Hi Even, I just read your COG notice from May 3rd. Are you aware of the approach I've used to achieve the same outcome in MRF JPEG? Most of the code is in https://github.com/OSGeo/gdal/blob/master/gdal/frmts/mrf/JPEG_band.cppIt inserts an RLE encoded bit mask in theJPEG itself as an application specific chunk (Zen) instead of having it as aseparate entity. I usesthe mask to only keep the locations with zero fixed (thus the Zen name),instead of exposing the mask to GDAL. Then the zero value can be declared as NoData, eliminating the noisy JPEG edge issue.This service uses it: http://www.arcgis.com/home/webmap/viewer.html?useExisting=1&layers=c1c4c750a2154842ae523c984cc14fa5
The Zen chunk could be exposed as a mask, but then you have the headache of getting themask and the normal bands read and written together, which is not trivial asfar as I can tell. It could also be exposed as a full alpha band. As far as I can tell, the NoData approach has significantly less overhead, no extra band blocks to handle. The Zen chunk in JPEG is ignored by older decoders, so it is backwards compatible. Lucian
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