On 08/07/2020 13:23, Even Rouault wrote:
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.
So that explains why it works with the mapserver GROUP keyword, as it
does the blending of the sub-layers.
Long time wish / todo.
Ok, good to know.
Your best option with VRT would be to use a NoData value instead if you can.
You mean setting a nodata value when building the COGs (ie
gdal_translate -a_nodata 0 -of COG dataset.vrt dataset.tif) instead of
the alpha/mask band, and then using this nodata value as -srcnodata 0
parameter to gdalbuildvrt ?
Or use gdalwarp to create a materialized mosaic.
That would be sub optimal (ie double storage) as i also need to be able
to use the distinct campaigns independently.
Landry
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