Re: Is this a scum dot?
Re: Is this a scum dot?
- Subject: Re: Is this a scum dot?
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Wed, 20 Nov 2002 10:29:25 -0600
Bruce Fraser wrote:
>
I scan the capture target at different gamma settings, and build a
>
profile from each one. Then I assign the corresponding profile to
>
each captured target and convert it to Lab. I compare these Lab
>
values with those in the target description file, and choose the one
>
that gives the lowest delta-e values, and use that gamma setting (or
>
tone curve in a drum scanner) for all subsequent scans.
Graeme Gill responded:
>
Interesting. Of course if the profiling system was perfect, you
>
shouldn't see much difference between the results with different
>
gamma settings, since it should be exactly compensating for
>
any changed you make. Do you think in doing this you're finding
>
the "sweet spot" of the profiling system, or are you optimizing
>
the quantization of the RGB encoding used between the output of
>
the scanner and the input of the CMM ?
Following this tangent of what has become a multi-headed hydra of a thread
(Scum dots? We don't need no stinkin' scum dots!) relates to tone curves (or
gamma settings) used to obtain optimal 8-bits per channel from a high-bit
scanner prior to profiling. This is an an alternate strategy to Bruce
Fraser's technique for finding the "sweet spot" of a scanner. It involves
using curves or gammas such that the scanner produces RGB values for the IT8
grayscale that are linearly related to the L* values of the steps on the
target. This spreads the RGB values out in a more uniformly perceptual way
that may be more amenable to scanner profiling software. You can compute
these "ideal" RGB target values like this (use a spreadsheet if you want):
For each IT8 grayscale step i (i = 1 through 22 across the bottom of the IT8
target):
RGBi = ((RGBmin - RGBmax)*(A16 - GSi))/(A16 - L16) + RGBmax
where:
RGBi is the target RGB value for grayscale step i
RGBmax is the RGB value you want for Dmin (e.g. 245)
RGBmin is the RGB value you want for Dmax (e.g. 5)
A16 is the L* value for patch A16 (from the IT8 reference file)
L16 is the L* value for patch L16 (from the IT8 reference file)
GSi is the L* value for grayscale step i (from the IT8 reference file)
So for example, looking at step 10 of the IT8 grayscale, then i = 10, and
let's use RGBmax = 245, RGBmin = 5, A16 = 87, L16 = 7, GS10 = 51. The target
RGB value for step 10 of the grayscale would then be:
RGB10 = ((5 - 245)*(87 - 51))/(87 - 7) + 245
= 137
If you repeat that calculation for *your* IT8 reference file values and
*your* desired RGBmin and RGBmax, you get a table of target RGB values for
all 22 IT8 grayscale steps. Then adjust your scanner tone curves or gammas
so that these target RGB values are most closely achieved (making sure
though, that you always get separation between adjacent grayscale steps!).
Note that this technique is best used for a bit reduction tone curve (e.g.
16-bits down to 8-bits). Probably not such a good idea for 8-bit to 8-bit,
since in that case you will lose tones.
Just an idea for the more adventurous types...
--
Bruce J. Lindbloom
email@hidden
http://www.brucelindbloom.com
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