Inconsistency between contact frame and its visualization

[jjyyxx]

Intro

Hi!

I am a graduate student at HKU, I use MuJoCo for my research on robotic manipulation.

My setup

MuJoCo 3.2.6, Python and C, Linux

What's happening? What did you expect?

Mujoco is using the X axis as the contact normal direction in mjContact.frame, as explicitly stated in the documentation:

Thus, given that MuJoCo uses row-major format, the contact normal axis (which is the X axis of the contact frame by our convention) is in position mjContact.frame[0-2], the Y axis is in [3-5] and the Z axis is in [6-8]. The reason for this arrangement is because ...

However, when visualizing with mjFRAME_CONTACT, the normal direction becomes the Z axis in blue, as explicitly switched in

mujoco/src/engine/engine_vis_visualize.c

Lines 134 to 137 in 4d82ab5

	// mat = contact rotation matrix (normal along z)
	mju_copy(tmp, con->frame+3, 6);
	mju_copy(tmp+6, con->frame, 3);
	mju_transpose(mat, tmp, 3, 3);

Both conventions are perfectly fine for me, but the inconsistency may create confusion during debugging, especially when dealing with anisotropic friction in the way recommended by documentation:

For a predefined contact pair we know the two geom types in advance, and the corresponding collision function always generates contact frames oriented in the same way – which we do not describe here but it can be seen in the visualizer.

Steps for reproduction

Minimal model for reproduction

No response

Code required for reproduction

No response

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[yuvaltassa]

Ya, that's valid. Do you want to send a PR to fix?