Re: Is this a scum dot?
Re: Is this a scum dot?
- Subject: Re: Is this a scum dot?
- From: bruce fraser <email@hidden>
- Date: Mon, 18 Nov 2002 15:45:06 -0800
At 4:49 PM -0600 11/18/02, Bruce J. Lindbloom wrote:
I think we are having a communication problem between us -- time for a baud
rate check. Let me restate my position again...
<snip>
You're absolutely right. What I meant to say is that a scanner
profile that maps RGB 255 to something other than L*100 is by *my*
definition, broken -- I didn't mean to invoke standards that don't
exist.
My compromise is a bit different from the one you suggested, and it's
one that's well-supported by most profiling tools, which generally
don't map the A16 patch to L*100 -- they do in fact look at the RGB
values and make sure that RGB 255 maps to L*100.
I profile the scanner's wide-open 16-bit/channel behavior, and scan
everything wide open. I experiment to find the tone curve that will
produce the best agreement between the actual values in the target
and the predicted ones I get by assigning the profile to the scan,
converting to LAB, and comparing with the target values.
The resulting scans come in a little flat and a little unsaturated,
but with accurate gray balance and hue. It then becomes very simple
to optimize the contrast and saturation by editing the images. Until
we get an image-aware CMM, I don't think any other approach is going
to work any better in practice.
This approach is of course technically wrong, but since all the
technically correct ones I've tried so far fail fairly horribly in
practice, it's the best I've been able to do, and I have a lot of
clients who are happy with it. I'd be delighted if guys like you and
Graeme came up with a better one, but I'm not convinced that the
current ICC implementation allows it. And if we're going to make
image-specific adjustments, I'd rather do so post-scan, where I can
see all the pixels, than prescan on a postage-stamp-sized preview.
I always want RGB 255 to map to L*100 -- it's the only way I know to
let me decide where my highlights go from specular to diffuse while
using all the bits.
Bruce
--
email@hidden
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