On Friday 15 May 2009 20:31:17 Durk Talsma wrote:
> I'm not sure about this, but my estimate is that the trouble doesnt arise
> when the mag-compass is part of the user aircraft, but only when it's part
> of the exterior world, i.e. when part of an AI aircraft. Also, it's
> possible that the instrument by itself may be okay, but triggers an error
> in interaction with other scene elements.
>
I've been checking a little more today, and it looks like there's more than
meets the eye. In retrospect, the cessna / mag-compass models themselves are
probably okay. It's probably only that the mag compass showed up because it
was the first object to be subjected to a test containing bad preconditions
already.
I'm still not fully understanding the finer details of the ground cache, but
in essence, if works by trying to shoot a line through every triangle in the
scene graph. Then it returns the distance of the closest intersecting point.
To make the process efficient, the scenegraph is traversed node by node, and
triangles obviously not relevant are quickly discarded, quite similar to the
way the culling algorithm works.
Since the scene graph is composed of models that may each have their local
coordinate systems, it is necessary that the line that is shot through the
scene graph is also transformed accordingly, so for each level of the
scenegrahp, a new transformation matrix is created, using a popMatrix
function.
While trying to trap bad data in this the popMatrix function, I just noticed
that a bad transformation matrix is already set up relatively early in the
process, only a few levels deep at the stack. I haven't been able to relate
this to any meaningful object yet. (All that came up was the name "Scene").
So, it looks like a transformation error early on blows up the intersect line
vector(s) already. and scenegraph is traversed further down, OSG keeps happily
multiplying already corrupted data with valid transformation data further down
the line, restuling in an intersect line, composed of NaNs. This goes
unnoticed, until the error is finally picked up at the first possible occasion
where there's a nan error check. That is, in trialintersect.
I hope to continue this investigation later, and hope to be able to traverse
the bad data to their true source.
In the mean time, I'm sorry to all the model developers for prematurely
raising a red flag. I still find it rather curious that the error only seems
to occur when AI traffic is activated, which still seems of indicate a
critical role for the c172p model. However, until further notice, it doesn't
look like the problem is as straightforward as I thought yesterday.
Cheers,
Durk
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