[argyllcms] Re: Using a Sequel Chroma 4

On Dec 7, 2007 12:30 PM, Graeme Gill <graeme@xxxxxxxxxxxxx> wrote:
> > The last hack was made to give white level around 140 Cd/m^2 using
> > dispcal -yl -r.
> > This also gives correct reading for maximum brightness according to
> > monitor specifications.
>
> It's hard to justify this unless you've got another instrument
> that you trust to compare it with though. (ie. the monitor
> specs could be quite different to the actual monitor behaviour).

Sure, it was just a quick hack to get a somewhat sensible reading.

>
> The right spot for an absolute scale factor is in 
> i1disp_take_XYZ_measurement().
>
> > It works like a dream, though I cannot rely on the absolute value of gamma.
> > Setting -g1.75 in dispcal gives a final gamma of 2.2 (according to my
> > gamma swatches, downloaded from Timo Autiokari).
>
> It's probably more reliable to check for yourself using dispwin and spotread.
> Read the Y for 100% and 50%. Gamma = log(50%Y/100%Y) / log(0.5)
>

Oh yes. I have noticed before that different gamma swatches often have
rather big errors. I have made correction layers to the gamma swatches
I use in Photoshop. Then I can drag the swatches across the screen and
can see the variations in error.
I have just changed video card, because I wanted to test Nvidia's GPU
computing, and found that the old card had a broken fan. After I
changed the card, I may have messed up with old .cal files. I have
made new calibration files now, and they work perfectly, mostly
hitting target gamma within 0.05 according to spotread/dispcal.
I have also uninstalled all irrelevant icc profiles, after I noticed
that Microsofts Picture and Fax Viewer displayed images using
ColorMatch instead of an embedded sRGB profile.


> > I'm leaving Nvidias settings from installation time untouched.
> >
> > I do not understand the black point setting, as I see it in the
> > calibration file:
> > BEGIN_DATA
> > 0.0000 0.019102 0.018301 0.017533
> > 3.9216e-003 0.025279 0.024478 0.024011
> > 7.8431e-003 0.031338 0.030534 0.030370
> >
> > I don't understand why is it not going down to more near zero.
>
> It doesn't necessarily follow that zero device value is the threshold
> where the display starts to turn on. Some devices have a dead zone
> from zero where nothing happens, so it's important to
> set the calibrated zero at the threshold where something starts
> to happen, otherwise the display characteristic isn't invertible,
> and a calibration curve can't be created.
>
> Having said that, it doesn't mean that I think the current
> calibration algorithm is perfect in what it achieves for
> the black point.
>

Got it!

> Thanks for the feedback on the Chroma 4 - I'll see about
> adding your changes into the code, if you can clarify what
> the actual version number and sub-model ID is.
>

Version no: 5.008
Sub model id (vv): -1
USB driver inf file: MonacoOptix.inf

Thanks for your help
Preben Soeberg

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