When making a btTransform, why is setIdentity() necessary?

[Jonathan]

What does btTransform::setIdentity() do?

I had some code that wasn't generating any collision pairs, and after doing a little digging around, I realized the btTransform I was passing to my custom MotionState hadn't called setIdentity(). After calling it everything works fine, but why?

If there is some 3D math term I can Google, just let me know what the term is, because I wasn't finding much with my combinations of 'identity'.

[Mako_energy02]

"SetIdentity" really just sets a safe default value for each of the data members, usually (0,0,0) on a Vector3, or (0,0,0,1) on a quaternion. Without calling it the data members in the transform can have any value. Imo, SetIdentity() should be called in the constructor.

[Jonathan]

"SetIdentity" really just sets a safe default value for each of the data members, usually (0,0,0) on a Vector3, or (0,0,0,1) on a quaternion. Without calling it the data members in the transform can have any value. Imo, SetIdentity() should be called in the constructor.

Ah thanks! Makes sense... just weird that there must be a situation where default initializing the values of a transform are somehow too expensive to do automatically in the constructor.

[Flix]

just weird that there must be a situation where default initializing the values of a transform are somehow too expensive to do automatically in the constructor.

Probably when you initialize long arrays or containers (e.g. 10000 or more elements) at once...

[laadams85]

"SetIdentity" really just sets a safe default value for each of the data members, usually (0,0,0) on a Vector3, or (0,0,0,1) on a quaternion. Without calling it the data members in the transform can have any value. Imo, SetIdentity() should be called in the constructor.

Well that's not entirely correct. Setting the identity for a transform is setting the rotation matrix (basis) equal to the identity matrix, you can google that, and setting the origin to zero. This represents no rotation and no displacement. Transforming a vector using the identity transform will give you that vector back.