[gdal-dev] VRT of VRTs with overviews. They do not seem to be used.

[Even Rouault]

Christoph,

gdaladdo always start from the full resolution layer (or the preceding 
overview in case of cascading overview), hence what you observe. And even if 
it did start from the nearest existing overview level, in your full scenario 
of a VRT of VRT, with each 'bottom-level' VRT having overviews, that wouldn't 
ause the overall VRT to be able to expose overview bands.

That said, there's a trick you can use

If you do:

gdal_translate second_level.vrt tmp.tif  -outsize 3.125% 3.125% -of GTiff
mv tmp.tif second_level.vrt.ovr

you'll get a 1/32 overview, and this will be able to use the overviews of the 
bottom-level VRTs

and you can then run gdaladdo on second_level.vrt.ovr

Regarding the fact the gdalinfo first_level.vrt only reports overviews of size 
500x500, 250x250, this is because of an optimization of the VRT driver that do
not report overviews smaller than 128x128.

Even

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