Add a hue curve transform

[doug-walker]

OCIO has a basic set of color correction transforms:

• GradingPrimaryTransform
• GradingToneTransform
• GradingRGBCurveTransform

However, when comparing to various software packages that allow color correction, there is one missing piece of basic functionality which is the ability to make HSV type adjustments using curves. For example, to have a spline curve that would allow moving the hue or saturation of a specific hue range.

In order to be compatible with OCIO, this would need to have two features that are often not found in these types of tools:

• The ability to perform well on scene-linear color spaces
• The availability of a closed-form inverse

The implementation should use dynamic parameters to allow adjustments to be made without creating a new processor object.

A note on scope: OCIO is not trying to emulate a dedicated grading application since there is no ability to use mattes to apply results selectively to one part of an image, or to represent animation. However, a set of basic grading controls has proven to be very useful since it allows simple color correction adjustments to be represented parametrically rather than needing to bake them into 3d-LUTs.