[gdal-dev] VRT and COG

[Vincent Sarago]

Thanks Even ;-) 

> Le 26 oct. 2018 à 10:11, Even Rouault <even.rouault at spatialys.com> a écrit :
> 
> On vendredi 26 octobre 2018 09:09:34 CEST Vincent Sarago wrote:
>> Goal: I’d like to see if there could be a way to use VRT of COG to create a
>> mbtile like system to read tiles.
>> 
>> If I remember well when a VRT points to COG you still need to create
>> external overviews for the whole VRT because it won’t fetch de overviews
>> from the COG itself when trying decimated read.
> 
> No, that's not exactly true. If a VRT contains more than one source, GDAL 
> won't be able to expose a proper overview externaly, but it will use the 
> overviews of sources, when present, for decimated read (*). However if your 
> VRT has a lot of sources (let's 1024x1024 tiles of 256x256 pixel each), if you 
> ask for just a 1024x1024 output buffer of the whole extent, there is no magic: 
> GDAL will have to open each source dataset to compute its contributed single 
> pixel. In that case, you may want to pregenerate an overview of the whole VRT.
> 
> Even
> 
> (*) and actually even if those don't expose overviews proper, it will let the 
> underlying driver handle that at best: so a VRT of VRT of COG would work
> 
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