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Re: [css3-images] Proposed Gradients changes

From: Chris Lilley <chris@w3.org>
Date: Wed, 24 Nov 2010 17:37:13 +0100
Message-ID: <1944658140.20101124173713@w3.org>
To: Alan Gresley <alan@css-class.com>
CC: "Tab Atkins Jr." <jackalmage@gmail.com>, Anne van Kesteren <annevk@opera.com>, Brad Kemper <brad.kemper@gmail.com>, www-style list <www-style@w3.org>
On Wednesday, November 24, 2010, 2:40:14 AM, Alan wrote:

AG> On 24/11/2010 12:00 PM, Tab Atkins Jr. wrote:
>> On Tue, Nov 23, 2010 at 4:41 PM, Alan Gresley<alan@css-class.com>  wrote:
>>> This may be true for current display devices that shows a sRGB gamut but
>>> some display devices now can show scRGB. Also future displace devices may
>>> have much better gamma correction.

>>> I would like to ask you a simple question. At what point does the below
>>> gradient use imaginary colors of scRGB gamut?

>>> background: linear-gradient(left, rgba(-20, 270, 250), transparent);

>> The transition here is obvious and doesn't care what colors you are
>> specifying.  You just first map both colors into premultiplied color
>> 4-tuples, which gives you (-20,270,250,1) and (0,0,0,0).  Then you do
>> a naive interpolation, and map the results back into sRGB.

AG> Really! What about on a display devices that uses scRGB? Mapping colors
AG> from scRGB into sRGB 

At no point in CSS processing are colours in a device gamut mapped *into* sRGB.

Once the colours are calculated, if the display is calibrated then the colours can be mapped *from* sRGB into the device gamut.

AG> will cause the start of the transition to be within
AG> the sRGB gamut. Is this what we desire with scRGB displace devices?

>> Then, at actual value time, you map any colors outside your device's
>> gamut, if this is known, to colors inside the gamut. 


>>  CSS doesn't
>> define how this must be done, and so it is recommended that you only
>> use colors within the gamut defined by sRGB (that is, only colors with
>> components between 0 and 255).

AG> CSS doesn't have to define how it's done. A display device that only 
AG> shows a sRGB gamut will clamp  (-20,270,250)  to  (0,255,250). I'm 
AG> talking about what happens along a gradient in a display device that 
AG> support a scRGB gamut.

Tab already gave the answer to this question.

If the sRGB value (-20,270,250) falls within the device gamult it will be displayed, otherwise it will be clipped so that it falls inside the device gamut.

 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 Wednesday, 24 November 2010 16:37:23 UTC

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