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Based on a patch from Xyne, this should be a slightly less verbose way of safely calculating normals. Note that CGAL does have a unit_normal() function, but since the Gmpq kernel doesn't support sqrt(), this cannot be used.

felipesanches-svg
Marius Kintel 2012-01-03 21:55:44 +01:00
parent 7a044adfb8
commit 47c51d128a
1 changed files with 16 additions and 24 deletions
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40
src/export.cc
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@ -83,32 +83,24 @@ void export_stl(CGAL_Nef_polyhedron *root_N, std::ostream &output, QProgressDial
stream.str("");
stream << x3 << " " << y3 << " " << z3;
std::string vs3 = stream.str();
CGAL_Polyhedron::Traits::Vector_3 normal(1,0,0);
if (vs1 != vs2 && vs1 != vs3 && vs2 != vs3) {
// The above condition ensures that vs1-vs2, vs1-vs3, and their cross
// product are non-zero. Floating point arithmetic may however truncate
// small values to 0. This can be avoided by first scaling the components
// of vs1-vs2 and vs1-vs3. This has no effect on the resulting unit
// normal vector.
double dn[6] = { x1-x2, y1-y2, z1-z2, x1-x3, y1-y3, z1-z3 };
double maxdn = 0;
int i;
for (i = 0; i < 6; ++i) {
double dx = dn[i];
if (dx < 0) dx = -dx;
if (dx > maxdn) maxdn = dx;
}
for (i = 0; i < 6; ++i) dn[i] /= maxdn;
double nx = dn[1]*dn[5] - dn[2]*dn[4];
double ny = dn[2]*dn[3] - dn[0]*dn[5];
double nz = dn[0]*dn[4] - dn[1]*dn[3];
double nlength = sqrt(nx*nx + ny*ny + nz*nz);
// Avoid generating normals for polygons with zero area
double eps = 0.000001;
if (nlength < eps) nlength = 1.0;
// The above condition ensures that there are 3 distinct vertices, but
// they may be collinear. If they are, the unit normal is meaningless
// so the default value of "1 0 0" can be used. If the vertices are not
// collinear then the unit normal must be calculated from the
// components.
if (!CGAL::collinear(v1.point(),v2.point(),v3.point())) {
// Pseudocode: CGAL kernel must be set up to enable unit_normal and
// Vector type must be declared as Vector_3<Kernel>.
// http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Kernel_23_ref/Function_unit_normal.html
normal = CGAL::normal(v1.point(),v2.point(),v3.point());
normal = normal / sqrt(CGAL::to_double(normal.squared_length()));
}
output << " facet normal "
<< nx / nlength << " "
<< ny / nlength << " "
<< nz / nlength << "\n";
<< CGAL::to_double(normal.x()) << " "
<< CGAL::to_double(normal.y()) << " "
<< CGAL::to_double(normal.z()) << "\n";
output << " outer loop\n";
output << " vertex " << vs1 << "\n";
output << " vertex " << vs2 << "\n";