[QGIS-Developer] Point cloud renderer: tile borders visible?

[Martin Dobias]

Hi Andreas

The artifacts you are seeing are caused by the fact how the Entwine tool
works in the default configuration. When creating the octree index of point
cloud data, points are organized in a hierarchy of nodes (each containing
usually tens of thousands of points), with each level of the hierarchy
having smaller bounding volume and higher density. One optimization that
Entwine does is that if child nodes contain a small amount of points, they
get merged into the parent node in the hierarchy - in the hope this will
lower the number of nodes and therefore also the number of requests that
need to be done. See Entwine configuration [1] for more details -
especially the options like minNodeSize, maxNodeSize, overflowDepth,
overflowThreshold. So the artifacts could be simply fixed by reindexing the
dataset with modified configuration.

All that said, maybe none of that is necessary:
1. I would suggest lowering "Maximum error" in Layer styling to e.g. 0.5 mm
or even 0.3 mm (assuming the default point size of 1mm) - this should be
giving you a rendering result with very little holes - it is much nicer to
look at and the artifacts should be gone at all scales  (it will increase
the rendering time, but I think it is worth it and I think we should lower
the default maximum error - the higher maximum error was initially used
mainly as a debugging tool for us to see if the point spacing in the
rendered map matches the expectations)
2. For indexing of point clouds, we will be internally using Untwine [2] -
a newer approach (also coming from friends at Hobu!) that has different
configuration and probably won't be showing these artifacts. Peter is
currently working on the integration [3]

Regards
Martin

[1] https://entwine.io/configuration.html
[2] https://github.com/hobu/untwine
[3] https://github.com/qgis/QGIS/pull/40404


On Tue, Dec 8, 2020 at 10:30 AM Andreas Neumann <a.neumann at carto.net> wrote:

> Hi,
>
> When testing the 2D point cloud renderer I noticed that in certain scales
> / zoom levels one can clearly see point cloud tile borders:
>
> see screenshot at
> https://www.carto.net/neumann/temp/point_cloud_artefacts.png
>
> There seem to be different densities involved in certain regions here, but
> clearly they correspond to tile borders.
>
> Is this an expected artefact? Can this be avoided?
>
> When I zoom in or out, these artefacts seem to disappear.
>
> Thanks and greetings,
>
> Andreas
>
>
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