Make luminosity and RGB curve interchangable (with proportional control).

What problem do you see this solving?

Adding a curve to an image can take some fiddling about to tune correctly with numerous control points. Sometimes after making an RGB curve (for example), I realise it should be a luminosity curve instead (or visa versa). Or I may just want to test if it would be better as the other curve type. Currently the only way to change the curve type is to recreate the curve as another type. Some curves can be complex to get right, so this becomes tedious. There are other times I find the best curve type would actually be in between RGB and Luminosity.

My request is to add a slider (or similar) with each curve which could change the proportion between RGB and Luminosity in any proportion you like. By default, there could still be an RGB and Luminosity curve, the difference would be that they could be transitioned to the alternative type with a slider (or similar). As a minimum, there would be a way to change the RGB curve to Luminosity, or visa versa, but I would find a proportional slider much more useful as the best position is often in between.

The only way to simulate the in-between position is to make two curves of half strength, one of each type but this isn't practical for more complex curves. Or the saturation slider can be adjusted, but this is also not ideal as the result is probably different and one big reason for having RGB/Luminosity curves is to be able to adjust tonal values without having to then make separate adjustments to the saturation whenever a curve is moved.

When was the last time you were affected by this lack of functionality, or specific tool?

Quite often when using curves. Working with art reproduction I find this more important than with regular images, but I think it would have good creative uses with regular images and save the user from having to re-adjust saturation when curves are adjusted.