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Documentation improvements.

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Fixes #598 #691 #1071 #1072

PiperOrigin-RevId: 579816915
Change-Id: Ica5e62516e2888d47030b0f44619efa2621f329a
This commit is contained in:
Yuval Tassa
2023-11-06 05:55:15 -08:00
committed by Copybara-Service
parent 5a9239e11c
commit 0f37deca0d
5 changed files with 100 additions and 22 deletions
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doc/overview.rst
+12 -7
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@@ -944,15 +944,20 @@ can be obtained as:
int qposadr = -1, qveladr = -1;
// make sure we have a floating body: it has a single free joint
if( bodyid>=0 && m->body_jntnum[bodyid]==1 &&
m->jnt_type[m->body_jntadr[bodyid]]==mjJNT_FREE )
{
// extract the addresses from the joint specification
qposadr = m->jnt_qposadr[m->body_jntadr[bodyid]];
qveladr = m->jnt_dofadr[m->body_jntadr[bodyid]];
}
if( bodyid>=0 && m->body_jntnum[bodyid]==1 && m->jnt_type[m->body_jntadr[bodyid]]==mjJNT_FREE ) {
// extract the addresses from the joint specification
qposadr = m->jnt_qposadr[m->body_jntadr[bodyid]];
qveladr = m->jnt_dofadr[m->body_jntadr[bodyid]];
}
Now if everything went well (i.e., "myfloatingbody" was indeed a floating body), qposadr and qveladr are the addresses
in qpos and qvel where the data for our floating body/joint lives. The position data is 7 numbers (3D position followed
by unit quaternion) while the velocity data is 6 numbers (3D linear velocity followed by 3D angular velocity). These
numbers can now be set to the desired pose and velocity of the body.
The semantics of free joints are as follows. The linear postions of free joints are in the global frame, as are
linear velocities. The orientation of a free joint (the quaternion) is also in the global frame. However, the rotational
velocities of a free joint are in the local body frame. This is not so much a design decision but rather correct
use of the topology of quaternions. Angular velocities live in the quaternion tangent space, which is defined locally
for a certain orientation, so frame-local angular velocities are a natural parameterization.
Accelerations are defined in the same space as the corresponding velocities.