Need help from Rhino experts.

[Alexanov]

I have two trimmed surfaces. First surface has 4x4 control points, second one 5x5 control points.
In second case Rhino can't calculate Area. What can be wrong? Iges file attached.

 

[pafurijaz]

I tested that with moi3D the little brother, but the file seem corrupted after remke it I can get the surface area.

here how looks your sample in Moi3D


and that after remake.

left area
2153790201,14 mm²
right area
2153790200,35 mm²

here the result from your original file with FreeCAD and moi3d

1815496126,75 mm²
1815496126,15 mm²

and with FreeCAD I have the same result of moi3d but edited one
2153790201,14 mm²
2153790200,35 mm²

But that is a bit strange result and I think the result of original file is wrong.
and below how looks the file exported as e mesh.

more test and I thinks this file has many issues..
inspection of original

edited inspection

 

[Dr34m3r]

Its looks like tolerance issue . rebuild the edge with increase tolerance . also note the free naked edge point between these two surface.

 

[Alexanov]

View attachment 161850

Its looks like tolerance issue . rebuild the edge with increase tolerance . also note the free naked edge point between these two surface.

Thank you! I will check it. What tolerance do you use for it?

 

[Dr34m3r]

i was playing with 0 , probably i used 0.000001

 

[pafurijaz]

Hi, I have made further checks on the file and I am convinced that the file has problems. the correct result is the following.

01 surface area =2492084275,52 mm²
01 trim area =338294074,39 mm²
01 trimmed surface area =2153790201,13 mm²

02 surface area =2492084274,85 mm²
02 trim area =338294074,20 mm²
02 trimmed surface area =2153790200,65 mm²

 

[DCockey]

Alexanov, the triangular hole is the source of the problem. Each surface has an extra set of edges around the triangular hole. The Split command also fails if the split line goes through the triangular hole. The What command in Rhino reports 11 edges when there should be 8 edges in total - 5 around the outside and 3 around the hole. If the hole is eliminated by Untrim then the area computes properly. I don't know why only one surface has the problems. It may be related to the surface with the problem having two spans with the hole located across the knot line.

I could submit the file McNeel to see if they can identify the problem via the Rhino forum.

What command results for the surface with the problems:

surface

ID: d0dd9b83-4997-4f7c-9a5b-aee13cadaffb (2747)
Object name: (not named)
Layer name: IGES level 0
Render Material:
source = from layer
index = -1

Geometry:
Valid surface.
trimmed surface with 2 hole(s).
NURBS Surface (SubD friendly)
"U": degree =3 CV count = 5 (0.000 <= U <= 2.000)
"V": degree =3 CV count = 5 (0.000 <= V <= 2.000)
Edge Tally:
11 boundary edges
Edge Tolerances: 0.000 to 0.000
median = 0.000 average = 0.000
Vertex Tolerances: all 0.000
Render mesh: none present
Analysis mesh: none present​

 

[Alexanov]

Alexanov, the triangular hole is the source of the problem. Each surface has an extra set of edges around the triangular hole. The Split command also fails if the split line goes through the triangular hole. The What command in Rhino reports 11 edges when there should be 8 edges in total - 5 around the outside and 3 around the hole. If the hole is eliminated by Untrim then the area computes properly. I don't know why only one surface has the problems. It may be related to the surface with the problem having two spans with the hole located across the knot line.

I could submit the file McNeel to see if they can identify the problem via the Rhino forum.

What command results for the surface with the problems:

surface

ID: d0dd9b83-4997-4f7c-9a5b-aee13cadaffb (2747)
Object name: (not named)
Layer name: IGES level 0
Render Material:
source = from layer
index = -1

Geometry:
Valid surface.
trimmed surface with 2 hole(s).
NURBS Surface (SubD friendly)
"U": degree =3 CV count = 5 (0.000 <= U <= 2.000)
"V": degree =3 CV count = 5 (0.000 <= V <= 2.000)
Edge Tally:
11 boundary edges
Edge Tolerances: 0.000 to 0.000
median = 0.000 average = 0.000
Vertex Tolerances: all 0.000
Render mesh: none present
Analysis mesh: none present​

The Iges file has been generated by Opencascade library. Intersting. To definition of trimmed surface I have to make outer contour an inner contour. May be this is create problem in Rhino. But why it is works different???

 

[DCockey]

The only reason for it to work differently that I can think of is the 4 point surface is single span and the 5 point surface has two spans. Should I ask McNeel (Rhino developers) to look into it?

 

[bhnautika]

David you're on the right track with the points but I think the degree has any influence also.I did a rebuild (see pic) and was able to get a surface area.

 

[DCockey]

David you're on the right track with the points but I think the degree has any influence also.I did a rebuild (see pic) and was able to get a surface area.

Both surfaces are degree 3. The difference is the number of control points and corresponding number of knots and spans.

 

[bhnautika]

I did find I could leave the points at 5 but change the degree’s to 4 I could get the area to work

 

[DCockey]

That is useful information. Changing the degree of the 5 point surface from 3 to 4 makes it a single span surface. That is consistent with the problem being associated with the problem surface being multispan.
(Number of spans = number of control points - degree)

 

[pafurijaz]

an answer from a developer...

It looks like the interior triangle hole boundary is malformed. There should be 3 edges there, in both MoI and Rhino I see there are actually 6 edges. It looks like it's 2 duplicate trim boundaries stacked up on top of each other.

It also isn't great that it has unusually large numeric coordinate values. It's not good to have coordinates like x = 153093.0 , values that are far from the origin like that will accumulate more rounding errors in floating point arithmetic. But that's minor compared to the malformed inner trim boundary.

 

[DCockey]

an answer from a developer...

You or someone else? A developer of what?

It looks like the interior triangle hole boundary is malformed. There should be 3 edges there, in both MoI and Rhino I see there are actually 6 edges. It looks like it's 2 duplicate trim boundaries stacked up on top of each other.

Same as I reported above. Both surfaces have duplicate trim curves around the holes. But only one surface causes a problem computing area.