[vex] pretranslate vs translate + Bullet?

[catchyid]

Hello,

I am trying to understand what is the difference between pretranslate and translate (in order to understand the code below which applies points transforms to RBD primitives). Note, in the code below, orient, pivot and P attributes come from the points attributes

matrix xform = qconvert(orient); //convert quaternion into 4x4 rotation matrix, checked and 4th column is [0 0 0 1], i.e. no translation

pretranslate(xform, -pivot); //what does it mean? maybe xform= -pivot * xform *pivot

translate(xform, P); //does it mean xform = xform * P, i.e. just add the translation component to the existing one in the matrix?

v@P *= xform;

//--

I have a question: I assume Bullet uses center of mass for rotation, hence we have "pivot" attribute?  and "P" is just a pure translation in world coordinates?

 

Thanks,