[C++] how to respond to Zoom Extents for custom object bounding box

Rhino Developer
,
1

One of the custom objects that we use has a larger “perceived” bounding box than its constituent geometry, due to part of the object being drawn in a mirrored way.
I have tried to override GetBoundingBox(CRhinoView* pView) and that gives good results for clipping (i.e. the virtual, mirrored part is not clipped upon zooming into that part).

I find, however, that the zoom extents all command (ZEA) does not take this larger bounding box into account. It always zooms to the geometry, whereas I want it to zoom to the geometry AND the virtual, mirrored part. How do I tell Rhino that my object should be shown the way I want?

In ASCII art:

[ ] viewport
G only geometry
M|G geometry with mirrored part
M|[G] - current result of zoom extents all, only the G part is in the viewport
[M|G] - desired result of zoom extents all, both geometry and mirrored part are visible

(I hope this is clear?!)
2

Hi Menno,

On your custom object, override the CRhinoObject::GetTightBoundingBox virtual function and return the correct bounding box.

Does this help?

– Dale

3

This did not help in the first try, but when I take into account the supplied transformation, it works.

Currently, this is my implementation:

bool CMyObject::GetTightBoundingBox(ON_BoundingBox& tight_bbox, bool bGrowBox, const ON_Xform* x) const
{
    if (CRhinoSurfaceObject::GetTightBoundingBox(tight_bbox, bGrowBox, x))
    {
        ON_Xform mir;
        ON_3dPoint origin(ON_3dPoint::Origin);
        ON_3dVector normal(ON_3dVector::YAxis);
        
        if (x && x->IsValid())
        {
            origin.Transform(*x);
            normal.Transform(*x);
        }        
        mir.Mirror(origin, normal);

        ON_BoundingBox mbb = tight_bbox;
        mbb.Transform(mir);        
        tight_bbox.Union(mbb);
        return true;
    }
    return false;
}
4

@stevebaer @dale

The use of TightBoundingBox on a custom object gives problems when the constituent geometry is asked for a bounding box in .NET. I filed a bug for this

http://mcneel.myjetbrains.com/youtrack/issue/RH-30605

5

Hi there,
I am interested to understand how TightBoundingBox works, is there any documentation regarding this common? @dale
Thanks

6

Hi @bianchini,

There is no document for how tight bounding boxes are calculated. Is there a particular entity you are curious about?

– Dale

7

Hi @dale
I was curious to see how an arc bounding box was defined.

8

Hi @bianchini,

For arcs, the bounding box is calculated from it’s NURB form. For normal bounding boxes, the NURBS cv’s are used. For tight bounding boxes, the cv’s from Bezier spans are used.

– Dale

9

Thanks @dale,
I used the extremes, based on the derivatives to find all the points to define a tight bounding box for a nurbs.
Thanks again for the message.