[geos-devel] Geometry/Rectangle Intersection: line touching rectangle

[Mika Heiskanen]

On 09/09/2014 05:56 PM, Sandro Santilli wrote:
> Another case where output differs from ST_Intersection:
>
>    Rectangle: 0,0 10,10
>     Geometry: LINESTRING(10 0, 20 0)
>     Expected: POINT(10, 0)
>     Obtained: EMPTY
>
>    +----------+
>    |          |
>    |          |
>    |          |
>    +----------O------------+
>
> That is, the intersection of a rectangle and a linestring whose endpoint
> is on one of its corners is expected to yeld the corner point.

Well, this is unfortunate. As implied earlier, I did not check any 
standards on how such things are resolved, I merely programmed it
as quickly as I could from the perspective of rendering. Our
algorithm produces only linestrings and polygons for respective
input. Points are generated only for pointlike input.

If this is how clipping is to be defined, it is going to have
a serious impact on the algorithm. There aren't that many special
cases, but handling them within the already complex if-else
sequences could be a nasty task.

The special cases are as far as the current code would be concerned are:

* a vertex is on a rectangle edge
* a vertex is on a rectangle corner
* an edge passes through a corner of the rectangle

The RectangleClipBuilder API would have to be enlarged to
handle individual points. The implementation should probably
merge all such points into a multipoint.

I'd like to see a reference where all such special cases
are defined explicitly. (No, I never looked for one, as explained
before.) In your previous example a linestring was split in half
because a vertex is on the rectangle edge. This is kind of consistent
with the interpretation that the vertex is considered to be out.
However, the example presented here is consistent with the
interpretation that the vertex should be considered to be in.

Regards,

Mika Heiskanen / FMI