Please remove this address for David McKenzie from your mailing list.
He passed away December 2016.
Thank you,
M.H. McKenzie
May 2019
From: Lucian Plesea
Sent: Friday, May 17, 2019 4:14 PM
To: gdal-dev@lists.osgeo.org ; Even Rouault
Subject: [gdal-dev] COG w/JPEG YCbCr mask
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.cpp
It inserts an RLE encoded bit mask in the JPEG itself as an application
specific chunk (Zen) instead of having it as a separate entity. I uses the
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 the mask and the normal bands read and written together, which is not
trivial as far 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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