[gdal-dev] building a vrt to stack overlapping COGs with a mask/alpha band

[Landry Breuil]

Hi,

i'm converting a stock of tiled aerial pictures (from distinct 
years/campaigns) to COG (using gdal 3.1.0), and trying to stack them via 
a vrt. the COG arent 'full', in the sense that they have a bbox, but 
there isnt data *everywhere*. Some of those campaigns/COGs have 
overlapping BBOXes.

the COG are built from vrts (assembling all the tiles for the campaign) 
built with -addalpha, and then using gdal_translate -of COG -b 1 -b 2 -b 
3 -mask 4 to have the mask band in the resulting COG, ie i get 3 bands 
with a 'per-dataset mask':

Band 1 Block=512x512 Type=Byte, ColorInterp=Red
   Overviews: 148000x172000, 74000x86000, 37000x43000, 18500x21500, 
9250x10750, 4625x5375, 2313x2688, 1157x1344, 579x672, 290x336
   Mask Flags: PER_DATASET
   Overviews of mask band: 148000x172000, 74000x86000, 37000x43000, 
18500x21500, 9250x10750, 4625x5375, 2313x2688, 1157x1344, 579x672, 290x336

i have zero issue loading the separate COGs in QGIS (locally or via 
vsicurl), the nodata areas are properly displayed as transparent, 
loading several COGs on top of each other properly render the 
overlapping areas, ie if at some coords there's data in the lower COG 
and nodata in the toplevel COG, then the data from the lower COG is 
rendered. That can also be achieved via a mapserver GROUP, loading the 
layers on top of each other, as seen in https://imgur.com/9BgZSTH.png

Trying to achieve the same via a vrt file stacking the COGs fails (plain 
gdalbuildvrt img1.tiff img2.tiff), ie for the same coords, nodata will 
be rendered if there's nodata in the toplevel COG (even if there's data 
in the lower COG), and that for the complete BBOX of the toplevel COG 
(ie outside of the toplevel COG BBOX, data from the lower COGs is 
displayed). See https://imgur.com/a/HiEfjxY for an example (here, it's 
via WMS loading as jpeg, using png the white area would be transparent).

I tried playing with the various gdalbuildvrt (-addalpha, etc..) options 
but i still cant figure out what is the correct incantation to achieve 
my goal (ie tell gdalbuildvrt that there's some kind of 'cutline' around 
the given COGs ?), so i'm resorting to find hints on the list :)

TIA,

Landry