On mercredi 8 juillet 2020 10:12:31 CEST Landry Breuil wrote: > 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 :)
Landry, The behaviour you describe is indeed how VRT stacking works currently. Alpha / masks bands are dealt as regular bands. No blending is done. Long time wish / todo. Your best option with VRT would be to use a NoData value instead if you can. Or use gdalwarp to create a materialized mosaic. Even -- Spatialys - Geospatial professional services http://www.spatialys.com
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