[gdal-dev] Question on gdalwarp, srcnodata and dstalpha

[Daniele Romagnoli]

Hi List,
I was checking a gdalwarp (GDAL 2.1.2) invokation with a colleague and I
have a question.

I know that when warping a geotiff with internal NODATA_VALUES=0 0 0
metadata, only RGB pixels = 0 0 0 are handled as nodata whilst pixels like
"0 2 4", "50 0 0" will not right?

In the past, I had issues using -scrnodata 0 instead of embedded
NODATA_VALUES=0 0 0, getting results with sparse nodata around due to
band_i = 0 being interpreted as a nodata pixels. This is why I usually
suggest to define the NODATA_VALUES metadata to make sure only the exact
triplet is handled as nodata.

However, today we have processed a simple UTM RGB dataset (without
NODATA_VALUES metadata defined) using this command:
gdalwarp -t_srs EPSG:4326 -srcnodata 0 -dstnodata None -dstalpha

And the result is properly rendered: the alpha mask is zero only where the
input pixels are "0 0 0" which is exactly what we needed.
Do something has been changed in the latest versions? Is the alpha band
mask being set to zero only when all input pixels are zero?

Thanks for the clarification.
Cheers,
Daniele

-- 
==
GeoServer Professional Services from the experts! Visit
http://goo.gl/it488V for more information.
==

Ing. Daniele Romagnoli
Senior Software Engineer

GeoSolutions S.A.S.
Via di Montramito 3/A
55054  Massarosa (LU)
Italy
phone: +39 0584 962313
fax:      +39 0584 1660272

http://www.geo-solutions.it
http://twitter.com/geosolutions_it

-------------------------------------------------------

*AVVERTENZE AI SENSI DEL D.Lgs. 196/2003*

Le informazioni contenute in questo messaggio di posta elettronica e/o
nel/i file/s allegato/i sono da considerarsi strettamente riservate. Il
loro utilizzo è consentito esclusivamente al destinatario del messaggio,
per le finalità indicate nel messaggio stesso. Qualora riceviate questo
messaggio senza esserne il destinatario, Vi preghiamo cortesemente di
darcene notizia via e-mail e di procedere alla distruzione del messaggio
stesso, cancellandolo dal Vostro sistema. Conservare il messaggio stesso,
divulgarlo anche in parte, distribuirlo ad altri soggetti, copiarlo, od
utilizzarlo per finalità diverse, costituisce comportamento contrario ai
principi dettati dal D.Lgs. 196/2003.



The information in this message and/or attachments, is intended solely for
the attention and use of the named addressee(s) and may be confidential or
proprietary in nature or covered by the provisions of privacy act
(Legislative Decree June, 30 2003, no.196 - Italy's New Data Protection
Code).Any use not in accord with its purpose, any disclosure, reproduction,
copying, distribution, or either dissemination, either whole or partial, is
strictly forbidden except previous formal approval of the named
addressee(s). If you are not the intended recipient, please contact
immediately the sender by telephone, fax or e-mail and delete the
information in this message that has been received in error. The sender
does not give any warranty or accept liability as the content, accuracy or
completeness of sent messages and accepts no responsibility  for changes
made after they were sent or for other risks which arise as a result of
e-mail transmission, viruses, etc.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/gdal-dev/attachments/20161103/ec78246a/attachment.html>