Re: Is this a scum dot?
Re: Is this a scum dot?
- Subject: Re: Is this a scum dot?
- From: Graeme Gill <email@hidden>
- Date: Tue, 19 Nov 2002 14:53:06 +1100
bruce fraser wrote:
>
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.
Profiling software that does this is seriously broken. In general
no scanner will exactly read RGB 255 given a (not practically realizable)
Absolute L*100 reflective sample. I can imagine scanner makers
aspiring for their scanners to behave that way (although they might
get themselves in to trouble with fluorescent samples reflecting
more than 100% light at certain wavelengths), but the whole purpose
of characterizing a scanner is to compensate for how it actually
behaves, as opposed to how it ideally behaves.
If the scanner is manually calibrated of course, then you
can get whatever behaviour you like out of it in this regard
(including getting RGB 255 to whatever relative L*100 you like).
>
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.
By "wide open" you mean running in its uncalibrated, "raw" RGB mode ?
By "tone curve", you mean a per channel calibration curve applied to
the raw RGB data before it hits the CMM ?
I'm not sure I understand where the profile comes from, that you
are using to get Lab values, to set up your tone curves. Seems
like a chicken and egg problem. You can't create the scanner
profile until you've decided on tone curves, yet you're using
some profile to choose your tone curves. Can you clarify how
this works ?
>
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.
If you are calibrating the scanner, then you can do anything you want.
I'm a bit puzzled about "what" L*100 you are getting RGB 255 to map
to. Absolute L*100 (how do you test it ?). If a Relative L*100, which
media - the IT8 chart white (I presume not, from your comments), the
white of the particular item being scanned ?
If the latter, doesn't this imply calibrating the scanner to
each print you are scanning (viable for high value work, not
viable for more casual scanning).
Graeme Gill.
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