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Re: CSS3 Color Comments: Clarification of hsl() hue parsing

From: Chris Lilley <chris@w3.org>
Date: Thu, 13 Mar 2003 11:20:59 +0100
Message-ID: <11167025328.20030313112059@w3.org>
To: www-style@w3.org, David Woolley <david@djwhome.demon.co.uk>

On Thursday, March 13, 2003, 8:24:17 AM, David wrote:


>> 
>> DW>  This seems to give HSL a smaller gamut than
>> DW> sRGB.
>> 
>> It has the exact same gamut as sRGB.

DW> The way I read the description of sRGB is that it is clipped in
DW> device space

Yes, values outside the gamut can be expressed and are only clipped
once they are transformed into the device colorspace where, in fact,
they may be inside that gamut.

Which cannot, it seems, be done with the HSL space.

However, the colors outside the sRGB gamut are still outside the sRGB
gamut.

DW>  (with the possible implication that clipping is dependent
DW> on the values of components other than that being clipped), whereas 
DW> what seems to be being said here is that clipping occurs in HSL space.

DW> Maybe the description of sRGB clipping needs to be clarified, as it
DW> it appears to imply that values outside [0,100] are have a meaning
DW> other than their clipped values.

Yes, they do. But those values are still outside the gamut.

>> All light sources stimulate the optic nerve. Not sure what you are
>> trying to say here. Are you referring to additivity?

DW> But if you only add three components, the cross talk between receptors
DW> means that you will never be able to get all combinations of output
DW> from the three types of receptor.

Sorry, this is nonsense. The receptors in the eye do not so much have
'crosstalk' as huge overlap, which is why they immediately form
difference signals at the next level of retinal processing. The number
of components used to produce the light source is orthogonal to the
number of receptors in the eye, and they can be of any color.

DW> THe reference to light sources was because the colour of dye based
DW> reflective systems depends on the detailed spectrum of the illuminating
DW> source, making a source, where everything is controlled a simpler case.

As is this. All colors, whether reflective or emissive, present a
spectrum of light to the eye.

-- 
 Chris                            mailto:chris@w3.org
Received on Thursday, 13 March 2003 05:23:15 GMT

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