How to change saturation on a curve in RGB mode, in Photoshop?

Question

In Lab mode it's easy to change saturation independently of luminosity, by using curves on the *a* and *b* channels, but how can you do it in RGB mode? In RGB mode it's still easy to change luminosity independently of saturation by chosing **luminosity** layer blend mode. Photoshop also has a **saturation** layer blend mode, however, how do you get a *saturation map* to blend with? Of course you can use a duplicate layer that you do your adjustement on, and using saturation blend mode you'll only get the change in saturation, not in luminosity, but unlike Lab mode, you *can't grade on a curve*, so to speak. You can't put a steeper curve on the saturated colors while keeping more neutral colors closer together (opposite of vibrance, basically), or can you? Is there a way to get a *saturation map* from a Lab copy of the document, and use that somehow in RGB mode?

Answer 1

Yoi could always use the saturation layer type