Its been way too long since I posted a new tutorial, so to usher in the new year, I submit this step-by-step video on using Lab channels as the source for luminosity masks. This type of masking is all the rage right now, and you can find many tutorials about the general procedure all over the internet. There are even action that automatically generate multiple, mostly useless, layers with contrast variations in masks from the RGB channels. My humble offering here is a bit different in that I show how to use a mask generated from the “b” channel of Lab, to isolate a subject based on color rather than tone!
Here is my base image:
This exposure works overall, especially for the sky! The mood is about what I want, though is is a bit under exposed for the trees and houses. Fortunately I made another exposure for the foreground:
Now… I could do a simple blend with a soft layer mask, replacing the dark foreground with the lighter exposure, but the result would have the sky directly behind the trees too bright. Using, say, the blue channel to generate a mask would provide a very detailed mask for the entire foreground, but it ends up being very difficult to keep from showing light halos around all the little tree branches—here’s what a blue channel mask would look like:
At first glance, this looks like it would be an ideal solution, but the result looks fake!
A close up shows that the bare tree branches have a subtle washed out look – manually editing the mask just makes everything worse!
I decided that what I really wanted, was to isolate the yellow and red leaves of the trees, and let the tree branches go dark. This may seem like a difficult task, and it certainly would be, if I were to look for a starting place in the RGB channels. But by making a copy in Lab, it becomes very simple to end up with a mask that does exactly what I want. Additionally, editing the color in Lab gives me a great way to enhance the saturation and color of the leaves that really enhances the image – here’s the final result:
I prepared a video showing the complete step-by-step process with a little bonus on selective sharpening that I think you will find useful, here: