[gdal-dev] What is the purpose of -nosrcalpha in gdalwarp?

Sun Nov 8 08:33:20 PST 2020

Hi,

I noticed this question from gis.stackexchange https://gis.stackexchange.com/questions/378557/gdalwarp-does-not-use-lanczos-resampling-on-image-with-alpha-channel/378569?noredirect=1#comment618912_378569

It seems that alpha band has some bad effect on the result of lanczos resampling and that can be avoided by using -noscralpha option. From the gdalwarp manual page:
"Prevent the alpha band of a source image to be considered as such (it will be warped as a regular band)".

The option has been added to GDAL version 2.2 but with no mention about any bug that it resolved. What is the difference between warping a band as an alpha band or as a regular band? How users are supposed to know when it is appropriate to use -nosrcalpha switch?

-Jukka Rahkonen-
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