Visual Comparison of 2 Raster Images

FIgure 1

The visual comparison of raster data can be complicated by images that have non-NULL values at each grid cell. Transparency can used to "see" both images at once, but the results can be misleading. An alternative method suggested in the GRASS Book (Neteler and Mitasova, 2003), uses a couple simple raster algerbra tricks to make a "checkerboard" image composite of both input raster files. Figure 1 illustrates a simple comparison between an IKONOS false infra-red image, and a classified vegetation image via this checkerboard approach.
 
Example Code

#create checker pattern with some raster algebra-fu
r.mapcalc " v=if( (sin(5*row()) + sin(5*col())) > 0.0, null(), veg_11 )"
r.mapcalc " i=if( (sin(5*row()) + sin(5*col())) > 0.0, ikonos.rgb, null() )"
 
#assign the original colors to the new files:
r.colors map=v rast=veg_11
r.colors map=i rast=ikonos.rgb
 
#patch them together with r.patch, or view with:
d.rast v
d.rast -o i

References:
Neteler, M. & Mitasova, H. Open Source GIS A GRASS GIS Approach. Kluwer Academic Publishers, 2003.