From the simulation, I found it is a right-hand z-up coordinate system. But I am not quite certain about what the x, y, z axis mean in the bone space, like the definition of the joint on humanoid.xml? <joint name='abdomen_z' type='hinge' pos='0 0 0.065' axis='0 0 1' range='-45 45' damping='5' stiffness='20' armature='0.02' /> Is there any visualization on the axes definition? I am curious about this problem because I would like to calculate the inverse dynamics from motion captured data. It is very important to match the angles correctly.
You can enable frame visualization for bodies, geoms and sites. The frame rendering is red for x, green for y, blue for z.
I enabled the joint visualization with Code: option.flags[mjVIS_JOINT] = 1; option.flags[mjVIS_TRANSPARENT] = 1; But the coordinates are rendered all in blue.