RGB vs CMYK for gum


Gregwen@aol.com
Sun, 24 Jan 1999 01:19:41 -0500 (EST)


     I think to understand the difference between RGB, CMY and CMYK you have
to go back and look how pre digital processes handled these conversions.
Digital programs hide what is occurring. The process can be broken down into
two serial processes, a conversion from RGB to CMY and then if you are
printing by press a conversion to CMYK.
      In film based separations whether for photomechanical, Dye Transfer, or
carbon, or whatever, a low contrast negative was made thru an appropriate
color filter. This was then put over the transparency and the final Sep neg.
was made thru the another appropriate filter. For instance a negative made
with the red filter was registered over the transparency and then that
combination was printed thru a green filter for a final green separation
negative. This was necessary because pigments and dyes don't absorb only their
complementary color but also some adjacent color.
     For a practical example in Photoshop make a new file in RGB, split it
into three separate channels. Then make foreground white and background black,
do a gradient tool from one edge to the other of the red channel. Combine this
back into an RGB file using the channel with the gradient as the red channel.
This will give you a cyan colored tablet. Print this RGB file using whatever
process or printer you normally use. Photograph this, scan it and open it in
Photoshop. The difference between this file and the original will be the color
error of the complete process. When you view this file the cyan image will be
markedly darker and more magenta (bluer). In the original there is no image in
the green channel but If you look at the green channel in the reproduction the
gradient will be very visible (about 50% of the red channel). The use of the
masking negative when making separation negatives will remove this color
error. In this illustration the neg. exposed thru the red filter in register
with the transparency will print less magenta where cyan is being printed.
       If you are printing with a process that will give a good black you can
stop here and print CMY. If you are using a process that will not produce a
good black (printing press) then you must generate a black printer.
      When you convert a RGB file to CMY, the file should be changed if you
change inks or pigments. In practice if you are using a non commercial process
you will have to experiment with the various printer options in the Photoshop
separation setup to find one that you think is best. Almost any printer
selection though will be preferable to printing a straight RGB file.



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