex;
// What vertex would be exposed if another edge were deleted?
sidecasing.Copy(ref nextedge);
nextapex = nextedge.Apex();
if (nextapex != null)
{
// Check whether the edge is Delaunay.
badedge = RobustPredicates.InCircle(lowerleft, lowerright, upperright, nextapex) > 0.0;
}
else
{
// Avoid eating right through the triangulation.
badedge = false;
}
}
}
}
if (leftfinished || (!rightfinished &&
(RobustPredicates.InCircle(upperleft, lowerleft, lowerright, upperright) > 0.0)))
{
// Knit the triangulations, adding an edge from 'lowerleft'
// to 'upperright'.
baseedge.Bond(ref rightcand);
rightcand.Lprev(ref baseedge);
baseedge.SetDest(lowerleft);
lowerright = upperright;
baseedge.Sym(ref rightcand);
upperright = rightcand.Apex();
}
else
{
// Knit the triangulations, adding an edge from 'upperleft'
// to 'lowerright'.
baseedge.Bond(ref leftcand);
leftcand.Lnext(ref baseedge);
baseedge.SetOrg(lowerright);
lowerleft = upperleft;
baseedge.Sym(ref leftcand);
upperleft = leftcand.Apex();
}
}
}