[QGIS-Developer] MinimumRectangle implementation

[Nyall Dawson]

On 6 June 2017 at 18:43, Paul Jobling <paul.jobling at gmail.com> wrote:

> I am implementing geo in the Kognitio WX2 database using GEOS as a backend,
> though I am using my own code for many things. One of those things is
> ST_MinimumRectangle, which I have implemented as the rectangle with the
> smallest area that encloses the geometry. This is different to the
> implementation that I see you have recently added, which extrapolates from
> the MinimumDiameter property.
>
>
>
> For example, ST_MinimumRectangle ('POLYGON((1 2, 3 0, 5 2, 3 2, 2 3, 1 2))')
> has an area of 9.6 (and a perimeter of 12.65) using the MinimumDiameter
> implementation, whereas my implementation finds the actual minimum – with an
> area of 8 (and perimeter of 11.31) - it is POLYGON ((5 2, 3 0, 1 2, 3 4, 5
> 2)) rather than POLYGON((1.4 3.2, 0.6 0.8, 4.2 -0.4,5 2, 1.4 3.2)).
>
>
>
> My question is why has this implementation been decided on – is it just that
> MimimumDiameter is already present? Or is there some other reason due to
> some standard?

Did you mean to post this to the GEOS mailing lists instead?

Anyway, some background from QGIS' perspective. A while ago I was
looking for a minimum rect implementation, so ported JTS'
implementation over to GEOS. I then exposed this to the QgsGeometry
API, but to my suprise found that the results were not ideal. At the
time there was some python code in processing which performed a MBR
calculation, and I found that the area of rectangles calculated by
this python code was smaller then JTS' algorithm. Additionally, I
thought it created subjectively 'more aesthetically pleasing'
rectangles (when used as atlas feature extents, and also output the
angle/width/height of these rectangles (the GEOS implementation
doesn't expose this). So I ported the Python code over to c++ as
QgsGeometry::orientedMinimumBoundingBox and updated the processing alg
to use this c++ method instead. The guts of this algorithm sits in
https://github.com/qgis/QGIS/blob/master/src/core/geometry/qgsgeometry.cpp#L890
.

Does that clarify?

Nyall