[pdal] Colorization using a large VRT raster is slow, but using a small VRT raster is fast ... but why?

[Michael Smith]

Michael,

VRTs are not spatial indexed. I wonder if you did a tileindex and created a
vrt on that (as pdal wouldn't open a tileindex directly) if it would work a
lot faster since a tileindex is spatially indexed.

Mike

-- 
Michael Smith
Remote Sensing/GIS Center
US Army Corps of Engineers

From:  pdal <pdal-bounces at lists.osgeo.org> on behalf of Michael Rosen
<michael.rosen at gmail.com>
Date:  Wednesday, October 11, 2017 at  12:48 PM
To:  pdal <pdal at lists.osgeo.org>
Subject:  [pdal] Colorization using a large VRT raster is slow, but using a
small VRT raster is fast ... but why?

> 
> 
> 
> I am using the filters.colorization with a .vrt built by gdalbuiltvrt.  The
> raster dataset actually contains 2200 files which I believe are all correctly
> georeferenced and non-overlapping.  Of those, there are only 19 that actually
> intersect the point cloud being colorized.
> 
> What I'm observing is that if I build the vrt with only the 19 files, the
> colorization runs in less than two minutes.  However, if build the vrt with
> all of them, then it takes an unacceptably long time (I've not watched it
> finish but I'm keeping an eye on the open file handles ... it's finding right
> files but it's just moving through them really slowly).
> 
> I would expect that the VRT would be able to immediately provide the pixels
> from the right raster tile, making the number of tiles in the mosaic
> irrelevant.  That's clearly not the case (does it not do some sort of indexing
> here?).  Can anyone offer an explanation / fix?  This is important because I
> actually have many LAS tiles to colorize and while all of them are contained
> in the bounds of the large mosaic, they each have a different extent.
> 
> I guess a work around would be to build small VRTs based on the geographic
> extent of each LAS tile.  But how?
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