Re: On ProPhotoRGB
Re: On ProPhotoRGB
- Subject: Re: On ProPhotoRGB
- From: Marco Ugolini <email@hidden>
- Date: Sun, 26 Feb 2006 12:56:53 -0800
In a message dated 2/26/06 2:20 AM, email@hidden wrote:
> Draw a circle on a piece of paper, then draw a triangle that encompasses every
> single point along that circle's radius within its perimeter. You'll notice
> that in order to bring the entire circle into bounds, you have to push the
> triangle's verticies out beyond the circle's edge. The same thing happens when
> you try to define a color space with only three primaries on the CIE diagram:
> the spectrum loci is curved, while the triangle you get by connecting the
> three primaries together is not. A triangular peg isn't ever going to fit a
> circular hole perfectly, so if you want to get something on the edge into
> bounds, you need to push those bounds places they probably should not go, such
> as the realm of imaginary colors.
Hi, xji1ocw02 (whoever you may be).
Thank you for taking the time to reply.
So far it all makes good sense.
> You could also define additional primaries, but putting such a model into use
> would be impractical at our present level of display technology (I did read
> something about somebody somewhere putting together a monitor that used 6 or
> so primaries recently, so maybe someday).
Something analogous to what multi-inks do in commercial printing? Would they
still be based on a tristimulus-type model, though?
> Some of the colors that can be captured on traditional film lie along the edge
> of the spectrum loci, so if ProPhoto wants to be able to record them, it needs
> to push its green and blue primaries out a bit into la-la land.
It makes sense in theory. But what are the practical effects of going into
la-la-land? I mean the the *negative* ones as well as the positive ones you
already mentioned.
> The consequences are that you end up wasting a portion of your tonal range on
> colors that cannot be seen, recorded or reproduced in any way, leaving you
> with fewer real colors to work with despite the visibly more expansive gamut.
> This occurs only because of the finite nature of digital information storage,
> and not because of any natural phenomenon. When you can place any one of 256
> possible values in a single pixel and no more, throwing away any portion of
> those possible values on colors that cannot be seen effectively wastes them,
> and posterization can result. This is why it is generally recommended that you
> work with 16-bit images when using wide gamut working spaces, because then you
> have 65536 possible values to assign to every pixel, and can afford to throw a
> handful away if necessary without introducing any objectionably abrupt tonal
> transitions.
To sum up, you are saying that the act of stretching the primaries to
la-la-places outside the spectrum locus (that being the only way for the
triangle to encompass colors *inside* the locus) also stretches the distance
between the steps defined by the 256 values in 8-bit editing, with the
posterization attendant to that. Hence the need for higher-bit editing, in
order to cover all the colors in between that would be lost otherwise. If
that is what you mean, it's all copacetic.
> If none of the colors in your image lie in any of the additional
> areas ProPhoto covers, then there is no reason to use it.!
Right, but what about any possible "funny math" triggered by the presence of
these imaginary out-of-spectrum-locus colors? Nothing to worry about there?
> The debate seems to arise amongst photographers who for some reason or another
> have convinced themselves that these colors are rarely encountered in the
> "real world" no matter how many times you show them areas of bright greens
> and/or blues clipping in Adobe RGB.
Yes, the never-ending jabber on "that other forum" (harrumph!). I tuned that
one out a long while ago as a waste of my time and a source of no additional
knowledge whatsoever.
> There was once something of a negative stigma associated with 16-bit images
> since Photoshop was severely crippled when it came to working with them, but
> that is no longer the case, and the only good reason not to is the increased
> resource consumption.
That position has less and less credibility, given the processing speed and
massive storage capabilities of today's CPUs (and growing).
Still, your clear and helpful reply is not addressing the other issues I
raised, namely:
- the uneven distances between steps in ProPhotoRGB (image 5 at
<http://home.mindspring.com/~marcoug>)
- the pinched hue line end at point 1 in the same image
- the crossover at point 2
- the messy-looking overlap at point 3.
These are the things I still do not know whether to interpret as negative or
of no consequence (which would surprise me).
Best regards.
--------------
Marco Ugolini
Mill Valley, CA
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