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Re: filmscanners: negative and skin tones

rew@impulse.net (Robert E. Wright) wrote:

> ----- Original Message -----
> From: Maris V. Lidaka, Sr. <mlidaka@ameritech.net>
> To: <filmscanners@halftone.co.uk>
> Sent: Saturday, March 31, 2001 7:47 AM
> Subject: Re: filmscanners: negative and skin tones
> >
> > Another problem that comes to mind is that scanners export the image 
> > in
> > and desktop printers (without exceptions that I am aware of) require 
> > RGB
> > input, performing the RGB-CMYK conversion internally.  As there is 
> > loss in
> > color in the RGB-CMYK conversion and the subsequent CMYK-RGB 
> > re-conversion
> > many try to avoid having to color-correct in CMYK despite the 
> > benefits of
> > the black channel.
> >
> > Maris
> The change in going from RGB to CMYK is quite observable, and expected 
> due
> to the implied change in out put media (to print), but is a further 
> loss in
> going from CMYK back to RGB demonstratable?
> I just tried an experiement on one image. I first converted from RGB to 
> (difference observable). I then duplicated the CMYK image and converted 
> it
> to RGB. Finally I copied the RGB into the CMYK image and used difference
> blend mode, result complely black. I also copied the CMYK image into 
> the RGB
> file. Same result.

This is a statistical phenomenon I think, caused by the nature of 
overlapping colour spaces.

When you convert a picture from one colour space to another, you end up 
with a picture that is more likely to be made up of colours common to both 

This is because colours not expressible in the RGB space aren't in the 
original picture. 

Any colours not expressible in CMYK are converted to the nearest 
equivalent during the conversion.

However there is much more overlap between the colour spaces than 
inexpressible colours, so statistically the new colour selected is more 
likely to be still expressible in the original colour space.

So when you convert the picture back, there is an excellent chance that 
all the colours are now expressible in both colour spaces and you will be 
able to convert backwards and forwards as many times as you like without 
further changes.


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