How to compute the pinAndPivotFrame matrix on CustomJoints

[pr0t0z]

Hi all,

I am currently trying to change the NewtonHinge standard Joints for some CustomJoints. However, the interface is slightly different and the problem I have right now is how do I transform a the Pin vector I use into the pinAndPivotFrame matrix required to compute the Local Matrixes (only the rotation matrix, since the position (pivot) is straightforward).

Best Regards,
Carlos

[JoeJ]

For me it's easier to ignore the pnaAndPivotframe param from the custom joints constructor, and set both the local matrices directly afterwards.
Something like:

Joint *j = new CustomXXX (..., temporal identity for pin&pivot, ... bodies etc.);
j->localMatrix0 = myJointspaceForBody0;
j->localMatrix1 = myJointspaceForBody1;

... usually you already know the positional part und can find proper orientation by 'pin is mostly x - axis'-rule, or looking at joints submit function or trial and error.

Hope that helps.

[JoeJ]

Maybe not the best aswer...

looking at joints submit function there is:

sinAngle = (matrix0.m_up * matrix1.m_up) % matrix0.m_front;

...which means it measures hinge angle from up & front vector.
So the pinAndPivot for constructor should be in worldspace and:
left vector is aligned with hinge axis,
front vector should point in direction where limit angle is zero (if i'm wrong, some trial and error remains)

[Julio Jerez]

in the engine all joint use a coordinate frame of reference to operate. This frame is passed by the user in for of a matrix.
On that frame the first row is the principal axis, the second row is the secondary axis, the third row is the cross product of the first an second axis and the matrix position is the origin of the joint.
This matrix must be in global space.

for example the parameter value of a hinge are the origin o, the pin dir, and initial wings orientation up.

you take those values and you make a 4 x 4 matrix by setting the first row to dir, the second row to up, the third row to cross (dir, up) and the fourth row to origin.
dir and up must be perpendicular and unit vectors.

[pr0t0z]

That was exactly what I was looking for, perfect.
Thanks so much for the replies, I'll try it and reply the results.

Carlos