[Source] [X3D-Earth] Xj3D property setting for origin manager, for inclusion in X3D-Edit

[Rex Melton]

chris wrote:
> thanks Rex, yeah that all makes sense, just one more query on the last 
> point:
>
>
> 2009/6/6 Rex Melton <rex.melton at comcast.net 
> <mailto:rex.melton at comcast.net>>
> ....
>  
>
>     A straight line calculation certainly is more efficient than the
>     orthodromic distance. However, the elevation threshold is set
>     rather high by default (50km) for a variety of reasons. A straight
>     line distance from the established origin to the viewpoint would
>     incorporate the elevation. Adding that component to the distance
>     threshold would cause recalculations more frequently than would
>     really be necessary. In other words, in order to keep the amount
>     of recalculation down, only the 'lateral' distance from an
>     establish origin is considered (and necessary I think) and not the
>     vertical. Hope that makes sense..... If not, let me know and I'll
>     try to explain it again.
>
>
> So if you make the threshold distance greater, you could use the more 
> efficient calculation, e.g. comparing distance^2 to threshold^2? I 
> remember that geovrml scaled speed based on elevation, so one could 
> also scale threshold similarly.

The orthodromic distance calculation only happens once per frame. 
Overall, it's not that expensive. On the order of a matrix multiply.

What IS expensive is recalculating all the geo-coordinates that are 
loaded in the scene to account for a newly established local origin.

The trade-off you suggest may produce a miniscule performance increase 
per frame. But will likely introduce a recalculation penalty more 
frequently. Overall, I think it would negatively impact performance.

> Presumably, the content developer could also tweek the distance based 
> on amount of jitter and framerate.

At present, the thresholds are configured with properties at browser 
instantiation time. They -could- be exposed for an application to manage.

I don't consider this approach to be more than a temporary proof of 
concept. Spec wise - GeoOrigin will likely be removed or deprecated. How 
that gets managed by a browser should be transparent to the user.