[x3d-public] bboxDisplay considerations

[Andreas Plesch]

Following up, I looked up the bboxDisplay implementation in x_ite, and
it does pretty much follow what I outlined, eg. using an additional
internal, on demand Shape. Instead of wrapping in a Transform, for
scaling it multiplies in a local transform matrix to the model view
matrix. it is not strictly part of the scene graph but gets added
during render traversal but not picking traversal. The bbox shape is
predefined as a white IndexedLineset but presumably could be
generalized/made customizable.

I hope it is possible to avoid special treatment during traversal and
just manage as a regular, inert scene graph participant. Let's see.

Ok, this was helpful. Andreas

On Fri, Mar 24, 2023 at 12:10 PM Andreas Plesch <andreasplesch at gmail.com> wrote:
>
> Please forgive me if I use this message to plan strategies for a
> possible implementation of bboxDisplay for x3dom. There will be a few
> questions at the end.
>
> To start, it is already quite straightforward to generate visual
> feedback for selection purposes in x3dom. Here are a few examples:
>
> https://andreasplesch.github.io/Library/Examples/html5/highlight/turnyellow.html
> https://andreasplesch.github.io/Library/Examples/html5/highlight/withBBox.html
> https://andreasplesch.github.io/Library/Examples/html5/highlight/outline.html
>
> Thinking about generalizing to any X3DBoundedObject leads to the
> following observations.
>
> In x3dom, only Shapes are collected for rendering during scene
> traversal. Displaying a bbox for groups etc. breaks this basic
> pattern.
>
> Although a custom shader would be most performant, this means it is
> not really advantageous to expand the shader to include an option for
> bbox rendering because it is restricted to shapes. Also a bbox needs
> to be displayed even if the shape is not visible which complicates
> logic.
>
> In turn, this means it appears to be necessary to add an internal bbox
> Shape for each X3DBoundedObject to the scene graph which is managed by
> the engine. Since the bbox size needs to be updated at each traversal
> this will add some overhead. I guess if the updates are only performed
> when the bbox is visible this may be ok but the check is still
> necessary. Also the bbox shapes need to be created for a lot of
> objects, perhaps on demand only which could be an optimization. It
> will be necessary to be careful in cleaning up when a X3DBoundedObject
> is deleted from the scene graph.
>
> Finally, I am considering adding an SFNode field to X3DBoundedObject
> "bboxTemplate" which would be a Shape node or a Grouping node and
> would let the scene designer customize the kind of bounding box used
> for the display. By default it would be a green cage or such. It is
> expected to be of unit size, and scaled internally (by a Transform
> wrapper) to the actual bbox size. ( Hm, this would probably mean
> allowing subShapes to Shape nodes as children. It may be possible to
> deal with bboxes for Shapes as siblings, or in general as siblings,
> eg. child of parent ).
>
> By default the size of the displayed bbox would be a tight fit but it
> would look probably better to make it a little larger. So another
> custom field could be SFFloat "bboxMargin", zero by default.
>
> What strategies do FreeWRL, x_ite or castle use to implement
> bboxDisplay while keeping overhead at a minimum ? Check at each
> traversal and insert a scaled bbox ? As an option in the shader ?
>
> Depending on the strategy, would it make sense to support a custom
> bounding box ?
>
> A somewhat unrelated question is if a displayed bounding box is
> eligible to be sensed by Sensors such as a TouchSensor ? Presumably,
> it should be completely inert and be ignored but I am not sure if this
> is how the spec. intends to be read. I think in x3dom it should be
> possible to make sensing eligibility configurable.
>
> Thanks for any feedback,
>
> Andreas
> --
> Andreas Plesch
> Waltham, MA 02453



-- 
Andreas Plesch
Waltham, MA 02453