[postgis-devel] ST_GeneratePoints randomness

[Martin Davis]

Perhaps there is a use case for both functions?  One that generates a
reproducible set of pseudo-random points, and one that generates a
different set each time called.

If so, it sounds like the "really random" function would have to be a
separate VOLATILE declaration.

On a related note, generating random points using a simple random function
produces quite "clumpy" point sets. There's lots of approaches to creating
more evenly spaced quasi-random pointsets, such as Halton points [1] or
using a distance tolerance [2].  It might be nice to provide one or more of
these as functions as well.

[1]
https://lin-ear-th-inking.blogspot.com/2012/09/halton-sequences-at-last.html
[2]
https://lin-ear-th-inking.blogspot.com/2015/07/even-distribution-random-point-and.html

On Fri, Jan 18, 2019 at 5:51 AM Darafei "Komяpa" Praliaskouski <
me at komzpa.net> wrote:

> Hi Felix,
>
> would you prefer the function to always return the same set of random
> points, or get the fact that it can return different set of points on
> invocations in different transactions documented?
>
> I'm not sure treating it same as random() is a great idea, I use it more
> like "give me a point cloud that fills this polygon" converting it for
> usage in point-accepting functions, like calculating the median of the
> polygon. I'd prefer it cached like it is now for such cases.
>
> What are you using your random geodata for?
>
> On Wed, Jan 16, 2019 at 1:11 AM Felix Kunde <felix-kunde at gmx.de> wrote:
>
>> Hi list,
>>
>> I thought that this topic better belongs on this list.
>> I'm trying to create some random spatial data using the ST_GeneratePoints
>> functions. I wanted to reuse it also for lines and polygons but stumbled
>> over the issue that multiple calls of the function in one transaction
>> produce the same result. I tested different query constructs - even looping
>> over a PL/pgSQL function - but no luck. So whereas:
>>
>> SELECT
>>   random() AS random_a,
>>   random() AS random_b;
>>
>> creates different results, this:
>>
>> SELECT
>>   ST_AsText(ST_GeneratePoints('POLYGON((0 0,1 0,1 1,0 1,0 0))'::geometry,
>> 2)) AS random_a,
>>   ST_AsText(ST_GeneratePoints('POLYGON((0 0,1 0,1 1,0 1,0 0))'::geometry,
>> 2)) AS random_b;
>>
>> will produce the same. There are workaround to generate random geodata,
>> of course.
>> Just wanted to point it out here and see, if you are aware of this. I
>> couldn't found any ticket regarding this behaviour.
>>
>> Best, Felix
>>
>>
>
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