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

On Sat, May 26, 2012 at 11:49 PM, Tab Atkins Jr. <jackalmage@gmail.com>wrote:

> 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.
>

That is not the only reason but I think transitioning hue if there are
discontinuities will not give a desirable effect.
I'm not sure if this is a small issue...


>
> 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.
>

Did you mean green and RGB(255, 0, 255)?
Just green and red will produce RGB(128, 128, 0) which isn't gray.

I haven't heard anyone complain about interpolating colors. Maybe you're
thinking of the muddy appearance of gradients because of the sRGB transfer
curve?


>
>
> >> 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?
>

You can specify them in HSL (both in the app and in the model), but if you
transition or manipulate them, they will always be converted to RGB first.
Otherwise, a gradient with 2 HSL color stops with different hues, will
display a bunch of intermediate colors. For color conversion, there are no
HSL profiles since it's just a simple formula on the raw RGB values, which
is why it's always converted to RGB.


>
>
> >> (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.


Really?
I've attached a simple gradient that has a transparent midpoint. Does it
not look natural to you?


> This hack is *impossible* to do when
> creating 2d gradients like the mesh gradients we're talking about for
> SVG.


Hopefully meshes won't use premultiplied alpha... Do they allow alpha?


>
> In general, RGB-space transitions are done with premultiplied colors
> in CSS.  We won't be changing that.


I know, It's unfortunate that that decision was made but I don't feel like
starting that fight over again...


> It would just be nice if there
> was some similar transformation that could be done for transitioning
> in HSL or a similar colorspace.


Yes! Let's add Lab and have the color interpolations separate from the
alpha there.