New article on color management

Started Apr 26, 2013 | Discussions thread
Hugowolf
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Re: Yeah...
In reply to gollywop, Apr 28, 2013

gollywop wrote:

Great Bustard wrote:

gollywop wrote:

Great Bustard wrote:

gollywop wrote:

Great Bustard wrote:

...this is something I've always wondered about.  Whenever you expand the range of colors, you necessarily decrease the distance between colors, for a given bit depth.  So, the question, then, is when the range of colors matters more than the gradations of color, and vice-versa, as a function of existing colorspaces.

The simple solution, of course, is to use 16-bits.

I take that as a given!

That does fine even with ProPhoto RGB.

So 16 bits per channel gives at least enough color separation in PPRGB?  Is the implication, then, that 16 bits is overkill of less expansive color spaces like sRGB?

Well, there's more to it than that – as you well know, there always is.  The real advantage of 16-bit images is the ability of the colors to survive tonal manipulations without posterization.  This would apply equally well to sRGB as to a broader space.  I doubt you can ever consider the potential of 16-bits overkill, but I suspect, even with sRGB you can consider the potential of 8-bits to be underkill.

Here's a nifty demonstration of the "survivability factor"

http://www.photoshopessentials.com/essentials/16-bit/

What I'm asking, I guess, is if 16 bit sRGB will ever have any practical advantage over a larger 16 bit colorspace due to the finer gradations of the colors it does represent.

Well, I have certainly encountered significant posterization in large expanses of blue skies under aggressive tone mapping when PP-ing an 8-bit sRGB jpeg.  This is not an uncommon experience.

And, unfortunately, once an image starts life as an 8-bit image, you don't gain a heck of a lot just converting to 16 bits; all those gradations in between don't suddenly get created.  If you're going to use 16-bits with sRGB, you want to shoot raw and process with 16-bits from the outset.

As to just what color differences humans can see, and how fine practical gradations can be, we humans are apparently much more sensitive to small shifts in blues than reds, and more in reds than greens. So the whole notion of JND (just noticeable difference) in color is very wavelength related. Clearly going to 16-bits is a huge boost. It gives a 256 fold increase in the number of divisions over 8-bits, and none of the color spaces is anywhere nearly 256 times larger in linear dimension.

You can get an idea of all this from the MacAdam ellipses:

http://en.wikipedia.org/wiki/MacAdam_ellipse

but beware in assessing the diagram they give there that they have exaggerated the sizes of the ellipses ten fold.

Interesting as it may be (MacAdam et al) it doesn’t address metameristic differences: that colors may be indistinguishable within their range, but may produce visual differences when placed next to other colors.

And on the use of either or both 16 bit and a large space like ProPhotoRGB, surely the concern is not with final reproduction (print, screen, projection) but with the ability to manipulate without clipping during editing – working space as opposed to output space. There are many operations which when performed under a space as restrictive as sRGB, could cause a push out of gamut during operations, which could (had they been in a more expansive space) subsequently landed them back in the expected output gamut.

And, although the spaces are based on integer values, processing is done with floating point numbers.

Brian A

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