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From: shiva@well.sf.ca.us (Kenneth Porter)
Newsgroups: comp.lang.postscript
Subject: Replacing the transfer function to get a negative
Keywords: currenttransfer settransfer setgray negative
Message-ID: <24507@well.sf.ca.us>
Date: 30 Apr 91 08:48:57 GMT
Distribution: comp
Lines: 23


Remember that when changing the transfer function, you should
carry along the old definition.  It may contain code to
compensate for a non-linear marking engine, or it may have been
previously modified by an enclosing application to achieve a
special effect.
 
================================================================
% We must call existing transfer function after ours
% for proper mapping of user grays to device grays.
/oldtransfer currenttransfer def
 
% Function just subtracts requested gray from 1, then
% calls device transfer function.
{ 1 exch sub oldtransfer } bind settransfer
 
% init page to logical "white"
gsave
  1 setgray clippath fill
grestore
================================================================
 
Ken (shiva@well.sf.ca.us)