CGAL users discussion list

[Sebastien Loriot <>]

Welcome on the mailing list!

As documented here:
https://doc.cgal.org/latest/Kernel_23/classCGAL_1_1Plane__3.html#ad8f355a87bd5ca0fec2d423dbf5909d7

`returns a point q, such that to_2d( to_3d( p )) is equal to p.`

Since (-1,0,0) and (1,0,0) are both converted by to_2d() to (0,0),
there is no way to guarantee that to_3d(to_2d(p3)) == p3 (it is
not a bijection).

Best regards,


Sebastien.




On 1/31/22 5:46 PM,  wrote:
> Hello!
>
> First time writing on the mailing list, so I hope it is the right place
> to discuss, since I'm not super convinced, whether my finding is a bug
> or not. I transform a 3d point into 2D using a plane and transform it
> back. I would have expected approximately the same point.
>
> I'm using CGAL with Python SWIG bindings in the newest version (commit
> 2479e35) on Arch Linux. I took a look at the bindings code and could
> not find much code, that could do something wrong. Therefore I suspect
> the error being somewhere in the C++ part.
>
> Here is my example:
> ```python
> from CGAL.CGAL_Kernel import Plane_3, Point_3
>
> # plane with distance 1 in x-direction
> plane_3d_x = Plane_3(1, 0, 0, 1)
>
> # this point obviously lies on the plane
> point_3d_unit_x = Point_3(1, 0, 0)
>
> # results into a (0,0)
> point_2d = plane_3d_x.to_2d(point_3d_unit_x)
>
> print(str(point_2d))
>
> assert point_2d.x() == 0
> assert point_2d.y() == 0
>
> # i would have expected the same point (except for minor numerical
> errors)
> point_3d_retransformed = plane_3d_x.to_3d(point_2d)
>
> print(str(point_3d_retransformed))
>
> # this assertion fails, since x == -1
> assert point_3d_retransformed.x() == 1
> assert point_3d_retransformed.y() == 0
> assert point_3d_retransformed.z() == 0```
>
> Can anyone tell, if this is a bug or explain to me, why this is
> correct?
>
> Thanks in advance!