Norman Vine wrote:
Mathias Fröhlich wwrites:This should not necessarily be a requirement for the 3D model. It is, however
On Sonntag, 15. Februar 2004 10:49, Erik Hofman wrote:
Jon Berndt wrote:I hope you don't!
I give up. Sort of.
No need to IMHO. I think we now have an excellent solution. Could someone file a patent request for this?
There are some gotcha's involved which could mean some other
points/locations should be exposed also in the future, but that's about it.
And even then, JSBSim is now able to satisfy *every* request in that direction.
I also did not understand why this simple feature is such a big question!
The main remaining problem is that the rest of the subsystems have yet to take this adjustment into account also.
Also from my reading of the current Model code it appears as if the absolute positioning of an aircraft with the world will be *very* dependent on the designer's choice of the VRP and I am also not sure if just with this addition the current code will fix the 'issue' that prompted the incarnation of this thread again. i.e. the aircraft appears to be rotating about the nose
But since this is an abstract thing it is good to have a 'public' thrashing so all can gain an understanding of the issues involved please note this is non-trivial to most and not just a simple feature request which has far reaching consequences on the very 'core' of most everything within the SIM
so to outline what I have gleaned so far
1) the 'modellers' coordinate frame will be [ X ] longitudinal increasing aft < i.e. > smallest at the nose [ Y ] latitudinal increasing right < i.e. > smallest at the left wing tip [ Z ] vertical increasing up < i.e. > smallest at the ground
what the model must be translated to with the /offsets/ property.
The center of the bounding sphere shouldn't be relevant in this discussion.
2) within this frame a VRP < virtual refrence point > will be designated to represent the origin of the coordinate frame with respect to the 'tip' of the nose of the aircraft.
3) The SSG will assume a 'similar frame' but keep in mind that the Object.getBSphere()->getCenter() will return the point that is 'magically' computed to be at the center of the 'Geomettry' when passed to SSG at Object creation time. Most likely this will not be the VRP and in any case can *not* be assumed to be, and is .
IMPORTANT: This *is* the point ultimately used by the Rendering Engine from which any rotation and / or translation is derived.
The IG shouldn't be used to position the 3D model. If it being used, that's wrong.
4) The FDM will compute the center of 'Gravity' WRT to the frame and pass it's offset WRT to the VRP back to FGFS. note this is not and thus can *not* be assumed to be the same as the center of 'Geometry' that will be used by the Rendering Engine.
5) AFAICT FGFS will then reposition the AirCraft using the offset provided by the FDM and rotate the AirCraft to align itself with the orientation.
Here is where I get confused and ... I am probably missing the obvious but nowhere in the above is the offset from the possibly corrected VRP WRT the Object.getBSphere()->getCenter() accounted for ??
Also I can not find in the code the mechanism that will rotate the AirCraft about any point other then the point returned by Object.getBSphere()->getCenter() as adjusted by the translation WRT the VRP which appears to be set at Model load time from the OffSet from the nose and not the Geometrical center about which rotations will occur. In order for this to occur you need two complimentary translations. One before and one after the rotation.
Also there is this issue about where exactly the [ lat, lon. alt ] reported back from the FDM each cycle refers to that is yet to be decided
Again I am probably missing the obvious here and should just go back to lurking so Erik doesn't start ignoring my posts too :-)
HTH
Norman
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