[gdal-dev] Compositing of overlapping sources with mask/alpha band in VRT

[Even Rouault]

Hi,

Just wanted to mention a new capability of the VRT driver
(https://github.com/OSGeo/gdal/pull/3303) to take into account the mask band 
of a source to determine which pixels must be taken into account. This is a 
generalization of the <NODATA> mechanism to the mask band concept. This can 
work with internal/external mask bands, and with alpha band too, with the 
limitation for the later that it will interpret any non-0 value as fully 
opaque (no alpha blending)

I was hesitating a bit if that new capability should be implicit or explicit. 
I decided for explicit, to be consistent with the existing NODATA mechanism. 
That said, if you create the VRT with gdalbuildvrt, it will default to using 
the new feature and adding the required <UseMaskBand>true</UseMaskBand> 
element.

Even

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