[pdal] EPT polygon query and octree in memory

[Andrew Bell]

There are many changes that are coming shortly in the EPT reader for
version 2.1 that should reduce the number of points retained.  In the more
recent code, we're trying to store the minimum number of points necessary
to meet the query.  That said, without knowing exactly what you're doing
and what you were doing before, it's hard to comment.  I think I would wait
and see if things are improved in version 2.1 before worrying too much
about taking other steps.

On Mon, Mar 16, 2020 at 8:27 AM Balázs Dukai <B.Dukai at tudelft.nl> wrote:

> Hello,
>
> I'm using PDAL's C++ API (v2.0.1) to query an EPT and get the point
> cloud within a polygon. I do this for millions of polygons that are
> often adjacent (building footprints), and the EPT covers a whole
> country. The EPT is stored as LAZ.
>
> I noticed that there was a relatively large increase in memory use
> (cannot tell exactly, sorry ) after switching to PDAL+EPT. This made me
> wonder what exactly is read into memory when doing a polygon query from
> EPT? I made a drawing of what I think is happening , could someone
> confirm or reject this? See
> https://www.dropbox.com/s/ppa24o666ulunxc/ept_octree.jpg?dl=0.
>
> In the image you'll see an octree of an EPT, with the root cell on the
> top and the leaf that contains the polygon on the bottom. The red
> splotch is the polygon. The darker blue cubes are the octree cells of
> the EPT that I think are kept completely in memory in order to get all
> the points within that red polygon. Is that correct?
>
> Then if this is the case, how could I reduce the memory use in PDAL? And
> speed up extracting the polygon-extents from the EPT?
>
> Many thanks,
>
> Balázs Dukai
>
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-- 
Andrew Bell
andrew.bell.ia at gmail.com
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