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Re: SVG Color 1.2 WD: Lab colors

From: Chris Lilley <chris@w3.org>
Date: Fri, 9 Jul 2010 14:39:24 +0200
Message-ID: <1406770817.20100709143924@w3.org>
To: Jeremias Maerki <dev@jeremias-maerki.ch>
CC: www-svg@w3.org
On Wednesday, June 23, 2010, 9:58:27 AM, Jeremias wrote:

JM> Dear WG,

JM> I'm now in the implementation phase for enhanced color support in Apache FOP
JM> and Apache Batik. I'm still building the color infrastructure for the
JM> PDF library, and that's where I noticed that the Lab colors in SVG Color
JM> 1.2 WD may be a bit underspecified. PDF and PostScript require a white
JM> point (D50, D65 etc. as XYZ coordinates) when defining a L*a*b* color
JM> space. The black point is optional and defaults to [0,0,0]. The spec
JM> doesn't say if a particular white point is assumed 

You are correct that its underspecified. I was assuming a D65 whitepoint but should have stated that. 

Furthermore, several widely used RGB colour spaces use a D50 whitepoint. It should therefore be possible to go from there to LAB without involving an adaptation transform.

So there is a need to be able to specify the whitepoint. 


JM> or if it is dependent
JM> on the user agent (meaning it's configured in the application). It may
JM> be useful to clarify that. I suspect that we don't have to care about
JM> the black point. I haven't seen any indication that this is widely used
JM> (relatively speaking).

I tend to agree that an implicit black point of 0,0,0 will be fine.

JM> I'm not sure if it makes sense, but the spec might be changed to:

JM> <fallback> cielab(<Lightness>, <a>, <b>,
JM> [<IlluminantName>|<WhitePointX>, <WhitePointY>, <WhitePointZ>])
JM> |
JM> <fallback> cielchab(<Lightness>, <Chroma>, <Hue>,
JM> [<IlluminantName>|<WhitePointX>, <WhitePointY>, <WhitePointZ>]) 

JM> <IlluminantName> is the name for a standard illuminant like "D50" or
JM> "D65" provided for convenience. Implementations should assume "D65" when
JM> no explicit white point is given 

Yes. To avoid needless verbosity, an implied white point should be provided if none is specified; and the commonest whitepoints should be accessible by keyword rather than having to provide the chromaticity coordinates.

JM> and should at least support the values
JM> "D50" and "D65" which are the most often used. 

Yes, those two are the most often used, with D65 the most common. A couple of oddball older spaces use illuminant C (which is a bad idea,  I think, but that can be expressed using the).

JM> Alternatively, an
JM> explicit white point expressed in CIE XYZ coordinates can be supplied.

There are (at least) four reasonable ways to specify an illuminant; I wonder which would be more useful?

Dnn (only for daylight illuminants, with a colour temperature of nn00 degrees).

Absolute X, Y, Z

Absolute Y and relative (chromaticity) x,y

Black body colour temperature, in K. (only for neutral blackbody radiators)

Lindbloom gives equasions to convert an arbitrary daylight illuminant Dnn to the chromaticity coordinates
http://www.brucelindbloom.com/index.html?Eqn_XYZ_to_RGB.html

of those, the Dnn notation is common and the absolute XYZ seems to be the next most common.

JM> As an implementation-note: neither PDF nor PostScript directly support
JM> CIE LCHab directly. Instead these colors will have to be converted to
JM> L*a*b* for PDF and PS.

Yes - but the conversion is trivial. C,H is the polar form of a,b.

CIE LCHab is useful as an interpolation colour space, and is useful for animation (hue and chroma (~= saturation) can be separately animated. So its a good fit for SVG and easily converted to the rectilinear form of LAB if producing PDF.

I am tending to believe that the spec should also specify the Bradford adaptation transform when converting colours between colourspsaces that use a different illuminant.

JM> References:
JM> - http://en.wikipedia.org/wiki/D65
JM> - http://en.wikipedia.org/wiki/Lab_color_space

JM> I hope that's useful.

It is, very much so. 

JM> Jeremias Maerki





-- 
 Chris Lilley                    mailto:chris@w3.org
 Technical Director, Interaction Domain
 W3C Graphics Activity Lead
 Co-Chair, W3C Hypertext CG
Received on Friday, 9 July 2010 12:42:04 GMT

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