[Plib-users] Matrix rotation question

[McEvoy, N. <nic...@ds...>]

Sorry matrix operations are not my strong point ... can somebody please
point out my stupid mistake !?

I'm developing a space sim with PLIB ... and I have a problem where my
spaceship behaves in a crazy manner when its heading is directly 'UP' and
tries to roll to the left or right.

// Say my initial my position is as follows: (ie. I'm pointing directly up
... NOTE: this only happens when pointing directory UP !!!)
sgCoord mPosition;
sgSetCoord(&mPosition, 0.0f, 0.0f, 0.0f, 0.0f, 90.0f, 0.0f);

// Say my direction change is as follows: (ie. I'm rolling to the left)
sgVec3 mDirChange;
sgSetVec3(mDirChange, 0.0f, 0.0f, -0.01f);

// I have a loop where the direction change is applied as follows to update
the hpr:
sgMat4 POS, ROT;
sgMakeCoordMat4(POS, &mPosition);
sgMakeRotMat4(ROT, mDirChange);
sgPreMultMat4(POS, ROT);
sgSetCoord(&mPosition, POS);

// Why does this loop toggle the hpr part of my mPosition as follows:
(causing the model to FREAK OUT flipping between ~0 and 90)
hpr:	0, 90, 0
hpr:	0, 90, 90.01
hpr:	0, 90, 6.18986e-006
hpr:	0, 90, 90.01
hpr:	0, 90, 1.38196e-005
hpr:	0, 90, 90.01
hpr:	0, 90, 1.38196e-005
etc ...

Any help appreciated,
Nick
mailto:nic...@as...
http://members.ozemail.com.au/~ndmcevoy/