Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ihnp4!homxb!genesis!hotlg!anumb!eao
From: eao@anumb.UUCP (e.a.olson)
Newsgroups: comp.graphics
Subject: Re: Color scanning of negatives
Message-ID: <169@anumb.UUCP>
Date: 10 Feb 88 13:20:23 GMT
References: <333@inuxf.UUCP>
Reply-To: adh@anumb.UUCP!mvuxq (a.d.hay)
Organization: Assorted Twerps & Troublemakers -- Big Loonies
Lines: 22

In article <333@inuxf.UUCP> matt@inuxf.UUCP (Matt Verner) writes:
>I am looking for references that indicate the proper way to convert
>'negative' RGB information to 'positive' RGB.  In other words if
>I take a film negative and scan it into an image using a color
>scanner what formula do I use to convert the image into a positive,
>properly color balanced, image?  The key here is proper color balance.
>
>Do I just subtract from the highest R, G or B value (255 in my case).
>or is it more correct to do a gamma correction on each value?
>
>Thanks for any help...

the negative isn't RGB, it's YCM (yellow cyan magenta) -
film is a subtractive color process; video is an additive color process.
so you'll need some transformation like
R = (MAX_COLOR_VAL * 2 - G - B) / 2	#	cyan	-> red
G = (MAX_COLOR_VAL * 2 - R - B) / 2	#	magenta	-> green
B = (MAX_COLOR_VAL * 2 - R - G) / 2	#	yellow	-> blue

Andy Hay		+-----------------------------------------------+
ATT-BL Ward Hill MA	|	Don't try to out-wierd ME, three-eyes!	|
ihnp4!mvuxq!adh		+-----------------------------------------------+