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[filmscanners] Re: RE:8bitsvs.16bits/channel:cantheeyeseethedifference

on 3/22/03 12:51 AM, Paul D. DeRocco at pderocco@ix.netcom.com wrote:

>> From: Roy Harrington
>> Interesting.  Maybe they figured there was a possibility of color
>> artifacts showing up.  I assume you mean an RGB where R=G=B ?  If they
>> were different the dithering would have to be different, too, wouldn't
>> it?
> I think the noise added to the three values is the same. If the fractional
> bits of the three colors are different, then how the three values round off
> to eight bits would vary among the colors.

Good call, Paul.  I decided to try this with an RGB of (50.2, 50.4, 50.6).
Sure enough all three colors dither between 50 and 51, but R=51 is a direct
subset of G=51 which in turn is a direct subset of B=51.

I think this rules out any of the more advanced dithering algorithms like
diffusing the error as you go -- i.e. each pixel is independent.

> If the difference were at all visible, which I doubt, I think having the
> colors separately dithered would be a slight improvement, for the same
> reason that an B&W image printed on an inkjet appears less grainy when
> printed with CMY dots rather than just black dots.

Don't know Adobe's reasons, but I think introducing color no matter how
little would be a no no.  If someone had a pure gray 16bit RGB file and
conversion to 8bit added color, I'd hate to be the one trying to justify
that.  It may be minor to start with, but with sharpening and other filters
who knows.

> --
> Ciao,               Paul D. DeRocco
> Paul                mailto:pderocco@ix.netcom.com


Roy Harrington
Black & White Photography Gallery

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