Geometry :

Subject: [ggl] what is an algorithm
From: Barend Gehrels (Barend.Gehrels)
Date: 2009-04-25 17:28:35

Mateusz Loskot wrote:
> Barend Gehrels wrote:
>
>> Now we've update the structure, I was thinking about the algorithm
>> folder. To crowded or fine?
>>
>
> A little :-) and there is potential it will get crowded in future.
>
>
>> It is sometimes hard to decide what is what.
>>
>
> For some of them it is not enough to decide their place based on
> algorithm defined as a procedure provided to operate on GGL
> data and types.
>
> Second issue is to help users to recognize algorithms
> easier by proper structure of folders.
>
>
>> Is "loop" an algorithm? Now in util.
>>
>
> It's unclear to me how it's different to for_each.
>
Sure, I was not clear enough. I didn't explain that for me also the
util/loop was strange. It used to be a sort of helper-algorithm and
therefore it was there. But it is about the same as for_each. Only
difference is that it can break out.
The loop was one of the first developed, being used in algorithms as
within, area etc. However, it makes them also more complex and I know
prefer using iterators.

>
>> OGC distinguishes algorithms:
>> - "query" contains within, touches, overlaps, disjoint, etc. This makes
>> sense to me
>>
>
> Aren't they called spatial predicates or just predicates?
> They return binary answer, so perhaps "predicate" is a good name for
> directory. Predicate is used in C++ std library terminology.
>
Good idea.

>
>> - "analysis" contains distance, buffer, ConvexHull, intersection, union.
>>
>
> Right.
>
>
>> This is also useful, there are two kinds:
>> - pointset spatial relationships (and, or, xor, not -->
>> intersection, union, sym_difference)
>> - other things as distance and buffer
>> - other, uncategorized such as envelope, dimension
>>
>
> All of them here and above are geometry algorithms - this could be
> an indicator for selecting how to structure them
>
>
>> GGL:
>> I've put some in core (e.g. topological_dimension, because it is
>> completely compiletime), some in util (loop) and most in algorithms.
>>
>
> for_each is also a general purpose algorithm, similar to loop.
>
Sure.
>
>> Besides OGC algorithms we have and get more:
>> - simplify
>> - spline
>> - diameter (defined by the most furthest point-pair of a polygon,
>> linestring, etc)
>> - closest_pair (idem but most closest point-pair)
>> - combine a bounding box growing if points are added)
>> - etc.
>>
>> The easiest is to have them all in one folder algorithms, users then
>> don't have to think about what kind it is. Do you agree with that? The
>> folder will have dozens of files, but that is how it is, the boost/mpl
>> folder contains 182 files.
>>
>
> It would be good to have it scattered in some categories, but on the
> other hand if categories are not obvious (or natural), then possible
> structure may be ambiguous to users.
> Looks like this is an issue in GGL, so I like the idea of single
> collection of files as you are suggesting.
> Or, may be a few obvious categories could be identified like
> OGC algorithms. This way GIS-oriented users could easily identify
> where to look for what they need.
>
I've thought about that also but the Boost folks are not that interested
in OGC. They just want geometry and things handling geometry. Creating
an OGC distinction is for GIS users maybe natural and handy, but other
users (who still want e.g. "distance") will look very strange at that.
Besides that many non-OGC things (as simplify) are great for GIS users
as well.

So I'm currently still favouring creating one folder. Bruno?
>
>> I'm using the GGL more and more in projects and have several pieces in
>> different states which will go there, coming time. As mentioned earlier:
>> touches, relate, disjoint, buffer, union but also diameter (see above),
>> remove_indentations, remove_small_holes (this last one should be
>> templatized by a selection struct), ...
>>
>
> Great.
> I'm still familiarizing with GGL, but next I'd like to get into WKB IO.
> What you think?
>
Would be great!

Regards, Barend

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