[argyllcms] Re: Difficulty profiling new iMac

On 2007 Sep 14, at 1:07 AM, Graeme Gill wrote:

> OK, here's a bit  of a clue. If I put up a  black test square on
> my MacBook with a linear Video  LUT at maximum brightness, I get
> the following with several different instruments:

Would it help you for me to do that, and how would I? I'm guessing
I'd have to build a profile starting with displin....

Anyway,  as  I mentioned  in  the  other  note,  I'm using  an  i1
Pro. I've got  an original  i1 Display stashed  away in  a drawyer
somewhere, if it would help you for me to do some tests with that,
as well.

> I'm, guessing that this machine uses LED backlighting.

I think  that's the case  for all MacBooks being  made today...but
LED backlighting is still too expensive for large displays to have
made  it into  the iMac. Unfortunately...I  was rather  hoping for
it....

> So  the  colorimeters  seem  to   be  rather  different  to  the
> spectrometers, as well as the difference between white and black
> color being  more extreme. An  explanation for  a "kink"  in the
> blue is that by default for  an LCD display, dispcal doesn't aim
> to make the whole curve neutral (ie. b* = -24, dE to black in b*
> of 17)  since this would lighten  up the black too  much, but it
> does so down to a point where it then crosses over to the native
> black (ie. it  does -k 0.0).  This isn't spread  evenly over the
> whole curve,  on the basis  that it is  desirable to get  a good
> neutral for more of the whole curve. The differences between the
> instruments may not mean much as  far as the visual effect goes,
> although it's another complicating factor.

That actually  does sound  like what  I'm seeing. Just  for kicks,
I'll try  the process again  later today, but  tell disp{cal,read}
that I'm using a CRT instead of an LCD.

> Another complicating factor with a test ramp starting at L* = 0,
> may be the clipping/gamut mapping  behaviour of the CMM when the
> L*  min of  the display  is 8. This  depends on  whether a  true
> gamut mapped  perceptual mapping is  being used, or  black point
> compensation, or not.

This has mostly  been in Photoshop CS3, but  I've observed similar
results in Apple's Preview. I don't know enough about the two CMMs
to comment further....

> I  can think  of two  approaches to  improve the  situation. One
> would be to  aim for a white point that  is more consistent with
> the color of  the black. (You'd have to convert the  black to an
> x,y value and  feed it into dispcal -w. Hmm. I should  add an xy
> reading mode to spotread!)

Sounds like  something I should be  able to play around  with this
afternoon.

> The  other thing  I  could do  is to  make  the transition  from
> neutralizing  to  not  neutralizing   more  gradual  and  spread
> out. I'm not sure if another parameter is needed, or whether all
> LCD's will be better off with a more gradual transition.
>
> Cutting the MacBook brightness back gives:

I've  actually  dimmed the  display  to  almost  as low  as  it'll
go. This thing is *BRIGHT*!

Cheers,

b&

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