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

[Rex Melton]

Hi Chris,

chris wrote:
> Hi Rex,
>
> That's an interesting implementation: I imagine the elevation 
> threshold approach helps when there is navigation paths between space 
> and the surface (and back again).

The threshold is really just to prevent unnecessary processing in the 
browser. The precision management is only really necessary as the user 
approaches the surface. If the user just spins around in orbit, 
transforming the coordinates would be wasted effort.

>
> a couple of observations:
>
> <snip>
>
>
>     Description
>
>     The dynamic origin management strategy currently implemented in
>     Xj3D is an extension to the original implementation. Internal to
>     the browser, all geo-referenced coordinates are converted into GCC
>     (Geocentric Cartesian) for use in rendering and transformation. The
>
> In the comments below, I am going to assume that coordinates are still 
> in double (or higher) precision immediately after this conversion.
>  

All the calculations/transformations are done in double. A single 
precision representation of the terrain geometry, etc. is produced 
finally to pass into the renderer.


>     original implementation would offset those coordinates by the
>     GeoOrigin position as nodes were instantiated. The functional
>     addition to the browser consists of a manager class that makes
>     determinations based on the user's Viewpoint on when and where a
>     new local origin is required, and notifies the loaded Geo nodes
>     that they must update their interal state to account for the new
>     setting.
>
>     Local origins will be configured at either the ellipsoid center,
>     or at the ellipsoid surface. The default origin manager's criteria
>     for switching the local origin setting are configured with two
>     parameters. An elevation above the ellipsoid is used as a
>     threshold for establishing an initial local origin. A Viewpoint
>     elevation above the threshold will cause the local origin to be
>     configured as at the ellipsoid center. An elevation below the
>     threshold will cause a local origin to be configured at the
>     Viewpoint's geodetic position on surface of the ellipsoid.
>
>  
>
>     Once a local origin has been established at the ellipsoid surface,
>     an orthodromic distance from that local origin will be used as a
>     threshold for reconfiguration. When the Viewpoint moves beyond the
>     threshold distance from the local origin a new origin is
>     established, again at the Viewpoint's geodetic position on surface
>     of the ellipsoid.
>
>
>  Observable jitter and other errors become evident on the graphics 
> display after the conversion to cartesian step mentioned above occurs 
> in the simulation pipeline. Therefore, given the assumption that 
> coordinates are still in higher precision immediately after the 
> conversion, a cartesian distance calculation is all that is required 
> to determine a threshold distance for reducing jitter. Would that 
> calculation not be more efficient than great circle calculation? It is 
> also less error prone: the more complex calculation has higher worst 
> case relative error. My research indicates that relative error has a 
> fluctuation property: occasionally spiking first in one direction then 
> the opposite.

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.