Re: [filter-effects] hue-rotate() and saturate() filters

On Mon, Oct 14, 2013 at 1:23 PM, Chris Lilley <chris@w3.org> wrote:

> Hello Rik,
>
> Monday, October 14, 2013, 8:42:46 PM, you wrote:
>
> > On Sat, Oct 12, 2013 at 8:53 PM, Michael Mullany <michael@sencha.com>
> wrote:
> > Has there been any thought of converting content to HSL before
> > applying these filter shorthands? Or adding true HSL primitives to
> > the filter toolbox? (Something like an feFuncH or feFuncS).
>
>
> > As Chris mentions, being able to switch the working colorspace to
> > Lab
>
> CIE Lab is a non-linear light, approximately perceptually linear,
> rectilinear colour space. Linear matrix operations in that space would
> not naturally be hue preserving or saturation/chroma preserving.
>
> As Michael pointed out, a polar coordinate space is a more natural fit
> there.
>
> > (not sure what CIELCHab is though)
>
> Rik, for someone who has implemented a color management engine in a
> commercial product you continually surprise me with what you do and
> don't know.
>

Really? Can you show me a commercial software product that uses this
colorspace?
A Google search reveals 31 webpages of mostly SVG mailing threads.
Maybe this is well known in academics?



> In 1976 the CIE produced two colourspaces - CIE L*a*b* and CIE L*u*v*.
> In normal conversation the asterisks are dropped and they are called
> Lab and Luv respectively.
>
> Polar forms of each were also produced, retaining the L axis and
> transforming a,b (or u,v) into a Hue angle H and a Chroma (similar to
> saturation) C. Because the two forms both have the same names for the
> three axes (L, C, H) they are disambiguated by a subscript ab or uv as
> appropriate - LCHab or LCHuv.
>

OK. Why should we push that complexity to the user?
CIELCHab doesn't sound like a different colorspace from Lab; it's more like
a simple transform (like hsl()/rgb() in CSS)


> Incidentally these polar forms were what inspired Tektronix et al to
> come up with HSL, HSV and similar polar forms of RGB. Unlike the
> measurement-based CIE forms, however, HSL and HSV are not perceptually
> uniform and have some odd behaviour - primary blue (#00F) and secondary
> yellow (#FF0) have the same lightness in HSL and the same value in
> HSV.


> > However, doing so would require a whole new set of formulas in the
> > filters specification and a lot of work in the browsers...
>
> Not really. In each case, you transform to the working colourspace,
> perform channelwise operations, then transform back.


The same channelwise operations? So the sepia, grayscale or saturate
formulas wouldn't change?
No, the formulas would have to be different.