How to patch images without color problem?

[Andrea Giacomelli]

>  actually I have to patch a mosaic of aerial photos together.
>  After rectifying them I want to have one big image
>  containing all the others.
> 
>  But now I run into trouble because the colors of all pictures
>  except one (the first I specified in r.patch) are wrong.
> 
>  Does anybody know how to patch images properly?
>  That should be possible in GRASS!
> 
I don't know if YOUR images are taken from a scanner, but I
had your problem in such a case.

If the images are scanned, it may well happen that you have a 
mismatch in the color lookup tables (i.e. the same color corresponds
to different cell values, scanner-gurus please comment).
For this reason you get only the first patched image looking ok,
while all the others go berserk.

if you r.mapcalc "img_new=#img_old" you create e new set of images
in which the correspondence between color codes and cell values
is re-established, and your "patchwork" should get a more
consistent look.
Clearly this procedure is applicable only for visualization purposes.

hope this helps!

                =========================================
                Andrea Giacomelli
                Centre for Advanced Studies, Research and
                Development in Sardinia
                Environment Division

                http://www.crs4.it/~andreag
                =========================================