curves: restore old DCT_Spline, add new DCT_CatmullRom type

[agriggio]

Hopefully this will make everybody happy. The new curve type (which I simply called "Custom new" in the GUI -- better names are welcome) is used only for histogram matching by default. Existing pp3's should pick up the old cubic splines without issues.

I coded this in about half an hour, so please test before merging (if you decide to merge)

[Beep6581]

We could call the cubic spline "Rigid" and the Catmull-Rom "Flexible", that would be pretty user-friendly.
Or... "Stiffy" and "Floppy" 😄

[agriggio]

As discussed on IRC, my vote for names goes to "Standard" for the old cubic splines and "Flexible" for the new ones. Regarding "Standard", I think cubic splines are sort of the de-facto standard when you talk about spline interpolation. In the photography/retouching world, it could be argued that "standard" is what the more popular tools do by default. I have never used Photoshop or Lightroom, but I just found this:

https://feedback.photoshop.com/photoshop_family/topics/point_editing_mode_for_tone_curve_control_hard_to_use_because_of_non_local_effects

Which makes me believe that the "point curves" are indeed cubic splines

[agriggio]

after a night of sleep, I think I can live with "smooth"/"sharp" if std/Flexi has no fans :-)

[agriggio]

OTOH, another argument in support of "Standard". Quoting from https://discuss.pixls.us/t/curves-behaviour-changed-in-development-version/10141:

In V5.4 stable release, it behaves like any other curves adjustment tool I’ve used, most notably Photoshop

[Thanatomanic]

Just my 2 cents here, I would call the old version 'Standard' and the new one 'Precise' or 'Detailed'.

[TooWaBoo]

I suggest to use a different curve icon too.

[Beep6581]

• Photoshop CS2
  
• RT cubic spline
  
• RT Catmull-Rom spline
  
• GIMP-2.8.10
  

As the Photoshop curve resembles RT's cubic spline, I'm fine with "Standard" for cubic spline and "Flexible" for the centripetal Catmull-Rom spline.

I committed a new icon, curve-catmullrom.png and curve-catmullrom-small.png

[agriggio]

Can this be merged? If so, should I merge on releases first and then port to dev?

[Beep6581]

@agriggio please merge to dev, then I'll cherry-pick to release-5.5.

[agriggio]

pushed