I'm a little unclear on generating correct UDIMs in Houdini and perhaps someone smarter than me can clarify it.

I'm using the UV Layout Node to generate my UDIMs, however if I up my Fixed Size parameter too high, I notice that my UDIM tiles start flowing in the wrong direction. I was under the impression that this is a no-no for UDIMs as everything should flow from tile 1001 to the right until the 10'th row, and the start again in the next higher column.

What I'm trying to figure out is, does this look correct for proper UDIMs, and if not, how do I fix it so that my rows and columns move properly from left to right and the up instead of down?

I hope I'm explaining the issue correctly. Many thanks for your insights!

those are simply islands that can't fit a single tile therefore they are placed outside of the UDIM area
you can check to output nonpacked group and maybe run another UV Layout on those with different settings where you are not forcing them to be larger than a tile

I take it that larger than 1 tile UV islands can't be split into several UDIM tiles?

As it is in my current example, would those UV islands simply be ignored and not receive any textures?

Midphase
I take it that larger than 1 tile UV islands can't be split into several UDIM tiles?

it's not considered a good practice to have any polygon intersecting uv tile boundary, you are asking for trouble, like not getting correct filtering across the seam where samples of a polygon sample one udim texture and neighbour samples another one

Midphase
As it is in my current example, would those UV islands simply be ignored and not receive any textures?

Texture VOP should return Default Color (whatever you set it to) if the udim is not found
texture() or colormap() VEX function don't expand udims on their own so you can use expand_udim() to get resolved udim path based on uv which can also check if the file exists (does it by default), so if it returns empty string use whatever you want for your color if it returns path sample the texture

EDIT: also UDIMs are sort of arbitrary, so technically you can have udims 999, 998, 997, … which would correspond to those below the 0, while not standard, Houdini should be able to resolve them so if found it would use those textures
but that's definitely not the intended workflow, they are simply put under 0 to be processed later as they didn't satisfy the rules

Cool, thanks for the good info!