How to set the value of a pixel based on its current value?

Question

I am attempting to set the value of a pixel based on its current value. I am using the Raster Calculator in Quantum GIS dev 1.7 trunk.

For example, if the value is 0, I want to set it to 255.

Based on an example that I found, something like this should work, but it doesn't.

myRaster@1=0 255 

The RasterCalc plugin appears to use a function syntax, but that doesn't appear to be valid either

eq( myRaster@1, 0, 255)

Any suggestions? Thanks.

Answer 1

As far as I know, this kind of query is not possible with the built-in raster calculator but only with "RasterCalc" plugin by gis-lab.

RasterCalc syntax is:

eq( [relief]@1, 50, 200 )

Compare: http://gis-lab.info/qa/rastercalc-eng.html

Answer 2

Technically I think you can do it. Say you wanted to assign 0 the value 255, 1 the value 142, 2 the value 719, and all other values to 0. I believe this would work:

((myRaster@1=0)*255 + (myRaster@1=1)*142 + (myRaster@1=2)*719)*(myRaster@1<3)

I used a similar approach to pull two values out of one raster, assign the composite raster 0 for both of those values, then do a specific calculation using two rasters for all other values in the first raster. I'm using QGIS 1.8.0.