[Qgis-user] Change value of alpha band in multiband raster

[dbobak]

Hello!

I have a four band raster, which is RGB raster with transparency (alpha)
channel. In raster calculator they are listed as:
"bbb at 1"
"bbb at 2"
"bbb at 3"
"bbb at 4"

I'd like to change the values of cells in "bbb at 4" channel (the alpha
channel) in the following way:
if value of the cell is smaller than 255, change it to 0
if it is 255, leave it unchanged

I do not want to change values of other channels.
And as a result, i'd like to have also a 4-band raster.

Would someone help me to create a rater calculator expression to get desired
result?

Darek Bobak



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