Oriented Minimum Bounding Box strange behavior

[caprieldeluca]

Describe the bug

By running the "Oriented Minimum Bounding Box" algorithm from the Processing Toolbox, on a polygons vector layer, in some (seemingly random) cases, the result is not as expected (it is visibly appreciated that an oriented bounding box of smaller area could be generated).

By creating a very small decimal radius buffer to the geometries of the problematic features, running the Oriented Minimum Bounding Box algorithm over the resulting geometry, gives the expected result.

How to Reproduce

1. Create a feature with the following geometry:
   Polygon ((264 -525, 248 -521, 244 -519, 233 -508, 231 -504, 210 -445, 196 -396, 180 -332, 178 -322, 176 -310, 174 -296, 174 -261, 176 -257, 178 -255, 183 -251, 193 -245, 197 -243, 413 -176, 439 -168, 447 -166, 465 -164, 548 -164, 552 -166, 561 -175, 567 -187, 602 -304, 618 -379, 618 -400, 616 -406, 612 -414, 606 -420, 587 -430, 575 -436, 547 -446, 451 -474, 437 -478, 321 -511, 283 -521, 275 -523, 266 -525, 264 -525))
   (Tested in a layer defined in CRS = EPSG:3857, with no projection applied to the project canvas. Although I think it doesn't matter.)

2. Run the Oriented Minimum Bounding Box algorithm to that feature.
   The output box attributes are: width = 361.00, height = 444.00, angle = 90.00.

3. Create a Buffer of Radius = 0.001 from the original feature.

4. Run the Oriented Minimum Bounding Box algorithm to the buffered geometry feature.
   The output box attributes are: width = 417.63, height = 298.02, angle = 162.68.

The area of the box returned by the buffered geometry feature is smaller than that returned by the original one.

QGIS and OS versions

QGIS version 3.10.1-A Coruña
Windows 10, 64bit (OSGeoo4W installation).

[lbartoletti]

Duplicate of #31371 ?

[caprieldeluca]

Duplicate of #31371 ?

@lbartoletti , I think not.

The problem descripted here is not related to the angle calculation. The angle is well calculated for the geometry returned.

The problem is with the geometry generated.

It was not the minimum (area) oriented bounding box.

[lbartoletti]

OK, confirmed.