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[CSS4 color][CSS4 transition] color pre-multiplied vs non pre-multiplied was (Re: [CSSWG] Minutes TPAC Tue 2012-10-30 AM I: Abstract Directions, Transforms, Transitions)

From: Alan Gresley <alan@css-class.com>
Date: Thu, 22 Nov 2012 21:39:56 +1100
Message-ID: <50AE00FC.5020107@css-class.com>
To: fantasai <fantasai.lists@inkedblade.net>
CC: "www-style@w3.org" <www-style@w3.org>
On 14/11/2012 5:58 PM, fantasai wrote:

>    <dbaron> http://dev.w3.org/csswg/css3-transitions/#animatable-types
>    dbaron: In section on animation of property types:
>    dbaron: colors in pre-multiplied space?
>    tab: I think we wanted to use pre-multiplied in all cases.


>    tab Need to be consistent with gradients, etc.
>    dirk: And with SVG
>    dbaron: But SVG 1.1 had opacity and color on separate properties
>    dino: Still has the same problem of interpolating in 4 channels.
>    dbaron: Gradient says pre-multiplied.

And what is seen on the right of the below screenshot is the result of 
pre-multiplied with a test where all colors are in the grayscale.


>    dbaron: Some OS's don't give you that.
>    dbaron: I'd be happy with pre-multiplied.
>    Rik: Prefer non-pre-multiplied.

I prefer two keywords. One being 'transparent' and the other being 
'pre-transparent'. Then authors have a choice and possibly the chance to 
learn that every single color point (over 16 million) has a 
corresponding transparent-color point.

>    Rik: Better for SVG and Canvas.
>    Tab: (How did Canvas end up different, I wonder...)
>    Tab: Because CSS gradient has been pre-multiplied for a while.

But still FF17, Chrome 23 and Safari 5.1.7 show gradients in non 
pre-multiplied for what is now longer that a year since it was resolved 
that they shouldn't because supposedly authors may not understand that 
'transparent' is shorthand for 'transparent-black'. The reasoning behind 
this resolution is ill founded.

>    Dino: benefit of pre-mul is you don't gray when animating to
>          transparent. And can solve it by going to rgb(...)

You do *NOT* go to gray with pre-mul unless the initial color is white 
or somewhere between white and black (within the grayscale).

If you going from color (not belonging to the grayscale) to transparent, 
you go to a color that has less saturation. It may look supposedly wrong 
if the composite color (part of which is the background color) is whitish.

At 25% and 75% in the below testcase are the midpoints between color and 
'transparent color'. Which one of these midpoint are gray?


>    <oyvind> I believe we encountered issues on the web when we did
>             non-pre-multiplied transitions
>    <oyvind> hovering comments on youtube looked weird, for instance

This is because the authors used transparent (or transparent black) 
instead of transparent-color. Advocacy would do wonders.

>    Tab: Can add some color stops.
>    Tab: But SVG is adding mesh gradients and you cannot do the same trick.
>    dbaron: I feel more strongly about animations being pre-multiplied than
>            about gradients.

I agree fully here. Transitioning of color is totally different from a 
gradient of color.

>    dbaron:. If an animation from/to transparent is ugly, that is a problem
>    Rik: Transparent is black, that is the problem


>    sylvaing: That's why we ended up with pre-multiplied, isn't it?
>    dbaron: If you animate from green 20% opaque to 100% opaque red.
>    dbaron: not the same issue as going through gray, but in
>            non-premultiplied space, the green will first get deeper
>            before fading.

The green (or lime) does not get deeper. At 50% it is close to olive and 
at 75% it is close to orange. All that is changing is the hue.

>    <leaverou> dbaron’s example http://dabblet.com/gist/3979232
>    dbaron: Our SMIL anim code is using pre-mul, I'm pretty sure.
>    dino: One old proposal was to transition in hsl.
>    * sylvaing doesn't think author expect transparent in a transition to
>               always imply going through black shades, whatever the
>               normative definition of the keyword says. Makes the keyword
>               somewhat useless in this context.
>    dbaron: I have a test case:
>    <dbaron> http://dbaron.org/css/test/2009/transitions/transitions-alpha
>    tab: webkit is non-pre-multiplied.
>    dbaron: FF is pre-multiplied.
>    dbaron: So actually everybody is doing pre-multiplied after all.
>    tab: [checking]
>    lea: Did FF change?
>    dbaron: No, we always did.
>    dino: So we all do the same. Let's specify it.
>    tab: Yes, chrome does pre-multiplied, too.
>    [dbaron adding a test]
>    RESOLVED: Animations of colors are in pre-multiplied space.


Alan Gresley
Received on Thursday, 22 November 2012 10:40:24 UTC

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