Re: [Flightgear-devel] Aerodynamic centre and 3D models

[Russell Suter]

Andy Ross wrote:

Russell Sutter wrote:
 

David Megginson wrote:
   

Andy Ross wrote:
     

I'm not sure exactly what this is for.  I can (and probably
should) export the C.G. position for the view code to use
appropriately.  But the VRP stuff seems like a double-correction.
It's basically identical to the view center offset stuff, isn't
it?
       

It's the location on the plane where the FDM reports the
lon/lat/alt.  It's kind-of a nifty idea, actually.
     

True, but...  This is a chunk of calculations running every frame.  In
the olden days, the cost would be too high.
   

It's a vector addition; I think we can handle it.  Seriously, YASim in
particular is doing *so* much more work than this that the overhead
would be literally immesurable.
 

That really wasn't the point, but whatever.  I don't question anyone's 
ability or
really even so much the cost.  I'm simply trying to point out 
alternatives. 

My objection is that it seems to be handled in multiple places, and we
should just pick one.  Right now we can adjust the model and view
origins with any/all of the following:
Yes, exactly!  I'm on your side here Andy, so be nice to me... ;-)

+ Move the model origin.  (Add an offset to all the vertices in the .ac file)
+ Move the FDM origin.    (Ditto for the FDM configuration)
+ Use the /view/config/*-offset-m properties (see the 747 for an example)
Adding the VRP is yet another mechanism, basically a direct analog of
the view offset stuff on the FDM side.  I just don't see the need.  If
we decide the VRP is the right way to do it, we should chuck the view
offset stuff for simplicity and orthogonality.
The VRP is a fine idea but if you change your point of view from the FDM 
and to
the model, as long as you have a fixed point calculated in the FDM, why 
not use it.
Translate the model to that point.  The translation can be done once 
when the .ac file
is loaded.  For us it was the Empty Weight CG.  If the IG software 
supports the concept
of an Static Coordinate System (SCS) node, the process is simple.  For 
those familiar
with SGI's Performer, you know what I'm talking about.  Then the VRP 
becomes meta
data for the model instead of the FDM.  We did this on the last two 
devices I worked on. 

--
Russ
Conway's Law: "The structure of a system tends to mirror the
structure of the group producing it."
     -- Mel Conway Datamation (1968)


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