[argyllcms] Re: negative primaries

  • From: Graeme Gill <graeme@xxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Mon, 07 Jun 2010 12:34:05 +1000

Ben Goren wrote:
On 2010 Jun 6, at 1:51 PM, Elle Stone wrote:

But really even ProPhoto isn't big enough for all images.

It's even worse than that, I'm afraid.

The spectrum locus lies at least partially outside both Lab and XYZ color 
spaces. That
is, there are huge swaths of spectrally-pure colors that no color space in use 
can encode without clipping.

Hi, I think you need to distinguish between the L*a*b* and XYZ colorspaces, and
the limits imposed by ICC profile encoding of these spaces. The spaces 
have no limits, since the numbers have no limits.

Some time back this whole point was driven home to me in a pretty emphatic 
manner when
I tried to create a digital rainbow. I created a .ti3 file that was all zeros 
for a single frequency at 100, and used fakeread to convert that to Lab colors. 
beyond the far red and violet portions of the spectrum, Argyll began reporting 
values well beyond 100 L* and 127 a* and b*. So, after some experimentation, I 
kept the
single peak at 100 but added values of 50 on either side. It brought the Lab 
closer to within range, but still waaaay out. It eventually took a very 
wide-band bell
curve to get something that would fit in Lab space -- and that, of course, was 
waaaaay outside any other color space.

argyll/gamut/GenVisGam.c is a similar exercise, to map the volume of the visible
gamut in Lab space.

Graeme Gill.

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