On Sat, 2004-01-10 at 19:20, Alan King wrote: > Jon Berndt wrote: > > >> I see what you're doing now. You are letting them just use > >>the nose, and > >>then shifting the FDM nose point until the FDM center is near the > >>visual center. > > > > > > Not really. The FDM still calculates the position of the CG of the aircraft. > > It's just that we know exactly where the agreed-upon MRP is at all moments > > of time, so we calculate that and give it to FlightGear. We could just as > > easily agree that the wingtip was the "common known point", calculate that, > > and have FlightGear place the 3D model in the view so that the wingtip was > > at the specified point. The only thing the FDM does as an auxiliary > > calculation to aid the 3D model placement is to additionally calculate the > > MRP position in world space and make that available to FlightGear. Or, at > > least, we will when I code it. > > > > Jon > > Ok that gets back to the same question then. What else besides the nose > point does FlightGear use to place the visual model? > > Say your CG is at the origin. Your nose is 10 feet forward. FG puts the > visual model's nose at the 10 foot position since that is the common reference > point. What makes sure that where the CG is on the visual model is at 0?
Nothing. It's not necessary. Every dimension supplied to the FDM is automatically referenced to the c.g. position. In other words, if the c.g. is at 10 feet and the reference point at 20, the only thing that's relevant to the FDM is the difference between the two. If you follow that, you'll realize that the location of zero is of absolutely no significance. > > You do a 20 foot long plane FDM, with CG at 10 feet. I draw a 3000 foot long > plane. The noses match. Does about 2980 feet of my plane sink into the ground? You need only tell the FDM that model's reference point is 2980 feet forward. > What makes sure that the CG is put at the right place in the visual model > with the nose as the only reference? Does FG just scale the total model, the > FDM also gave a length for the model, and FG assumes the visual model has the > wing in the right place? It doesn't need to be. If the reference point is known to the FDM, it can properly calculate it's location in space, and that's all that's needed. The CG location is 100% irrelevant to the visual model. Only six pieces of info are needed by the code that draws it in the scene: the x, y, and z of the reference point and the pitch, roll, and heading angles. If the x,y, and z are calculated correctly then the model will appear to rotate around it's cg once it's off the gear. > > Maybe FG uses your CG, matches the nose, then just guesses the CG of the > visual model of the overall size? There just has to be some other piece of data > that correlates. If not there is no telling if the real FDM CG is where it's > supposed to be on the visual model. An off center reference point like the nose > isn't enough if you have no other data to calculate back and match the center. > > If they're actually accurate together, there is something else matching. And > if there isn't anything else matching in some accurate way, then there is no > telling if the FDM CG and where it should be on the visual model are really > matching. What else is FG doing to put them in the same place? Nothing at all. It's not needed. > > Alan > > > > _______________________________________________ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel -- Tony Peden <[EMAIL PROTECTED]> _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
