[pgpointcloud] low compression with pure xyz data

[Rémi Cura]

Hey,
I'm a bit suprised by the poor perf of the dimensionnal compression on my
simple XYZ data.

XYZ are declared as int32_t, data is centered on (0,0,0),
ranging from -0.20 to 0.20 , precision 0.00001, 2 millions points.
Patches contains avg 1000 points, about 1.5k rows

It achieves a 2x compression ratio over binary ply, and roughly the same
compression ratio as zipping the binary ply.

More suprisingly, uncompressed patches occupy only 14% more (setting the
storage to external).

Any idea why  ?

(by the way, for the pcpatch type, the default value should be 'external'
and not 'main' I guess).

running 1.0.1

Cheers,
Rémi C
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