Path: utzoo!attcan!uunet!yale!cmcl2!phri!roy
From: roy@phri.UUCP (Roy Smith)
Newsgroups: comp.lang.postscript
Subject: Question on settransfer operator
Message-ID: <3648@phri.UUCP>
Date: 12 Jan 89 17:06:13 GMT
Organization: Public Health Research Institute, NYC, NY
Lines: 27


	When defining a transfer function, is it important to ensure that
the result is strictly in the range of 0-1?  What happens if it's not?
Here's my problem: I want to do contrant enhancement on an image with 8-bit
values (i.e. from 0-255), most of which fall in the central range.  What I
want to do is pick the 10% and 90% points and stretch them to be the full
0-255 range (i.e. I'm willing to put up with clipping at the top and bottom
to get more mid-range contrast).  This means that if I just do (30 is the
10% point, 185 is the 90% point, 30/255 = .1176, 255/(185-30) = 1.645):

	{ 0.1176 sub 1.645 mul } settransfer

I will ocassionally return values less than 0 or greater than 1.  Does
postscript automatically truncate grays to the 0-1 range or do I have to
use:

	{ 0.1176 sub 1.645 mul
	dup 0 lt {pop 0} if
	dup 1 gt {pop 1} if
	} settransfer

to ensure that the result stays within the legal range?
-- 
Roy Smith, System Administrator
Public Health Research Institute
{allegra,philabs,cmcl2,rutgers}!phri!roy -or- phri!roy@uunet.uu.net
"The connector is the network"