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

From: David Woolley <david@djwhome.demon.co.uk>
Date: Wed, 12 Mar 2003 21:36:33 +0000 (GMT)
Message-Id: <200303122136.h2CLaX401598@djwhome.demon.co.uk>
To: www-style@w3.org

Chris Lilley wrote:
> 
> On Wednesday, March 12, 2003, 8:22:26 AM, Boris wrote:
 
> mod 360, so 370=10 and so on

The algorithm provided needs to be modified, as it only works for
hues in the range [-120, +480).

> 
> BZ>  Further, how are saturation and lightness
> BZ> values outside the 0% to 100% range treated?
> 
> clipped

Do you really mean this?  This seems to give HSL a smaller gamut than
sRGB.  There are real life colours (e.g. the extreme violet end of the
spectrum) that require more than 100% saturation (they have a negative
sRGB green component, because the normal red and blue phosphors++ cross-talk
onto the eye's green receptors).

Also looking through it, I noted these points:

- it's inaccurate to say that the angles represent the rainbow colours;
  there are hues (reddish purples) that it can represent, but which are
  not rainbow colours;

- when one looks further, the whole concept of a circle  is somewhat 
  misleading, as what is actually represented (at l=0.5 and s=1.0) is
  a constant speed walk (in sRGB gamma) along all six edges of the colour
  cube in turn  (the walk has to be rather lumpy in subjective hue);

- I think there should be a prominent warning that subjective brightness
  depends on both hue and saturation (local peaks, as a function of hue,
  at 60, 180, and 240, with an envelope peaking around 120, and with 
  l=0.5 s=0.0 a lot darker than l=0.5 s=1.0, even for the hue minima, 
  because of gamma issues) - this might confuse people trying to obtain
  good contrasts without relying on hue;

- I've not convinced myself why gamma issues don't make subjective hue
  a function of saturation, but, assuming that there were no spurious
  gamma adjustments in the illustrations, I guess its OK;

- I think that the description of opacity may have been lifted from SVG,
  losing the context which included the algorithm, and not gaining a 
  hyperlink on the reference to masks - I am sure that transparency 
  requires forward and inverse gamma to be applied, around the actual
  merging.

++ any attempt to linearly combine three components in a physical system
   that doesn't directly stimulate the optic nerve is going to suffer
   from this sort of problem, even in a light source.
Received on Wednesday, 12 March 2003 16:37:06 GMT

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