Does AIS_Shape alter the geometry of TopoDS_Shape ?

Jim Williams
Fri, 08/26/2016 - 06:42

Forums:

I have a TopoDS_Shape, which I am displaying with this function:

void Display(TopoDS_Shape& bShape)
{

   Handle(AIS_Shape) ais = new AIS_Shape(bShape);
   anAIScontext->SetColor(ais, Quantity_NOC_YELLOW);
   anAIScontext->SetMaterial(ais, Graphic3d_NOM_GOLD, Standard_False);
   anAIScontext->Display(ais, Standard_False);
}

This works.

HOWEVER, I find the call " Handle(AIS_Shape) ais = new AIS_Shape(bShape);" alters the geometry of bShape!

I get the bounding box of bShape with:

Bnd_Box aBounds;
BRepBndLib::Add(bShape, aBounds);
aBounds.Get(aXmin, aYmin, aZmin, aXmax, aYmax, aZmax);

If get the bounding box before calling my Display() function, then after calling it, I get different results!

The difference is not huge, but bigger than it should be , e.g aZmax is 20.000001 before calling Display() ,and 20.003 after .

This surprises me a lot. I thought the "AIS_Shape(bShape)" function would just copy data from bShape, not alter it.

I want to rely on the data in bShape, even after it has been displayed.

Can anyone explain what is going on?

Or am I misunderstanding how things work?

Thanks

Jim

Roman Lygin
Fri, 08/26/2016 - 08:51

Hi Jim,

The matter of the fact is that computation of a bounding box depends on whether the shape does or does not contain triangulation. When you display the AIS_Shape for the first time, apparently it does triangulate your shape (which did not have triangulation before that). So the results differ.

This effect is explained here - http://opencascade.blogspot.com/2009/04/all-inclusive-bounding-box.html.

With that, you could pre-triangulate your shape prior to calling bounding box computation (using BRepMesh_IncrementalMesh).

Ideally you might want to pre-triangulate with exact same parameters that will be used during the display and avoid re-triangulation inside displaying the AIS_Shape. For that you would need to disable automatic re-triangulation using Prs3d_Drawer::SetAutoTriangulation (false), e.g.:

anAISContext->DefaultDrawer()->SetAutoTriangulation (false),

or likewise per individual AIS_Shape.

However, for simplicity and if performance is not an concern, just trust the AIS to re-triangulate as needed during the display.

Hope this helps.

Roman

P.S. You should rather prefer using "void Display (const TopoDS_Shape& bShape)".

Jim Williams
Sun, 08/28/2016 - 23:43

Thank you.

That's an excellent explanation.

Jim