[GRASS-dev] [GRASS GIS] #3130: r.out.gdal: add alpha channel support

[GRASS GIS]

#3130: r.out.gdal: add alpha channel support
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  Reporter:  neteler      |      Owner:  grass-dev@…
      Type:  enhancement  |     Status:  new
  Priority:  normal       |  Milestone:  7.2.0
 Component:  Raster       |    Version:  svn-releasebranch72
Resolution:               |   Keywords:  r.out.gdal
       CPU:  Unspecified  |   Platform:  Unspecified
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Comment (by mmetz):

 Replying to [comment:4 neteler]:
 > With Sentinel-2, also 0 is used as a data value (eg `Computed
 Min/Max=0.000,13668.000`). The black border mask can be retrieved from an
 auxiliary JP2000 file and applied as MASK in GRASS GIS. So far so nice.
 >
 > Still I am not sure if it now better to use `r.rescale` or the `#`
 operator of `r.mapcalc` with some formula in order to "free up" the 0
 value for paving the way of exporting with alpha as above?

 Using a nodata value means that any band values equal to the nodata value
 will be interpreted as nodata. Using an alpha channel means that all
 pixels with an alpha value of 0 are transparent. The alpha channel thus
 represents per-pixel encoding of nodata, just like the GRASS null file.
 That means when using an alpha channel, you do not need to "free up" any
 particular value, you can use any value you like to replace NULL cells, as
 long as the value can be represented by the Byte data type. It does not
 matter if that value is already present in any of the input bands.

 This is about exporting RGB data, not GIS data, therefore the `#` operator
 of `r.mapcalc` needs to be used. The result is a regular image (like a
 screenshot) that can be displayed by any simple image viewer coming with
 your OS and/or desktop environment. When exporting, do you want to provide
 something like a screenshot or the actual data?

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Ticket URL: <https://trac.osgeo.org/grass/ticket/3130#comment:5>
GRASS GIS <https://grass.osgeo.org>