[argyllcms] Re: Shadow detail problem
- From: Leonard Evens <len@xxxxxxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Fri, 01 Feb 2008 16:35:50 -0600
On Fri, 2008-02-01 at 13:49 -0600, Dave Wagner wrote:
> 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).
Yes, so do I.
> 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.
I now see what you are talking about. You are asking if what an
application using the display profile will show. I'm pretty sure my
display (shaper) profile does distinguish somewhere in the acceptable
range, but I would have to go back and check.
>
> 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."
The only comment I have about that---possibly irrelevant---is that some
of this advice is more relevant for CRT monitors. If you have an LCD
monitor like mine, I believe some modifications are necessary. I have
to admit I'm still confused about this because I'm not sure exactly what
my monitor does. For example, Dry Creek says that the only thing you
can really adjust directly on a LCD monitor is the backlight luminance
and that other adjustments have to be done with software to the video
LUT. But I can make adjustments with the monitor controls similar to
those for my old CRT. I strongly doubt that the monitor is
communicating backwards with my video card, particularly since my Linux
OS just thinks I have a generic laptop. I conjecture that the monitor
has a built-in LUT over and above the video card LUT, which the monitor
controls can make adjustments to, but I'm not completely sure. I
believe that the newest very expensive LCDs do have separate colored
LEDs to provide illumination instead of a backlight, and they may
function more like CRTs. I'm sure mine isn't such an LCD.
I must admit, although the subject is fascinating, I am developing color
management fatigue. I would like to go back to making pictures! It may
be time to declare a moratorium and just work with the profiles I've
managed to construct and then come back to refine them when I see how
they work in practice.
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