[postgis-devel] overlaps left

[Mark Cave-Ayland]

Kevin Neufeld wrote:

> The docs said that &> returned true if the bounding boxes overlap OR is 
> to the left of ...
> 
> That doesn't seem to be the case:
> SELECT tbl1.column1, tbl2.column1,
>   tbl1.column2 && tbl2.column2 AS overlaps,
>   tbl1.column2 &< tbl2.column2 AS overlapsleft
> FROM
>   ( VALUES
>     (1, 'LINESTRING( 1 2, 4 6 )'::geometry)) AS tbl1,
>   ( VALUES
>     (2, 'LINESTRING( 0 0, 3 3 )'::geometry)) AS tbl2;
> 
>  column1 | column1 | overlaps | overlapsleft
> ---------+---------+----------+--------------
>        1 |       2 | t        | f
> (1 row)

Yeah... I found this when I was debugging some of the R-Tree routines 
before. The R-Tree semantics for "overlap" are defined differently than 
you or I would expect.

Here are a set of examples to try and help explain this: if you imagine 
A sliding from the right to the left across B in the X dimension then it 
should make sense.


A:              -------
B:     ------

A &< B == False


A:        ------
B:     -------

A &< B == False


A:     ------
B:     ------

A &< B == True


A:   ------
B:     ------

A &< B == True


If you think of overlap as being "directly on top of" then this may help 
too. But feel free to come up with a better definition of course.


HTH,

Mark.

-- 
Mark Cave-Ayland
Sirius Corporation - The Open Source Experts
http://www.siriusit.co.uk
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