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SVG Color 1.2 WD: Lab colors

From: Jeremias Maerki <dev@jeremias-maerki.ch>
Date: Wed, 23 Jun 2010 09:58:27 +0200
To: www-svg@w3.org
Message-Id: <20100623092703.774C.60BA733C@jeremias-maerki.ch>
Dear WG,

I'm now in the implementation phase for enhanced color support in Apache FOP
and Apache Batik. I'm still building the color infrastructure for the
PDF library, and that's where I noticed that the Lab colors in SVG Color
1.2 WD may be a bit underspecified. PDF and PostScript require a white
point (D50, D65 etc. as XYZ coordinates) when defining a L*a*b* color
space. The black point is optional and defaults to [0,0,0]. The spec
doesn't say if a particular white point is assumed or if it is dependent
on the user agent (meaning it's configured in the application). It may
be useful to clarify that. I suspect that we don't have to care about
the black point. I haven't seen any indication that this is widely used
(relatively speaking).

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

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

<IlluminantName> is the name for a standard illuminant like "D50" or
"D65" provided for convenience. Implementations should assume "D65" when
no explicit white point is given and should at least support the values
"D50" and "D65" which are the most often used. Alternatively, an
explicit white point expressed in CIE XYZ coordinates can be supplied.


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

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

I hope that's useful.

Jeremias Maerki
Received on Wednesday, 23 June 2010 07:58:50 GMT

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