fuselage lofting issues with guide curves

[joergbrech]

The lofting of a fuselage fails if

• a segment contains guide curves, but none is located at relative circumference 0.
• a segment contains a single guide curve.

This issue can be reproduced using the following test case: ColaGuideCurve.zip. Without any modifications, the test case should work (TiGL creates the expected fuselage loft). The fuselage consists of a single segment with guide curves at relative circumference 0 and 0.5. Commenting out either of the two guide curves will cause TiGL to not successfully create the loft and return the following error:

ERR 03/30 12:09:05 CTiglMakeLoft.cpp:229] Could not create loft with guide curves. Error code = 2

It seems like the error originates in src/contrib/MakeLoops.cxx.

Click for some first debugging insights.

Case 1: A segment contains guide curves, but none is located at relative circumference 0

This can be reproduced with the test case by commenting out or deleting the guide curve with uID Duct_9_upperGuideCurveSegment1. This leaves a single guide curve at relative circumference 0.5.

ERR 03/30 12:09:05 CTiglMakeLoft.cpp:229] Could not create loft with guide curves. Error code = 2

Debugging reveals that the source of the error lies in this piece of code:

tigl/src/contrib/MakeLoops.cxx

Lines 178 to 215 in e5d4ee8

	bool foundStartingCorner = false;
	for (i = 1; i <= GridVertices.Extent(); i++) {
	Standard_Integer anExtent = GridVertices(i).Extent();
	if (anExtent == MinExtent) {
	// set current vertex as starting corner
	StartCorner = TopoDS::Vertex(GridVertices.FindKey(i));
	Standard_Boolean ForwardSectionFound = Standard_False;
	Standard_Boolean ForwardGuideFound = Standard_False;
	// iterate through all edges attached to the current grid vertex
	TopTools_ListIteratorOfListOfShape iter(GridVertices(i));
	for (; iter.More(); iter.Next()) {
	const TopoDS_Edge& anEdge = TopoDS::Edge(iter.Value());
	// get first vertex of the edge and check if it is the
	// same as the current starting corner
	TopoDS_Vertex aFirstVertex = TopExp::FirstVertex(anEdge, Standard_True);
	if (aFirstVertex.IsSame(StartCorner)) {
	// check if the edge is contained in the profiles
	if (myProfileEdges.Contains(anEdge)) {
	ForwardSectionFound = Standard_True;
	}
	// otherwise check if the edge is contained in the guides
	else { //<anEdge> is in <myGuideEdges>
	ForwardGuideFound = Standard_True;
	}
	}
	}
	// if the current vertex is attached to a guide and a profile edge,
	// both starting at the vertex, it is considered to be the starting corner
	if (ForwardSectionFound && ForwardGuideFound) {
	foundStartingCorner = true;
	break;
	}
	}
	}
	if (!foundStartingCorner) {
	myStatus = MAKELOOPS_FAIL_STARTINGPOINT;
	return;
	}

TiGL searches the grid of profiles and guide curves for a starting corner, which is a vertex with minimum neighboring edges. At this vertex, TiGL finds a "ForwardSection", i.e. the fuselage profile, but no "ForwardGuide" (presumably because none is defined at relative circumference 0). As a consequence, MakeLoops returns with status MAKELOOPS_FAIL_STARTINGPOINT.

Case 2: A segment contains a single guide curve.

Case 1 is a special case of this one. This can be reproduced by commenting out either of the two guide curves Duct_9_upperGuideCurveSegment1 or Duct_9_lowerGuideCurveSegment1. Let's assume that we comment out the second guide curve and keep the guide curve Duct_9_upperGuideCurveSegment1 at relative circumference 0, as this reveals a different bug.

Debugging reveals that in the first pass of the while-loop starting in

tigl/src/contrib/MakeLoops.cxx

Line 274 in e5d4ee8

while (true) {

the code will enter the true-branch of this if-clause:

tigl/src/contrib/MakeLoops.cxx

Lines 559 to 567 in e5d4ee8

	if (NextCorner.IsSame(StartCorner)) {
	StartCorner = NextStartCorner;
	NextCorner = StartCorner;
	if (IsFirstRow) {
	RowLength = myCells.Extent();
	IsFirstRow = Standard_False;
	}
	}
	}

But at this time, NextStartCorner is null, so that for the second pass of the while-loop, both StartCorner and NextCorner are null as well. At the beginning of the second pass of the while-loop

tigl/src/contrib/MakeLoops.cxx

Lines 275 to 278 in e5d4ee8

	// the four edges of the patch
	TopoDS_Edge E1, E2, E3, E4;
	// set current starting corner as the NextCorner from the previous iteration
	TopoDS_Vertex Corner = NextCorner;

Corner will be null, so that we never enter the true-branch of

tigl/src/contrib/MakeLoops.cxx

Lines 298 to 314 in e5d4ee8

	if (GridVertices.Contains(Corner)) {
	iter.Initialize(GridVertices.FindFromKey(Corner));
	for (; iter.More(); iter.Next()) {
	const TopoDS_Edge& anEdge = TopoDS::Edge(iter.Value());
	if (myProfileEdges.Contains(anEdge)) {
	TopoDS_Vertex aFirstVertex = TopExp::FirstVertex(anEdge, Standard_True);
	if (aFirstVertex.IsSame(Corner)) {
	E1 = anEdge;
	// save current vertex
	// however, since the orientation may have changed, get the object from the
	// gritVertices map (here, the Contains function seems to be orientation insensitive)
	vertex41 = TopoDS::Vertex(GridVertices.FindKey(GridVertices.FindIndex(Corner)));
	break;
	}
	}
	}
	}

and therefore E1 will remain null. Finally, this causes MakeLoops to fail here:

tigl/src/contrib/MakeLoops.cxx

Lines 330 to 334 in e5d4ee8

	if (E1.IsNull()) {
	if (NextStartCorner.IsNull()) {
	myStatus = MAKELOOPS_FAIL_FIRSTEDGE;
	break;
	}