A "Layers" question re: Local Contrast Enhancement

[Pirate]

I found the tutorial on local contrast enhancement very interesting, and decided to try it out on a picture I'm working on. I like the results a lot: subtle, but a noticable improvement. The following caught my eye at the end of the tutorial though:

Local contrast enhancement, as with sharpening, can also create unwanted color changes if performed on all three color channels. In addition, Local contrast enhancement can increase color saturation significantly.

All fair, something to look out for..... but wait, there's a tip to avoid the colour changes:

You can eliminate these unwanted effects by either performing local contrast enhancement in the lightness channel of the LAB color space, or in a separate layer (while still in an RGB working space) and blending using "luminosity" in the layers window.

Now I don't know anything about LAB colour, but I've had to learn about using Layers to put together a few different develops of a single RAW, in order to come up with a final image, as explained here, so I figured the second optios is probably the better one....

...BUT....

WHAT's supposed to be in the layer that I blend with? I tried a copy of the image but that didn't seem to be the right thing, I tried a neutral grey, but logical reasoning tells me that won't work, it has to be something based on the original image? What about a copy of the green channel, since it's histogram generally follows the luminosity histogram closest? Or am I way off track

I'd really appreciate some help!

:- P

[dendrophile]

Pirate,

I've got a small tutorial on using the LAB colour space, including local contrast enhancement at http://www.grahamd.co.uk/lab/lab.htm.

Note that this does not answer your question starting with ...BUT... :-)

What I think the luminosity blend thing is about is to create a new layer and copy your photo to it. (IE duplicate the background layer into a new layer.) Apply the local contrast enhancement to this new top layer, and then set the blend mode for this layer to "luminosity". This has the effect of taking the intensity from the new layer and applying that to the photo in the bottom layer.

HTH,
Graham

[Colin Southern]

Now I don't know anything about LAB colour

LAB Colour is great for certain types of images - far easier and more logical in my opinion.

If you're keep to pursue it then "The Canyon Conundrum" by Dan Margulis has become the industry standard text on the subject.