# Get surface parameters of an arbitrary point on a bounding curve

Hello fellow OCCT users, for a mesh algorithm I need to interpolate between the vertices of mesh cells. These mesh cells are lying on a OCCT surface and sometimes the vertices lie on a bounding curve of the surface (I added a picture to this post). In this case I know the parameters of the vertices on the curve, but not on the suface. This makes interpolating between the parameters of the vertices impossible. Therefore, I need to convert the parameter of a vertex on the curve to the parameters of the vertex on the surface. Currently, I use `ShapeAnalysis_Surface` and `GeomAPI_ProjectPointOnSurf` to do this, but even the combination of both algorithms doesn't give consistent results (I also added a code example). In some cases both algorithms cannot find a set of parameters, even if I know exactly, that the point is on the surface. Is there an alternate algorithm I haven't found yet? Or can I even use the parameter of the curve, the curve or the topology information to convert the curve parameter into the surface parameters?

``````int get_occ_surface_parameters (Handle_Geom_Surface surface,
double *coords,
double *param,
double tol,
double *near_param)
{
ShapeAnalysis_Surface surface_analyzer(surface);
const gp_Pnt pnt(coords[0], coords[1], coords[2]);
gp_Pnt2d uv;
/* Get parameters with ShapeAnalysis_Surface
* if we have some nearby parameters we use these as start point of the newton method */
if (near_param == nullptr)
{
uv = surface_analyzer.ValueOfUV(pnt, tol);
}
else
{
gp_Pnt2d next_uv(near_param[0], near_param[1]);
uv = surface_analyzer.NextValueOfUV(next_uv, pnt, tol);
}
/* Check the tolerance and if the parameters are inside the bounds */
double u1, u2, v1, v2;
surface->Bounds(u1, u2, v1, v2);
if (surface_analyzer.Gap() <= tol &&
((u1 <= uv.X() && uv.X() <= u2) || (u2 <= uv.X() && uv.X() <= u1)) &&
((v1 <= uv.Y() && uv.Y() <= v2) || (v2 <= uv.Y() && uv.Y() <= v1)))
{
param[0] = uv.X();
param[1] = uv.Y();
return 1;
}

/* If ShapeAnalysis_Surface fails we try the same with GeomAPI_ProjectPointOnSurf */
const GeomAPI_ProjectPointOnSurf proj_on_surf(pnt, surface, tol);
if (proj_on_surf.NbPoints() > 0)
{
if (proj_on_surf.LowerDistance() <= tol)
{
proj_on_surf.LowerDistanceParameters(param[0], param[1]);
return 1;
}
}

/* Return 0 if no parameters were found */
return 0;
}
``````
Attachments:

Hello Sandro,
have a look at the BRep_Tool class, there is a lot of methods there to access various sub-object, in particular BRep_Tool::Parameters() return the uv coordinate for a TopoDS_Vertex on a TopoDS_Face. If your TopoDS_Shape is correctly build and contains p-curves (ShapeFix_Edge::FixAddPCurve() ) you should be able to retrieve the uv coordinates.
Best regards,
François.