Adding linear interpolation in addition to cubic interpolation

[ronan-boitard]

In some context, it might desirable to interpolate linearly rather than using the cubic function. For example if you want to approximate a LUT.
I would suggest to add a flag indicated linear interpolation in addition to the cubic interpolation.

[ccameron-chromium]

I would want to see an example where linear interpolation of the gain curve (which is in $\log_2$ domain) is preferable. I wouldn't imagine it to perfectly approximate a LUT in linear domain or signal domain.

[ccameron-chromium]

With respect to the efficiency of linear interpolation versus cubic interpolation:

• The slowest part of sampling the curves is finding the interval that contains the input
• The evaluation of the cubics requires ~3 multiply-adds versus ~1 multiply-add for linear, both of which are negligable
• The cubic coefficients in formula in the spec can be pre-computed, and so the actual cubic evaluation is fast.

I also suggest that shader-based implementations use uniform buffers for the control points (rather than evaluating the curve and putting that in a texture).