[argyllcms] Re: Implementation of setting a black point

What I mean is this: say your display is capable of having a black point luminance as low as 0.12cd/m^2 or something like that. Now, your measuring device might be able to take readings as low as 0.15cd/m^2 (or even higher: the Spyder2 reported my black luminance as 0.4 when it actually was around 0.12 as measured with the DTP94b). What I want to be able to do (and most calibration software like iColor Display and basICColor Display allow - only to say that this is not uncommon!) is to set the desired black point to something above that, say 0.2 cd/m^2 or 0.3 cd/m^2 That way, you could avoid areas problematic for your measuring device, which should deliver better results for the calibration (no color casts in very dark tones as often exhibited). Also, alot of photographers I know want to set the contrast ratio of the screen to a specific value. For photo editing, a lot of people don't need very low luminosity values for black, as this would not reflect what you see on the actuall print. They want to be able to set the contrast ratio to something as low as 300:1 (or even lower) for better evaluation (of course, you could use soft proofing aswell, but that's not the main point). So, defining the black point luminosity at the same time as the white point luminocity does the trick. Needless to say this is against the advertising argument of the highest contrast ratio, but I hope you get my point. Of course I didn't mean to set the black point luminosity below the display's capabilities. Being able to do that would be some kind of magic ;-)
This is just an idea for a feature you might be interested in implementing in addition to the vast possibilities one already has.
By the way: I'm having some problems to achieve a truly neutral gradient from black to white with Argyll. It might be a problem with my monitor (Eizo S2431W). But, using the iColor Display software which came with the device, my results were far better. I don't know if it has anything to do with the bug in the current beta release, but I will take a look at it again when the next version is released. 

Steffen

Graeme Gill schrieb:

Steffen wrote:
I'd like to know if there are any plans in implementing a command line switch for setting a luminance for the black point with dispcal. As colorimeters (and spectrophotometers?) mostly don't provide very accurate results in low luminance situations, I would find it desirable to set the luminosity "pure black" should have. This way you could also calibrate to a distinct contrast ratio. 

Hello Steffen,
    I guess I'm not clear on how this would work. The calibration
can't make black any blacker than it is, so how would setting a black
luminosity help ? If I were implementing such a feature, my instinct is
to ensure that the target point is always equal to or greater than
the measured black, to avoid faulty behavior of the calibration
algorithm. Then there is the complexity of what to do about the black
point color (ie. -k option).

If inconsistency of readings is an issue, then I'd expect that the
inconsistency will have an effect on the calibration, irrespective
of the target value. What instrument(s) have you had this problem with ?

Note also that there is a bug in the current beta release that
has a subtle effect on the black point, and makes it not quite
as black as it should be.

Graeme Gill.

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