[x3d-public] bboxDisplay considerations

[Andreas Plesch]

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