Field Driver -driving Xpresso via User Data

[williambillcard]

Hi,
I have cloner, cloning Xpresso rigs that drive Tracer objects (think animated Spirographs). The rig has user data to control the wheel size and pen position. I want to use Field drivers and Random Fields to make every cloned rig unique.

I am trying to use a field driver on user data I have set up for my rig. The user data is connected to my Xpresso setup for the rig. This does not seem to work.

It allows me to add the user data into the field driver parameter but the changes to the user data don't seem to affect the Xpresso or objects.

Is this a limitation of Field Drivers?

many thanks in advance!

[Dr. Sassi]

Hi williambilcard.

Please take a look here and share with me what you notice is different.
CV4_2025_drs_25_MGsg_01.c4d
I will have a look into it with more information.

I have used the Linear Field with a wave curve to map data.

Files in the C4D format can be attached here as long as they are smaller than 1MB. If larger, please note that only a few cloud services are used, strictly based on security. (I will share the list and limitations, if needed)

All the best

[williambillcard]

@Dr-Sassi
Hi Dr. Sassi,
Thanks so much for taking a look at this. I can see your example works - but this is driving the user data of the matrix object, not the user data of an object within a cloner.
I attach an example that shows the problem. With the user data cloner if I switch it off the rectangle changes height. If you turn it on the changes stop. it does not seem to make a difference if I move the field driver tag to the cloner object.
user-data-xpresso-cloner_001.c4d

[Dr. Sassi]

Hi williambillcard,

To my knowledge, Cloner-Child based XPresso is calculated before the clone is newly generated. When it is generated, it is not adjusted from the Cloner Child level anymore; any further changes are either Effector/Field-based or on the Field Driver. Placing XPresso between Field Driver and Cloner Child signals that the Field Driver has worked with the XPresso and has no new run again.

The limitation that existed was the reason to have the Field Driver in the first place.

If you could share more about why this feature is important, perhaps we can find a workaround or use the same URL as below and utilize the "Share Your Idea!" option.

Perhaps check with Tech Support, in case I missed something:
https://www.maxon.net/en/support-center

Enjoy your Sunday

[williambillcard]

@Dr-Sassi
It would have been convenient for a project I am working on where I needed to re purpose the rig to create a large grid of unique spiro patterns. It's reassuring to know it is software limitation rather than (my usual) user error.

I disconnected the user data I wanted to drive and drove the object attributes (circle radius and pen position x) directly with field drivers. This kind of works - but there is now another issue where the tracer objects ignore the changed positions of their trace objects and all draw the same shape. It looks like some kind of evaluation order or priority issue but I haven't been able to get to the bottom of it. One day, when I have lots of time I will have a go at building the setup in scene nodes instead of Xpresso...

I attach a stripped down file in case you are interested
03_broken_spiro_cloner_001.c4d

but...
I do have a workaround, which is to create hundreds of separate setups to go inside the Cloner. Then select them all and use the "rnd" expression in the User data field to get unique values in each one. This works, but is a bit slow to set up and I was hoping there would be a more parametric approach.