On 11 October 2013 13:25, "Stefan Böttner" <virtual...@gmx.de> wrote:
> Now my point is this: You can think of this as non-uniformly scaling a
> circle in the color plane. You are able to specify scaling factors in a-
> and b-directions independently, causing your circle to become an ellipse.
> However, this way the principal axes of your ellipse will always remain the
> a- and b-axes, but more generally an ellipse could be rotated as well. We
> are currently unable to specify this rotation.
>
I get your idea. It's like singular value decomposition of a linear
transform vs only its scaling matrix. It is certainly more powerful. In
other words, it's hue rotation combined with chroma change vs just chroma
change. I'm not sure mixing these two modifications in the same tool is a
good idea usability-wise. Hue rotation is an orthogonal concept and belongs
to a separate tool, while ab curves should be used only to adjust chroma
(colorfulness). In fact, it is probably possible to achieve what you want
by combining ab curves and doing a uniform hue adjustment in colorzones.
Hue is supposed to be atan2(b*, a*).
The user needs to have reasonable expectations about tool's effect. Will
rotated color axes a' and b' be intuitive enough? The Lab curves themselves
are hard to master, but there are books and papers written about their use.
What's going to be the effect of applying a 2x scaling to a* rotated 15°?
The user needs to be able to choose tool's parameters based on observed
image and his mental previsualization of the desired result. Given some
color and targeting another color, what hue rotation angle he should
choose? How it would affect other colors? Adding another parameter to Lab
curves increases the dimensionality of the parameter space, without giving
visual guidelines to choose the right set of parameters.
Is it going to be used mostly for small rotation angles (slight hue
adjustments) or mostly large rotation angles (let say 45°)? The former case
is likely well serve by existing color correction tools applied
before/after standard Lab curves.
I suppose, it's necessary to review use cases for such a tool. Like real
life photos, and see if the desired result cannot be achieved with more
conventional and existing tools.
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