Open3D

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Checklist

• [X] I have searched for similar issues.
• [X] For Python issues, I have tested with the latest development wheel.
• [X] I have checked the release documentation and the latest documentation (for master branch).

Describe the issue

I have computed a point cloud of an object. The goal is to measure the object size in pixel, use a reference object with a known width and calculate the pixel/mm ration, to apply to other pointclouds of the same measure system.

But for that, the width, height, and length are necessary. I suppose one could calculate the rotation and translation, orient it to the origin of the coordinate system and just calculate the size then.

To make that shorter and faster, I was trying to use a bounding box. But it seems like, the bounding box is not the minimal bounding box, instead it's rotated and returns a big bounding box, which I can't use for measurement.

Maybe I have done something wrong, but the documentation pointed me this way, so I don't know.

Steps to reproduce the bug

import open3d as o3d

...

bb_pcd_ref = pcd_ref.get_oriented_bounding_box()

... 

o3d.visualization.draw_geometries([bb_pcd_ref, pcd_ref, coordinate_system])

Error message

No response

Expected behavior

I expected the output to be the minimal bounding box, but it was not.

Open3D, Python and System information

- Operating system: Windows 10 64-bit
- Python version: Python 3.8.13
- Open3D version: output from python: 0.11.2
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: conda

Additional information

No response

Small update here:

tried to compute the bounding box with the robust flag, like stated in the documentation, to see if that changes something:

get_oriented_bounding_box(self: open3d.cpu.pybind.geometry.Geometry3D, robust: bool = False)

Error:

TypeError: get_oriented_bounding_box(): incompatible function arguments. The following argument types are supported:
    1. (self: open3d.cpu.pybind.geometry.Geometry3D) -> open3d::geometry::OrientedBoundingBox

So there is no flag, as far as I can see.

If I'm looking at the right C++ source indeed it's not attempting to minimise the oriented box.

    /// Creates an oriented bounding box using a PCA.
    /// Note, that this is only an approximation to the minimum oriented
    /// bounding box that could be computed for example with O'Rourke's
    /// algorithm (cf. http://cs.smith.edu/~jorourke/Papers/MinVolBox.pdf,
    /// https://www.geometrictools.com/Documentation/MinimumVolumeBox.pdf)
    /// \param points The input points
    /// \param robust If set to true uses a more robust method which works
    ///               in degenerate cases but introduces noise to the points
    ///               coordinates.
    static OrientedBoundingBox CreateFromPoints(
            const std::vector<Eigen::Vector3d>& points, bool robust = false);

I ran into similar problem with get_oriented_bounding_box(). Using get_axis_aligned_bounding_box() worked better for me.

I ran into similar problem with get_oriented_bounding_box(). Using get_axis_aligned_bounding_box() worked better for me.

As far as I know, that creates a bounding box relative to coordinate axes. So if your pointcloud is not aligned to any axis, the result won't be minimum either.

For people running into the same problem as me:

I did not fix the problem with the bounding box functions. Instead, I used references in the pointcloud, calculated the plane with some RANSAC algorithm, calculated the plane orientation, rotated it, calculated the bounding box manually.

@lschirmbrand would you mind sharing the code of your work-around? I encountered the same problem and ended up using a different library to calculate the oriented bounding box (compas). I used the function oriented_bounding_box_numpy() . The implementation is here: https://github.com/compas-dev/compas/blob/main/src/compas/geometry/bbox/bbox_numpy.py You can find an example comparison below. Open3d is white, compas is black.

Closing this issue because it is not a bug. The discussion can continue here or in https://github.com/isl-org/Open3D/discussions . If the minimum bounding box is an important feature create a new feature request issue and consider contributing an implementation.