[pgpointcloud] RLE and SIGBITS heuristics

[Rémi Cura]

Hey Oscar (and Peter),
great series of articles, the benchmark was really interesting !

I personally am adamantly against reordering the points for storage
purpose, because the order can have a strong meaning (physical or
theoretical).

For instance I re-ordered the points of my patch so that the points follows
a LOD schema.
So reading more and more points gives a better and better approximation of
the patch.
This gives free LOD facilities when all the patch have been re-ordered
(free CPU wise, and no data duplication).

Now I have to admit that efficient storage and LOD are in conflict, because
storing is about putting the similar points together somehow, and LOD is
about the opposite (kind of sequential vs random access).
This has a strong impact, for instance reordering patches randomly will
greatly increase patch storage size in pgpointcloud.
It is funny because for LOD I considered using an inverse morton curve
(interleave X Y Z , then read is backward ! ).

I'm well aware that we could simply store the original order in an
attribute, and so compression based on morton on XYZ might be transparent
for the user.

Anyway, just to say that pgpointcloud is ultimately not about storing
points, but easing point cloud usage in my opinion.

Cheers,
Rémi-C




2015-04-17 11:02 GMT+02:00 Oscar Martinez Rubi <o.martinezrubi at tudelft.nl>:

>  Hi,
>
> About the XYZ binding for better compression. In our research in the NL
> escience center and TU Delft we have been thinking (not testing yet though)
> about one possible approach for this.
>
> It is based on using space filling curves. So, once you have the points
> that go in a block you could compute the morton/hilbert code of the XYZ.
> Since all the points are close together such codes will be extremely
> similar, so one could store only the increments which could fit in many few
> bits. We have not tested or compared this with any of the other
> compressions but we just wanted to share it with you just in case you find
> it useful!
>
> An additional improvement would be to sort the points within the blocks
> according to the morton code. Then, when doing crop/filter operations in
> the blocks one can use the morton codes for the queries similarly to what
> we presented in our papers with the flat table (without blocks), I attach
> one of them (see section 5.2). In a nutshell: You convert the query region
> into a set of quadtree/octree nodes which can be also converted to morton
> code ranges (thanks to relation between morton/hilbert curve and a
> quadtree/octree). You scale down the ranges to increments (like you did
> when storing the point of the block) and then you simply do range queries
> in sorted data with a binary algorithm. In this way you avoid the
> decompression of the morton code for most of the block. This filtering is
> equivalent to a bbox filter so it still requires a point in polygon check
> for some of the points.
>
> Kind Regards,
>
> Oscar.
>
>
>
> On 16-04-15 18:15, Rémi Cura wrote:
>
>  epic fail ! I had avoided html just for you
>
>    Dataset   |subset size  | compressing   | decompressing |
>              |(Million pts)|(Million pts/s)|(Million pts/s)|
> Lidar        |   473.3     |    4,49       |     4,67      |
>  21-atributes |   105.7     |    1,11       |     2,62      |
>  Stereo       |    70       |    2,44       |     7,38      |
>
>  Cheers
>
> 2015-04-16 17:42 GMT+02:00 Sandro Santilli <strk at keybit.net>:
>
>> On Thu, Apr 16, 2015 at 05:30:12PM +0200, Rémi Cura wrote:
>> > OUps
>> >
>> > Dataset        |  subset size(Million pts) | compressing (Million
>> pts/s) |
>> > decompressing (Million pts/s)
>> > Lidar           |            473.3                |               4,49
>> >               |             __4,67__
>> > 21 attributes |           105.7                 |
>> > 1,11                     |             2,62
>> > Stereo         |              70                  |                2,44
>> >                |             7,38
>>
>> These tables aren't really readable here.
>> Could you make sure to use a fixed-width font to write those tables
>> and to keep lines within 70 columns at most ?
>>
>> --strk;
>>
>
>
>
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>
>
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