I'm using Vegas Movie Studio 6, and I was wondering if there is a way to adjust the luminance of the image independantly from the chroma. I'd love to be able to use the curves fx to adjust the luminance without affecting the color/saturation of the image, and vice versa.

1- You can use levels to do that.

Any curviness in the curve will cause subtle shifts in hue/saturation (whereas if you have straight lines in it you won't).

Levels tutorial:
http://www.vasst.com/resource.aspx?id=a7a8c403-64dc-420d-97d0-90d2f8de9fc1

But it will, since luminance is a linear function of all three colors. This means you have to hold that ratio between the colors constant if you do not want to affect the color.

It's one of those omissions that leaves you stratching your head. Especially since video signals are natively in some form of Y'CrCb to begin with. It seems so obvious to me.

Emre: I don't think I'm following you.

adjusting levels or curves does change the saturation. The work around I do right now is I use the curves to adjust for the luminance I want, then I add a saturation effect to try and reduce the saturation change that occurs. It's not terribly important, but in after effects I like to use two adjustment layers over the footage. One for color, and another for luminance. It would just be great if I could find a way to mimic that in vegas.

Technically, if you want the color not to change, the hue and saturation have to be fixed, right? For example, if I increase the overall gamma, every pixel will be modified by the following formula:

(R',G',B')=(R^gamma,G^gamma,B^gamma)

but this will alter the ratio R:G:B, which is what determines the color.

Three equations, 3 unknowns...solve for gamma.
What's the problem.

The problem is that R:G:B is not the same as R':G':B'. What's needed is a Fade To Luminosity feature like Photoshop.

Emre: Ah, got it.

If you don't touch the gamma slider, you can do simple exposure adjustments.

2- The luminosity composite mode in AE+Photoshop, combined with curves, lets you alter stuff without affecting hue and saturation (kind of).

It's not altering luminance exactly.

The problem is we're talking about Vegas :)