Changing raster cell values within a polygon using QGIS GUI?

I would like to know how to change the values of raster cells which are within a polygon, leaving all cell values outside the polygon unchanged. I would like to subtract 1.5m from each raster cell value within the polygon, rather than make them all the same value.

I don't know how to use GRASS so I'm hoping there is a way to do it using the tools within the QGIS GUI, raster calculator, or processing toolbox. I'm using QGIS 2.0.

Thanks.

Answer

First, you need to convert your polygon to a raster. Create an integer field filled with "1" for the purpose of the conversion. (raster > conversion > rasterize)

Then, you can use the raster calculator to substract 1.5 where your polygon exists.

 yourdem@1 - ((poltoras@1 = 1) * 1.5 )