[x3d-public] inaccessible Background urls

[Andreas Plesch]

I am exploring if it is feasible to employ MS Babylon.js as an engine
to render X3D, first without any runtime. I implemented a few basic
nodes which went pretty well. While working on Background I noticed
that the X3D spec. seems silent on behaviour for the case if none of
the urls provided for a given face is available from the network, and
therefore the texture is left undefined.

9.3.2 X3DUrlObject (see links below) mentions that case and refers
back to the specific node type descriptions for further instructions.
But 24.4.1 Background does not deal with this case.

Some examples have comments that assume that a browser would use
skyColor et al. as a fallback. But this is not required it seems.

The tooltips do not offer guidance for this case.

For an implementation it may come down to if a non-accessible url
should be replaced with a transparent image (letting the sky show), or
a black image.

In Babylon.js, if any of the six images is not viable or null, the
whole skybox is rendered black. This is due to webGL cubemap
requirements. Of course, it is possible for a browser to check the
urls first and replace them, if necessary, with a transparent pixel,
for example. But perhaps this should be left as a responsibility to an
author.

A question to the group here would be if this case should be left unspecified ?

Outdated, obsolete urls may be common enough to justify tightening of
the spec. But GIGO may apply here and suffice as well.

-Andreas

Relevant links:

https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-CD/Part01/components/environmentalEffects.html#Background

https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-CD/Part01/components/networking.html#X3DUrlObject

https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-CD/Part01/components/networking.html#URLs

https://www.web3d.org/x3d/content/X3dTooltips.html#Background


-- 
Andreas Plesch
Waltham, MA 02453