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Re: [css4-color] unclamped values for RGB

From: Chris Lilley <chris@w3.org>
Date: Tue, 17 Jul 2012 15:23:50 +0200
Message-ID: <1067668777.20120717152350@w3.org>
To: Rik Cabanier <cabanier@gmail.com>
CC: Bjoern Hoehrmann <derhoermi@gmx.net>, www-style list <www-style@w3.org>
On Sunday, June 17, 2012, 9:18:56 PM, Rik wrote:

RC> The problem I see with the current spec, is that it is wrong and
RC> that it will make decent color management in the future
RC> impossible. 

I don't believe that it is wrong, nor that it makes color management impossible.

RC> CMS is primarily about mapping gamuts from one
RC> colorspace to another [1]. By removing the clamping, the gamut is
RC> made infinite so this becomes impossible.


The gamut is still there and is unchanged. Unclamped values just allows values which are outside the gamut to be specified, and to be treated, without multiple round-off steps.

(This is clearly easier in a linear space, because the transfer function outside the gamut is obvious, but it can also be done in gamma-companded space by reflecting the transfer curve about zero and using a linear extension above 1.0. The most common out of gamut values are small negative values in the red channel).

I understand that some color engines are unable to deal with out of gamut colors, and will clamp anyway. But that is not universal, and forcing better implementations to clamp when they don't need to artificially constrains the better implementations to do a less good job than they are capable of.

A useful paper on unbounded values in an ICC workflow is

Unbounded Color Engines
Marti Maria

(Marti Maria is the author of the LittleCMS engine, which is used for example in HP large-format inkjets).

Note that this is now a part of ICC v.4, with the Floating Point Encoding Range addendum. So I have to agree with the ICC (and thus disagree with Rik) on the assertion about unbounded values being incompatible with an ICC workflow :)

On the consumer electronics side, HDMI supports a wide-gamut colorspace, xvYCC. Note that this encodes an sRGB gamut (in the video range of 16..240 rather than the computer range of 0..255) for compatibility with older equipment, and encodes the out of gamut colors as ... out of gamut colors :) or in other words, unclamped values.

"Opto-electronic transfer characteristics [Extended]
?? Extended ITU-R BT.709 tone curve by
defining over-ranged values.
?? Unchanged for inside the ?0-1? range."

See this presentation, and in particular slides 14-16:

"The xvYCC color space permits YCC values that, while within the encoding range of YCC, have chroma values outside the range 16?240, or that correspond to negative RGB values, and hence would not have previously been valid. These are used to encode more saturated colors."

DisplayPort did not support that at first, although DisplayPort 1.2 added this.

There has also been some recent discussion of colorimetric characterisation of digital cameras; it turns out that negative values are needed for a full characterisation.

So various industry segments are supporting unclamped, out of gamut colors.

RC> I think if CSS implements the SVG color-profile property [2]
RC> (without the <name> keyword), that would be a great step forward.
RC> Old implementations would ignore it, so their colors will look
RC> slightly off but would most likely still be reasonable. 

Yes, implementing the color-profile property (and the rendering colorspace support) would be a big improvement. I don't see why you want to drop the name keyword though, since that is how you refer to the ICC profile!

RC> Lab would be really great since it corresponds to how we perceive
RC> color. This makes gradients and meshes more pleasing to the eye
RC> and has no need for profiles from the user's side.


RC> It's not unthinkable that in the near future there will be a race
RC> for best color displays (like there now is for resolution) so it would be good to be prepared.

This has already started.

Wide-gamut desktop displays have moved down in price from the multi-thousand dollar products of a decade ago. Products like the NEC spectraview are more competively priced, EIZO do a range of more affordable wide-gamut models. Mainstream manufacturers like Dell, HP and Asus (Ultrasharp, Dreamcolor and Pro-Art ranges, respectively) offer affordable wide-gamut displays that cover most of AdobeRGB (although without the display linearity circuits of the more expensive offerings).

Although these are marketed towards the graphics arts community, it is the larger amateur photographer market that seems to be driving this popularisation.

A couple of laptops (HP and Dell) have wide-gamut displays available in their 15inch and 17inch models.

The 'new iPAD' has a wider gamut (most of sRGB) than the very desaturated much-less-than-sRGB displays of the original iPad and iPad2.

 Chris Lilley   Technical Director, Interaction Domain                 
 W3C Graphics Activity Lead, Fonts Activity Lead
 Co-Chair, W3C Hypertext CG
 Member, CSS, WebFonts, SVG Working Groups
Received on Tuesday, 17 July 2012 13:24:33 UTC

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