 # Geometry :

Subject: Re: [geometry] what should be the minimum size of geometries supported in BG algorithms?
Date: 2015-01-13 11:46:56

Hi Menelaos

Menelaos Karavelas wrote:
> Hello all and happy new year.
>
Happy new year!

> I have been facing the following issue when implementing support for
> various algorithms in BG and for various geometries (especially with
> bg::distance): what should be the minimum sized geometry that should
> be supported by the algorithm?
>
> More precise questions:
>
> * should bg::distance be able to return a distance when one-point
> linestrings are passed to it?
> * should bg::distance be able to return a distance when one of the
> two input geometries is a closed polygon with less than four points?
>
> In both cases above the geometries are invalid (in the OGC sense), and
> this actually brings up a more general question. To what extend should
> we support invalid geometries in BG algorithms?
>
> In the current version of bg::distance if the uses passes an one-point
> linestring the algorithm sometimes returns something meaningful, and
> other times an assertion is triggered. Such a behavior is IMHO in some
> sense okay: BG algorithms are not guaranteed to work on invalid input
> (but they should work with valid input). So either returning something
> meaningful or triggering an assertion, or even returning something not
> meaningful is okay in the sense that the algorithm's behavior is
> undefined.
>

Only one remark, to be clear. By "work" do you mean "return valid
result" or "not blow up the whole program"?
I mean assertions should fail when a programmer's error was hit. In this
case the input data which could be loaded from some external source
represents an invalid geometry. At least it's my understanding. So in my
opinion in this case an algortihm could return some result (maybe in
some cases the correct one) or an exception could be thrown.

> Motivated by the above I decided to implement a new algorithm called
> is_below_minimum_size. It takes a geometry as input and returns true
> if the geometry's size is below the minimum acceptable valid size (see
> also the corresponding PR:
> https://github.com/boostorg/geometry/pull/193). In the PR there is a
> related new exception, and my intention was to use that exception
> instead of the empty_geometry_exception currently used in the
> bg::distance code. Using the new exception would avoid some assertion
> failures, and would treat geometries with very few points in a unified
> manner (through exceptions).
> On the other hand, it would limit the support for bg::distance on
> invalid geometries.
>
>

Some functions already handle such degenerated geometries somehow
(buffer?, centroid, area).
Other functions just ignore them (get_turns and therefore all relops and
setops).
But currently there is no function throwing an exception in this case.

Do you think that along with this change all other function should be
consistently changed to throw this exception?
Or should various functions handle such degenerated geometries differently?
Or something else?

AFAIU in BG a tradition is to throw an exception when there is nothing
else that could be done.
E.g. a centroid() of empty geometry can't be calculated in any way, but
for invalid geometry it can be, the result just may be invalid. In some
cases it'd be even correct or close.
area() is quite obvious case which can be calculated and even for an
empty geometry == 0.

What do you think about being in line with this approach and instead of
throwing an exception returning some (in some cases correct) result when
a geometry hasn't enough points?
E.g. just use the first point of a geometry (or rather sub-geometry),
e.g. returned by point_iterator<> or point_on_porder().
This'd give the correct result at least for linestrings and polygons
degenerated to a point.
For other cases like polygon containing too small number of points it'd
give more or less the correct result.

Another thing is that the function is_below_minimum_size() would just
return true if any of the sub-geometries was degenerated.
Maybe it could be convenient for the users if such geometry was ignored