Extracting B-spline basis information from U-spline json-export

Coreform Cubit Importing/Exporting
#1

Hello,

I want to extract B-spline patch data from a U-spline. Therefore, I assign each patch to a block so that it is easy to identify their corresponding elements in the .json export. But what are the knot vectors for these patches; is the knot vector always uniform with non-zero knots of size 1, or is there a relation to the element size?

Thanks.

Best regard,
Benjamin

PS: Here is a short journal file to construct a ring of 4 spline patches to clarify my question:

#’ set up geometry
create curve arc radius 1 center location 0 0 0 normal 0 0 1 start angle 0 stop angle 360 as 1
create vertex on curve 1 segment 4
split curve 1 at vertex all
curve 2 3 4 5 copy scale 2
create curve vertex 3 16
create curve vertex 4 18
create curve vertex 5 13
create curve vertex 6 14
create surface curve 13 6 2 12
create surface curve 5 12 9 11
create surface curve 8 10 4 11
create surface curve 10 7 13 3

#’ define patch domains
block 1 add surface 1
block 2 add surface 2
block 3 add surface 3
block 4 add surface 4

#’ meshing
surface all scheme map
surface all size auto factor 10
mesh surface all
block all element type QUAD4

#’ set up spline elements
set uspline surface all degree 2 continuity 1 creasing minimal
build uspline crease group
build uspline surface all
fit uspline all
draw uspline on
export uspline all json “splineExport”

#2

Hi Benjamin,

For the example you provided, yes you can assume uniform knot vectors with knot spans equal to 1. However, this is not always true; for many U-splines, there is a dependence on mesh size.

Hope this helps,

Luke

#3

Hi Luke,
I see. How can the knot span information be extracted from the JSON-file (or another export) for the general case?
Thanks,
Benjamin

#4

Hi Benjamin,

Unfortunately most of this information is lost once we convert to the extracted format, and can’t easily be recovered from the .json file. Can I ask what you’re trying to use the corresponding B-Spline patch data for?

#5

Hi Luke,
Ultimately, I want to generate and export regular conforming volumetric B-spline patches for an IGA solver that does not use Bezier extraction. Any advice on how to accomplish that with Coreform Cubit?
Best regards
Benjamin

#6

Hi Benjamin,

I don’t think this can be easily accomplished with the current version of Cubit. For many cases, you may get lucky, and have the resulting U-spline control points correspond to what would be built using a B-spline patch with uniform knot spans, but we can’t guarantee this. We will let you know if and when we add more structured information to U-spline export.

Best,

Luke