Yep, I found that in choosing a spline algorithm for my stuff. Since I’m math-challenged, I went with the easiest to use, a simple cubic spline class: http://kluge.in-chemnitz.de/opensource/spline/. The way I have it set up, it doesn’t behave symmetrically intersecting 0,0 vs 255,255, but I’m leaving that rumination for when I have more time, e.g., in retirement, probably…
My priority has been on insuring the displayed spline accurately represents the lookup application to the pixels. So, in old rawproc I use the same spline algorithm to build the LUT, in new rawproc I look up the spline interpolation for each pixel tone since the image data is float.
So, I think interpolation consistency between softwares is only realistic if we discuss the spline algorithms…