Path: utzoo!utgpu!water!watmath!onfcanim!dave
From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: comp.graphics
Subject: Re: Color scanning of negatives
Message-ID: <15562@onfcanim.UUCP>
Date: 2 Mar 88 21:27:53 GMT
References: <333@inuxf.UUCP> <169@anumb.UUCP>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 27

In article <169@anumb.UUCP> adh@anumb.UUCP!mvuxq (a.d.hay) writes:
>
>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

Film is indeed a subtractive process, using cyan, magenta, and yellow
dyes, but unless you work in a processing lab you can forget this, and
do everything in RGB.

The film's three layers are sensitive to RGB.  If you measure the density
of the three image-forming layers, you do it with RGB filters in the
densitometer, and the densitometer filters are labelled RGB, not CMY.
If you convert film to electronic form, either with a colour video camera
or a B&W sensor plus colour filters, you will have RGB information.
Nowhere will you need to convert to a CMY colour space.

Even if you are working with printing inks and need real CMY information,
you don't use formulae like the ones above - the CMY densities are,
more or less, just 1 minus the RGB densities.

Where *did* you get the transformation above from?  Have you actually
used it, or did you just make it up because it seemed like a good idea
at the time?