Negative Displacement Values and Subdivision

[CHawkes]

This is an issue that I come across quite frequently and have yet to find a way to solve it.

I like to use displacement maps that have 0 as their base point, this often means that I have negative displacement values.

I am aware that I need to add a VRay attribute in my file node to allow negative values.
On my geometry shape, I add a 'Subdivision and Displacement Quality' and a 'Displacement Control' attribute. If I render that, I get the results I'd expect but with some angular edges from the low res mesh.
The problem comes when I also add a 'Subdivision' attribute. Now when I render, the negative values are clipped rather than displacing the geometry inwards. The angular edges are gone though.

Does anyone have any thoughts on this?

[yolov]

Do you set explicit displacement bounds to the vray displacement control?

[CHawkes]

UPDATE:
I've been doing some tests and I've found I only get this problem if I'm using UDIMs in my displacement texture. Could it be that VRay is unable to figure out the displacement bounds when subdivision is on and UDIM is being used?

[CHawkes]

Hi Yolov, I didn't see your message before I posted mine. I was right and so are you!
I think VRay couldn't figure out the bounds with subdiv and UDIM going on. Explicitly setting the bounds has forced it to work. I used -1 to +1 is there any reason why I shouldn't use higher numbers to avoid any clipping? Does that make things harder for VRay?

[yolov]

Ah, then it should be clear. VRay can take the bounds automatically, but only for a single frame and without tiles. When you have a sequence of images and/or tiles (udim, uvtile), it is best to set the bounds explicitly. It shouldn't make things harder for VRay as far as I know.