[argyllcms] Re: Shadow detail problem
- From: "Dave Wagner" <dave.wagner@xxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 1 Feb 2008 13:49:26 -0600
On Feb 1, 2008 1:02 PM, Leonard Evens wrote:
> On Fri, 2008-02-01 at 12:21 -0600, Dave Wagner wrote:
> >
> > Here is what I am doing, I create a black (rgb 0,0,0) image in gimp
> > and then select a rectangle and bump it one rgb level at a time until
> > I can see a difference between it and the background. With just the
> > calibration loaded (.cal file or vcgt tag, it is visibly different at
> > 3,3,3 or 4,4,4 (of 255 for 8-bit in gimp). With the CMM loaded, it
> > isn't visibly different until 21,21,21.
>
> I haven't really been following this discussion, but do I understand
> that you are trying to use a gray scale with 256 steps? That doesn't
> seem to make much sense. To illustrate, at one point, for my setup at
> the time, essentially uncalibrated, I came up with the following Ansel
> Admas Zone System numbers in the 0..255 scale:
>
> O 0, I 25, II 50, III 75, IV 100, V 130, VI 165, VII 195, VIII 225, IX
> 245, X 255
>
> This is only a rough approximation, but it gives you some idea of what
> this means in terms of ordinary photography.
>
> If 0 is pure black, I is supposed to represent the first zone above
> black but still not high enough to show shadow detail. It seems to me
> to be able to recognize subtle differences of the kind you describe at
> the bottom of the scale is beyond what the normal eye can perceive.
>
> But maybe I just misunderstand the whole discussion.
>
I doubt it's your misunderstanding. It's more likely mine. On more
than one site, I've seen it suggested that a good monitor properly
calibrated will have distinguishable shadow values at around level
5-12 (on a 256 scale).
Both the CRT at home and the LCD at work do this with their
calibrations loaded (acutally around level 4 you can see a slight
difference). My question is, should an ICC profile preserve this
dynamic range of the monitor calibration? Or should it push
everything below level 20 to below 4 or 5 so that it looks
indistinguishable from black? The LUT profile on my LCD does this,
the shaper profile does not. With the shaper profile, the shadows are
still distinguishable at 4 or 5.
See these for reference, and flame them or me if they are bad advice,
or if it only applies to calibration and not to profiling:
http://www.drycreekphoto.com/Learn/Calibration/monitor_black.htm
http://www.xrite.com/product_overview.aspx?ID=756&Action=Support&SupportID=3558
Chapter 9 (page 218) of "Real World Color Management" (Peach Pit
Press, 2003 printing) gives similar advice for checking calibration.
This is speaking of evaluating calibration before evaluating profiles
and CMS, but it never makes it clear how CMS should change it.
Speaking of exactly the process I spoke of before: create a black
image in photoshop and then select a rectangle and adjust the black up
one level at a time in curves, they write...
"With excellent calibration systems, you may see a difference between
level 0 and level 1. More typically, you won't see a change until
somewhere around levels 5 to 7, or sometimes even higher. If you
don't see any change when cycling through the first twelve levels,
your black point is definitely set too low and you should recalibrate,
requesting a slightly higher black point."
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