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Re: [css3-transitions] colour space used for colour interpolations

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Sat, 26 May 2012 13:49:51 -0700
Message-ID: <CAAWBYDCwTLWP4PJoUucpo0tmxkV=28W9C2+d4J2TXDE05hPqZw@mail.gmail.com>
To: Rik Cabanier <cabanier@gmail.com>
Cc: Cameron McCormack <cam@mcc.id.au>, "www-style@w3.org" <www-style@w3.org>
On Sat, May 26, 2012 at 12:37 PM, Rik Cabanier <cabanier@gmail.com> wrote:
> On Thu, May 24, 2012 at 11:47 PM, Tab Atkins Jr. <jackalmage@gmail.com>
> wrote:
>> On Thu, May 24, 2012 at 9:44 AM, Rik Cabanier <cabanier@gmail.com> wrote:
>> > Reading a bit more on the wikipedia article [1] on HSL, I came across
>> > the
>> > following quote:
>> >
>> > Because hue is a circular quantity, represented numerically with a
>> > discontinuity at 360°, it is difficult to use in statistical
>> > computations or
>> > quantitative comparisons: analysis requires the use of circular
>> > statistics.
>> > Furthermore, hue is defined piecewise, in 60° chunks, where the
>> > relationship
>> > of lightness, value, and chroma to R, G, and B depends on the hue chunk
>> > in
>> > question. This definition introduces discontinuities, corners which can
>> > plainly be seen in horizontal slices of HSL or HSV.
>> >
>> > I believe it doesn't make sense to transition in HSL because of these
>> > issue.
>> > Who would want to create such a synthetic transition?
>>
>> Heh, who would ever want to transition in RGB?  It's infinitely worse. ^_^
>
> How so?

In the obvious way?  I'm confused.  In the quoted segment above, you
cite a small issue with HSL (the presence of "seams" in the hue every
60deg as a reason why you feel it doesn't make sense to transition in
HSL.

Transitioning in RGB is worse in every way.  It doesn't preserve *any*
quality of the endpoint colors when transitioning.  The RGB cube has
almost nothing to do with human eyes or perception, and straight lines
through the cube produce nonsensical color transitions.  Gray is *in
no way* halfway between green and blue, in *any* reasonable and
intuitive system.  But RGB produces that as a matter of course.


>> Ultimately, what matters is what the system graphics libraries
>> support, or can be extended to support.  HSL is a pretty shitty
>> colorspace, but it's simple to work with, and way better than RGB for
>> most things.  If we can convince the implementors working on graphics
>> to do a better colorspace, awesome, but if we can't, falling back to
>> HSL is acceptable in my mind.
>
> Even though we offer HSL as an option in our apps, interpolation is always
> done in the native colorspace (RGB or CMYK).
> So, I feel that it's a little odd that you transition through the hues.

I don't understand what you mean by this.  Are you saying that people
never choose to transition in HSL?  Or are you saying that you allow
colors to be specified in HSL, but always transition in RGB or CMYK?


>> (The major problem with HSL, more than anything else, is that there's
>> no analog of "premultiplied" colors like RGBA has, so transitions
>> to/from 'transparent' are going to be ugly.  I don't think any of the
>> better colorspaces have a solution to this either.)
>
> Split out the alpha. Then transition RGB and alpha separately and reapply
> the alpha on the transitioned colors.
> This is how we do it in InDesign, Illustrator and Photoshop and I haven't
> heard of any complaints about quality.

This doesn't help with transitions to/from 'transparent', the most
common transparent color.  It's also inconvenient when defining
gradients, for example - going from green to transparent to blue
requires *four* color-stops, as you have to double up the
"transparent" one with two different hues so each surrounding color's
transition looks natural.  This hack is *impossible* to do when
creating 2d gradients like the mesh gradients we're talking about for
SVG.

In general, RGB-space transitions are done with premultiplied colors
in CSS.  We won't be changing that.  It would just be nice if there
was some similar transformation that could be done for transitioning
in HSL or a similar colorspace.

~TJ
Received on Saturday, 26 May 2012 20:50:40 GMT

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