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

  • From: Milton Taylor <milton.taylor@xxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Thu, 23 Mar 2006 16:46:18 +1100



Graeme Gill wrote:
Milton Taylor wrote:

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.

The energy is approximately 1/CMF value, and since CMF's
go to zero out of visible wavelengths, the energy has to
go to infinity. I stopped at 380nm and at which the Y sensitivity
is 0.000039097450, so on the order of 26000 times the light
level at our most sensitive wavelength (555nm). At 380nm,
this may give you a really good sun tan.

Thought so!


Most color work simply assumes that it all looks
the same, as long as it's photopic, and doesn't assume a maximum.
Only some color appearance models (ie. Hunts model) and safety
folks worry about absolute light levels once it's out of
the scotopic region.

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?)

It's all in the Colorimetry theory. Basically a monochromic source
must be able to generate the most difference in cone responses, hence
most saturated colors (or two sources along the purple boundary).

So where does that leave us? What does that gamut chart you just plotted actually mean? That we can see virtually any emitted colour if it's strong enough? 

What happens if you redrew the gamut to also limit the maximum energy at any hue to something like the maximum typical brightness of an LCD?  (Say 250 cd/m2) knowing full well that this would only happen in practice if you did something like discussed below?

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?

It should be practical to create a LUT based profile that does this sort
of thing. The main problem is that the windowing system will sabotage
the effort. It will send direct RGB values to the screen via the RAMDAC
tables (for window decoration) making sure you're adapted to the 250 cd/m2
white point. You'd need a full screen application that used the profile to
be able to exploit this approach.
(I'm not sure any of the OS/windowing systems actually properly send
 everything through a display profile, nor is there a way of stopping
 some applications bypassing such a default behaviour.)

If you went with this approach, wouldn't the more extreme colours look brighter than white? Wouldn't that be perceptually rather confusing? Or would this idea have to be used with some subtlety so that the perceptual effect was preserved?

As for the technicalities, there are two issues there...firstly as you would probably know Windows doesn't use profiles for its own rendering, only it's video LUTs, so you'd have to also fiddle with those too in some similar fashion. This would have to mean that White=256,256,256 input, but output is 128,128,128.  Other colours could possibly generate individual channel output values > 128.

Secondly, re the windows fullscreen thing, I'm not worried about that because I'm assuming you'd be previewing full screen anyway, like in Photoshop's full screen mode.  Which also does use the profile to adjust what you're seeing.

Actually, there is considerable merit in this idea from another point of view. Most LCD monitors are too bright for color matching work. So halving the typical brightness for white point would be a great idea.  The only real practical problem I see is the 8bit limitation.

...Milt


Graeme Gill.


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Milton Taylor
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