Automate your color correction with a Perl script  Card Trick  ©
articular, Fotolia   Last month, I wrote about using reference
cards to correct the white balance in digital photos by taking a
test snapshot (see Figure 1) [2]. The black, white, and gray
plastic cards, which are available from any good photography
equipment dealer, should not generate any color values in a
digital image. This provides three calibration points for low,
medium, and high light intensity, which the GIMP photo editing
tool can then reference to correct your snapshots.  Perl Magic
How can a simple Perl script find out which pixel values the
three cards create, even though their position in the image is
not known, without using artificial intelligence?  If the
photographer manages to spread the cards in the center of the
image as shown in Figure 1, the script can follow an imaginary
horizontal line and identify the cards on the basis of pixel
values along the x axis.  The light intensity measured along
this line remains constant for