Bounding box corner question

Hi Mike,

I came across your Trimesh library for Python after trialling several other tools for generating signed distance functions from STL files. I’ve found yours to be by far the best for what I need, but I have a simple question that I couldn’t quite figure out from looking at the source:

Essentially what I want to do is to fill the bounding box with a 3D rectangular grid of points, and compute the SDF from these. I am able to generate points and calculate the SDF with no problems, but I don’t know how to get the coordinates of the corners of the bounding box. If I use:

box = mesh.bounding_box_oriented.primitive.extents

I get the half-lengths of each side of the bounding box, but if I use these to plot the 8 corners they form a box the same size and shape as the bounding box, but seemingly randomly oriented in space. How can I correctly get the coordinates of the corners so I can construct my meshgrid within the bounding box? I see that in bounds.py there is a function corners that seems to do what I want, but I don’t know where to get the bounds argument from.

Thank you for your time, Joel

[mikedh]

Hey, I think you may want to use mesh.bounding_box which is the axis aligned bounding box rather than mesh.bounding_box_oriented which is the oriented bounding box. Although in order to get the 8 corners of the OBB you could do: trimesh.bounds.corners(m.bounding_box_oriented.bounds)

The mesh.bounds property is computed and always axis- aligned, where the .primitive.extents is pre-transform (by .primitive.transform)

One thing you might consider, is that the oriented bounding box you’re using will probably have a lot less volume. You could generate the grid (axis aligned) the size of the OBB extents, and then transform the grid of query points using the OBB’s transform.

Some ipython noodling:

In [14]: trimesh.util.attach_to_log()

In [15]: m.bounding_box_oriented.primitive.transform
Out[15]:
TrackedArray([[   0.75983,   -0.49294,    0.42387,  831.14091],
       [   0.61234,    0.76166,   -0.21193,  437.48365],
       [  -0.21838,    0.42058,    0.88058,  636.10072],
       [   0.     ,    0.     ,    0.     ,    1.     ]])

In [16]: m.bounding_box.primitive.transform
Out[16]:
TrackedArray([[   1.     ,    0.     ,    0.     ,  831.14091],
       [   0.     ,    1.     ,    0.     ,  437.48365],
       [   0.     ,    0.     ,    1.     ,  636.10072],
       [   0.     ,    0.     ,    0.     ,    1.     ]])

In [17]: trimesh.bounds.corners?
Signature: trimesh.bounds.corners(bounds)
Docstring:
Given a pair of axis aligned bounds, return all
8 corners of the bounding box.

Parameters
----------
bounds: (2,3) or (2,2) float, axis aligned bounds

Returns
----------
corners: (8,3) float, corner vertices of the cube
File:      x:\trimesh\trimesh\bounds.py
Type:      function

In [18]: trimesh.bounds.corners(m.bounding_box.bounds)
[2017-05-12 18:21:23] DEBUG   (primitives.py:329) Creating mesh for box Primitive
[2017-05-12 18:21:23] DEBUG   (base.py:1212) Mesh transformed by matrix, normals restored
to cache
[2017-05-12 18:21:23] DEBUG   (util.py:686) triangles was not in cache, executed in 0.025009
[2017-05-12 18:21:23] DEBUG   (util.py:686) bounds was not in cache, executed in 0.025009
Out[18]:
array([[ 830.14391,  436.48424,  635.10141],
       [ 832.13792,  436.48424,  635.10141],
       [ 832.13792,  438.48306,  635.10141],
       [ 830.14391,  438.48306,  635.10141],
       [ 830.14391,  436.48424,  637.10002],
       [ 832.13792,  436.48424,  637.10002],
       [ 832.13792,  438.48306,  637.10002],
       [ 830.14391,  438.48306,  637.10002]])

[Emnaa12]

m.bounding_box_oriented.vertices works correctly, thank you !