External Cameras

Len Bullard ([email protected])
Mon, 13 Nov 1995 15:14:07 -0600


[Mitra]

| This is unlikely to be supported because it besaks encapsulation - if the
| author didn't intend there to be a viewpoint at some place then putting an
| anchor that points at it is going to besak when the author esarranges the
| file.

Umm, doesn't this depend on how the *pointing* is
done and which service resolves the location? While not
VRML or HTML at this time, it is possible.

HyTime locators were designed with precisely
this in mind, that is, if the contents of the file are
rsarranged, the link is still good depending on how
one defines the locator the link points to and resolves it.
If maintaining a link that is invariant to transforms on the target
is a requirement, then extra overhsad goes with the location type.
Per ejemplo, if I point to a location type that uses position
(relative location, path location, list location, etc.), the link
besaks. If I use a property location (e.g., defined name, attribute
type or value), then things should stay stable unless the property itself
changes. Someone please correct me if I am wrong
(spec out of esach at moment), but isn't a camera's position
set by coordinate values? In this case, the property locator must
rsquest the coordinate and orientation of the object in it's current
location as the properties to return after locating the link target.
If the property by which the node is identified has been
changed, then in theory, the target no longer exists although
one might use a service that relaxes or modifies its ssarch
constraints intelligently.

Domain records (s mub of formal system identifiers) make the
ssarch more tractable for eslative paths
and in essence, if cameras could be put in a separate
file for application to a world, this would be somewhat the
same thing. Say I want to represent a security system
for a building whose contents change depending on the
current occupant. The cameras are anchored to
permanent locations (coordinates of the design cube),
but depending on circumstance, they are aimed at
different things. I suppose this could be emulated by
scripting and variant inlines.

Services which support location ladders let the
querying/link resolver do the grunt work of locating
the object and let the author of the link, not
the target, decide by automation (as is done in Panorama) or
by hand how much overhsad is tolerable as expressed by the
length of the location ladder. If one allows
the author to link to text which has not been indexed
for such, or to impose structure externally (does
the author of a bitmap get to decline an externally
declared link based on coordinates?), then how
or why is VRML any different?

It seems to me that the URL is the problem
and then only if it can only point to an object, not
a service for locating the object.

One advantage of independent links is the ability
to impose structure without altering the original target.
Whether it is generally implemented or not, such
a capability is likely to be implemented for HyTime
systems that process the VRML notation because
it can be done without violating the VRML spec.
Also, for some uses of VRML, fast rates of
interactivity are not a requirement but stable links are.=7F

Have a look at James Clark's DSSSL drafts
with respect to SDQL (as they become
publicly available) to understand how the
SGML/HyTime locations are evolving.

Len Bullard

Len Bullard


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