removed convexity check for zero-volume quads

MeshLib/Elements/TemplateQuad.tpp

@@ -125,8 +125,10 @@ ElementErrorCode TemplateQuad<NNODES,CELLQUADTYPE>::validate() const
 	ElementErrorCode error_code;
 	error_code[ElementErrorFlag::ZeroVolume]  = this->hasZeroVolume();
 	error_code[ElementErrorFlag::NonCoplanar] = (!GeoLib::pointsOnAPlane(*_nodes[0], *_nodes[1], *_nodes[2], *_nodes[3]));
-	error_code[ElementErrorFlag::NonConvex]   = (!(GeoLib::dividedByPlane(*_nodes[0], *_nodes[2], *_nodes[1], *_nodes[3]) &&
-		                                           GeoLib::dividedByPlane(*_nodes[1], *_nodes[3], *_nodes[0], *_nodes[2])));
+	// for collapsed quads (i.e. reduced to a line) this test might result "false" as all four points are actually located on a line.
+	if (!error_code[ElementErrorFlag::ZeroVolume]) 
+		error_code[ElementErrorFlag::NonConvex]   = (!(GeoLib::dividedByPlane(*_nodes[0], *_nodes[2], *_nodes[1], *_nodes[3]) &&
+			                                           GeoLib::dividedByPlane(*_nodes[1], *_nodes[3], *_nodes[0], *_nodes[2])));
 	return error_code;
 }