0031290: Modeling Algorithms - Center of mass of the TopoDS_Edge of an ellipse seems to be wrong

View Issue Details [ Jump to Notes ]

[ Issue History ] [ Print ]

Project

Category

View Status

Date Submitted

Last Update

0031290

Community

[OCCT] OCCT:Modeling Algorithms

public

2020-01-01 21:43

2020-01-15 12:15

Reporter

b_walter

Assigned To

msv

Priority

normal

Severity

minor

Status

new

Resolution

open

Platform

Mac

OS Version

Product Version

[OCCT] 7.3.0

Target Version

Fixed in Version

Summary

0031290: Modeling Algorithms - Center of mass of the TopoDS_Edge of an ellipse seems to be wrong

Description

I use opencascade 7.4.0_1

If I create an ellipse with the following code

```
  gp_Pnt center(0., 0., 0.);
  gp_Dir zDir = gp::DZ();
  gp_Ax2 axis(center, zDir);
  gp_Elips ellipse_gp(axis, 4000., 40.);
  TopoDS_Edge ellipse = BRepBuilderAPI_MakeEdge(ellipse_gp);
```

then calculating the center of mass by

```
  GProp_GProps Properties;
  BRepGProp::LinearProperties(ellipse, Properties);
  gp_Pnt c = Properties.CentreOfMass();
```

leads to (-81.295066, 0.000000, 0.000000)

I would have expected (0, 0, 0).

What happens here?

Steps To Reproduce

ellipse el 0 0 0 4000 40
mkedge e el
lprops e

Center of gravity :
X = -81.2951
Y = 1.40172e-15
Z = 0

Tags

No tags attached.

Test case number

Attached Files

Relationships

Notes
(0089836) msv (developer) 2020-01-01 23:25	This seems to be a bug in calculation of linear properties of an ellipse. Workaround is to create a face of that ellipse and to compute surface properties of that face. The computed center of such face is (0, -3.91083e-15, 0).

(0089837) b_walter (reporter) 2020-01-02 15:48	As a side note, if I approximate the ellipse with points (python code for illustration) ``` def ellipse(maj_r, min_r, alpha): return (maj_r * math.cos(alpha), min_r * math.sin(alpha)) alphas = np.linspace(0, 2*math.pi, 3601) points = [ellipse(400, 10, alpha) for alpha in alphas] ``` and build a closed wire, the center of mass is correct. So it is not a matter of the shape dimensions or the shape form, but of the ellipse itself.

Issue History
Date Modified	Username	Field	Change
2020-01-01 21:43	b_walter	New Issue
2020-01-01 21:43	b_walter	Assigned To	=> msv
2020-01-01 23:25	msv	Note Added: 0089836
2020-01-01 23:26	msv	Steps to Reproduce Updated	View Revisions
2020-01-02 15:48	b_walter	Note Added: 0089837
2020-01-15 12:14	kgv	Summary	Center of mass of the TopoDS_Edge of an ellipse seems to be wrong => Modeling Algorithms - Center of mass of the TopoDS_Edge of an ellipse seems to be wrong
2020-01-15 12:15	kgv	Relationship added	related to 0030449