[gdal-dev] FW: Don't we have any ideas for GSoC 2017?

[Casper Børgesen (CABO)]

Yes this is about spatial indexing :)

Without looking at the actual VRT code I would guess that parsing the VRT file is one thing, but having a VRT file with +100,000 tiles would require GDAL to look through each tile every time a pixel is requested?
Even, if your idea below could be implemented into the VRT driver to be able to build the index file and gdaladdo to build the overview index files, it would be a huge step forward.

Is it possible to build the index on the fly into memory when opening the VRT file? Is it too memory or time consuming?

/Casper

From: gdal-dev [mailto:gdal-dev-bounces at lists.osgeo.org] On Behalf Of Even Rouault
Sent: 1. marts 2017 13:25
To: gdal-dev at lists.osgeo.org
Cc: Rahkonen Jukka (MML)
Subject: Re: [gdal-dev] FW: Don't we have any ideas for GSoC 2017?


On mercredi 1 mars 2017 11:43:12 CET Rahkonen Jukka (MML) wrote:

> Forwarding to the list because Jordan's trial was not successful.



Sounds a good idea. I think potential students should be more pro-active in proposing and developing their own ideas.



Regarding "indexing on VRT files", I guess this is about spatial indexing of sources in a VRT file. My bet would be that the size of VRT files that would need such indexing would be so large than their parsing would be the most limiting factor. But I can be wrong.

Some people who want to make a mosaic from a huge number of tiles do that by making VRTs of VRTs of VRTs with a pyramid structure to avoid parsing a lot of XML and still having good performance.

Another option that has been mentionned in the past would be to have a variation of the VRT syntax that would reference a tile index (shapefile or any OGR datasource) and would thus benefit from its spatial indexing & filtering capabilities



Something like:



<VRTDataset rasterXSize="20000" rasterYSize="20000">

<SRS>EPSG:26711</SRS>

<GeoTransform> 4.4072000000000000e+05, 6.0000000000000000e+01, 0.0000000000000000e+00, 3.7513200000000000e+06, 0.0000000000000000e+00, -6.0000000000000000e+01</GeoTransform>

<TileIndex>

<Filename relativeToVRT="1">index.shp</Filename>

<!-- <Layername>index</Layername> -->

</Tileindex>

<Overview rasterXSize="10000" rasterYSize="10000">

<TileIndex>

<Filename relativeToVRT="1">index_ovr_2.shp</Filename>

</Tileindex>

</Overview>

<Overview rasterXSize="5000" rasterYSize="5000">

<TileIndex>

<Filename relativeToVRT="1">index_ovr_4.shp</Filename>

</Tileindex>

</Overview>

<VRTRasterBand dataType="Byte" band="1">

<ColorInterp>Red</ColorInterp>

</VRTRasterBand>

<VRTRasterBand dataType="Byte" band="2">

<ColorInterp>Green</ColorInterp>

</VRTRasterBand>

<VRTRasterBand dataType="Byte" band="3">

<ColorInterp>Blue</ColorInterp>

</VRTRasterBand>

</VRTDataset>



Even
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