Find link index

[sven1904]

Am I right that there's no way to programmatically find out the link index by the link name used in the URDF?
Something like pybullet.getLinkIndex('urdf_link_name') or vice versa pybullet.getLinkName(1)

Background:
I'm trying to setup some contact constraints between links, but I don't have a clue how bullet introduces link indices since they do not always match the order in the URDF file.
For example, if you load this model, the resulting link index->name map is this:

-1 -> world
 0 -> link_AB
 1 -> link_BE
 2 -> link_CD
 3 -> link_EC

although the URDF order is different for the last two elements and the URDFParser goes through the file link by link.

Any information would be appreciated.

PS: I know that the gazebo tags are ignored in the URDF file, it is only an example file from the web.

[rohit-kumar-j]

@sven1904 you can either create a utility function or a utility class that does that programmatically when the robot is loaded in or compare the p.getJointInfo[i] values (where 'i' is the index of the name of the joint).

....Something like that as far as I remember

[sven1904]

hello @rohit-kumar-j,

thanks for your suggestion.

yes, for the joints there is the method getJointInfo(i) where you also get the information on the joint's name.
unfortunately, there is no such function for the links, but i need the link indices for the creation of constraints.
even if i go with the workaround via getJointInfo(i) which also gives me the index of the parent link and the name (why?) of the child, i still don't know the index of the child.

maybe a short example for explanation:
i have a model of a robot hand with five fingers and want to add a contact constraint to the pointer finger tip and the tip of the thumb.
with getJointInfo(i), i can find out all indices of finger links except for the tip.
so, there are still 5 possibilities left for each of the tips.
a workaround for this would be to add another "virtual" joint and link to each of the fingers and get the parent index of that joint via a loop over getJointInfo(i) with joint name comparison.
but this seems quite complex for such a simple information and messing around with the model is not that nice for that kind of stuff.

this is why i asked if there is already a functionality (without editing the model) to find out which link index refers to which link name.

[sven1904]

after having a look here, i'm wondering if the child link always has the same index as the joint to which it is appended.
because although createConstraint(..) takes link indices as parameters, there are joint indices given.

@erwincoumans as the author, can you confirm that this is the case?

[rohit-kumar-j]

@sven1904
Yes, it is cumbersome to find the link indices manually, however in my experience when you load the URDF, even if you do not mention the p.URDF_MAINTAIN_LINK_ORDER argument to the p.loadURDF() function, the links spawn with the same indices.

I simply color the links with p.changeVisualShape() to input the link that I want to color, I think the links are +or- 1 to the joint index. That way I can find the link Index. It is a little troublesome, but only once per URDF.

I had created constraints here: https://youtu.be/0hfkm7vv5o8

Also See: #3924