Hi Volker,

Volker Schöch wrote On 13-10-2014 18:28:

The more I think about it, the more confused I get. What are the exact pre-conditions and post-conditions of your algorithms?


On http://www.boost.org/doc/libs/1_56_0/libs/geometry/doc/html/geometry/reference/algorithms/difference.html it says: “Check the Polygon Concept for the rules that polygon input for this algorithm should fulfill”.


On http://www.boost.org/doc/libs/1_56_0/libs/geometry/doc/html/geometry/reference/concepts/concept_polygon.html it says “There should be no cut lines, spikes or punctures”. It also says “If the input is invalid, the output might be invalid too”. Fair enough.

I can imagine it is not that simple, and we probably should describe it better.


What it doesn’t say is that a polygon without area is invalid. Let’s take that at face value, though: It is trivial to imagine, e.g., the difference of two non-empty, valid polygons that results in empty polygon. Does this mean that, e.g., the difference algorithm can take valid input and return invalid output?

No: the difference algorithms outputs a multi-polygon, and not a polygon. The multi-polygon does not contain any polygons, and is a valid geometry. So the output is valid.

Having said that, an empty polygon is also considered as valid by several database packages. Our implementation (is_valid is just released) reports false to empty polygons, we probably should fix that.


If I a have a computation that consists of multiple steps, I would like to make sure that the input is valid, and then run my computation as a sequence of calls to geometry algorithms. Given the above – do I have to verify every intermediate result, and make sure that if I find an invalid polygon, the algorithm returns half way? What is your recommended best practice to deal with this? I am sure that calling correct, unique, remove_spikes (in which order?) and is_valid all over the place cannot be the answer…

No, if the input is valid, the output should be valid too. If it is not valid, these cases may be reported.

Thanks for your input.

Regards, Barend