[x3d-public] Which color space for Background color interpolation

[Michalis Kamburelis]

CGE implementation takes care to interpolate hue along the shortest
path (i.e. sometimes clockwise, sometimes counter-clockwise). See
https://github.com/castle-engine/castle-engine/blob/master/src/base/castlecolors.pas#L373
.

Indeed the code is a little ugly, needs to look separately at 3 cases:

- if HueDiff > 3 then
- f HueDiff < -3 then
- otherwise

Regards,
Michalis

wt., 13 paź 2020 o 06:32 Andreas Plesch <andreasplesch at gmail.com> napisał(a):
>
> While trying to linearly interpolate in HSV space, a question came up
> for me with regards to Hue. Hue is represented as an angle in a color
> circle. This means there are two ways to linearly interpolate, in the
> acute segment defined by the two hues, or in the obtuse segment. Say
> the first hue is 10 degrees and the second 300. At a fraction of 0.5,
> the interpolated value may be 155 ( in the obtuse segment ) or 335 (
> in the acute segment ).
>
> One rule may be to always go counterclockwise in the circle, eg. from
> 10 to 300 in the example. But then it is hard to predict if the
> interpolation between two similar shades of a color (say for a blue
> sky) results in similar shades, or actually goes around most of the
> full color circle, generating a very colorful result.
>
> So a more reasonable rule may be to always interpolate in the acute
> segment, clockwise or counterclockwise as necessary. I think slerp
> always takes the shorter path as well.
>
> I hope that there is a more straightforward interpretation of what it
> means to interpolate in HSV space, and I just overlooked something.
>
> -Andreas
>
> On Mon, Oct 12, 2020 at 6:17 PM Andreas Plesch <andreasplesch at gmail.com> wrote:
> >
> > The Background specification in
> >
> > https://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/enveffects.html#Backgrounds
> >
> > states that
> >
> > "The sky colour is linearly interpolated between the specified
> > skyColor values." and
> > "The ground colour is linearly interpolated between the specified
> > groundColor values."
> >
> > The ColorInterpolator specification in
> >
> > https://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/interp.html#ColorInterpolator
> >
> > states that
> >
> > "A linear interpolation using the value of set_fraction as input is
> > performed in HSV space."
> >
> > The ColorInterpolator language is more precise in giving the space in
> > which linear interpolation should be performed.
> >
> > I think there is a need to also become more precise for Background
> > color interpolation. The consistent approach would be to choose HSV
> > space there as well:
> >
> > "The sky colour is linearly interpolated in HSV space between the
> > specified skyColor values." and
> > "The ground colour is linearly interpolated in HSV space between the
> > specified groundColor values."
> >
> > HSV space probably also leads to more aesthetically pleasing backgrounds.
> >
> > [x3dom currently performs all color interpolation in RGB space].
> >
> > Do we need such a clarification ?
> >
> > -Andreas
> > --
> > Andreas Plesch
> > Waltham, MA 02453
>
>
>
> --
> Andreas Plesch
> Waltham, MA 02453
>
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