[argyllcms] Re: sRGB/AdobeRGB98 vs Lab (was Verifying profile quality...)



Graeme Gill wrote:
Milton Taylor wrote:
You could still compare the gamuts of the various devices with both 'eye' gamuts...in fact it may be more useful because you would be able to see what parts of the gamut the device might produce that were for all practical purposes irrelevant. 

OK - attached is the gamut I ended up with by simply plotting the
spectrum locus in L*a*b* space, up to Y = 100. See what you think :-)
I suspect it's so big that it dwarfs most practical output devices.

Hmm interesting!   :-)

Question for you: what energy levels are required to generate the colors that are at the extremes of this chart? Are they dangerous? If so, then I'm not sure this chart is all that useful...it would need to be bounded by a safe/reasonable energy limiting shape.

(I believe we would have this as it applies to reflective viewing, as per conditions I suggested in my last message)

On the other hand if it's all safe...then does it not answer the original question about what colors the eye can see? (Also, how do you know this is what the eye can see? i.e. what is the basis behind this shape?)


Yes that's true, but I don't see a problem with that. If you do this with an LCD, and it's set to maximum backlight brightness you're doing this anyway since any adjustment in the monitor's white balance color temp is just ultimately fiddling with the input values, so one might as well let the profile do it all.

Well, on some exotic LCD's they actually adjust the backlight white point
(those using RGB LED's as backlighting),
Well I was just salivating over NEC's newest model sporting this technology the other day, but it's a bit out of my price range!

but ignoring this, the usual
way of adjusting white point is to limit the maximum RGB channel input
values and scale the input to the limit. This won't expand the gamut
in the way I was suggesting above. To exaggerate the suggestion,
say you're LCD was capable of putting out 250 cd/m2, but you set
the white point at 125 cd/m2 in the profile (ignoring the clipping
this would do to the relative transformed ICC table data). When
viewing images on the display, you're eye will be adapted to 125 cd/m2,
but the display is then capable of displaying R, G or B channel values
at up to twice that brightness. For instances, say this is an sRGB
display:

(Using the ICC convention of Y = 1.0, and adapting CIE values to D50
white point)

White:
    XYZ 0.964300 1.000000 0.825100 -> Lab 100.000000 0.017285 -0.016162

Normal Red:
    XYZ 0.436100 0.222500 0.013900 -> Lab 54.291260 80.824574 69.917360
Red with twice brightness:
    XYZ 0.872200 0.445000 0.027800 -> Lab 72.561438 101.832582 88.090353

Normal Green:
XYZ 0.385100 0.716900 0.097100 -> Lab 87.819161 -79.278000 80.981276
Green with twice brightness:
XYZ 0.770200 1.433800 0.194200 -> Lab 114.803947 -99.884022 102.030014

Normal Blue:
    XYZ 0.143100 0.060600 0.714100 -> Lab 29.563338 68.330825 -112.054083
Blue with twice brightness:
    XYZ 0.286200 0.121200 1.428200 -> Lab 41.406208 86.091445 -141.179297

and so on. Note that no input gamut would request a color with L > 100
(So the green with twice brightness would never be used displaying
an image, etc.)

A disadvantage with this approach is that almost all frame buffers and
displays only have 8 bits per component, so the larger gamut will come
at the cost of poorer precision.


Now that's a very interesting suggestion. Suppose you leave the panel's backlight at 250 cd/m2, why could we not get the profiler to take advantage of this fact, still scaling white to 125, but allowing the other colors that the eye is less sensitive to, to go brighter, to the max of 250? Or am I missing something here?

Re the resolution of the displays - yes true - and this will be a limiting factor because using this approach, weve just chucked one of those 8 bits away for typical images.

Cheers...

Milt


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