[gdal-dev] VRT and COG

[Even Rouault]

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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