Vermont House
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Lab Luminosity Masks

Editor’s Note: Lee presents a very powerful way of enhancing color and sharpening an image in this post. The video that concludes this post is an excellent tutorial with step by step instructions and explanations. These are advanced techniques. 

Here I submit, for your consideration, 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:

Farm house with trees-1
Starting point from a shot done in Vermont this past fall

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:

Landscape with trees-2
The lighter exposure works for the trees, but detail in the sky is lost.

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:

Blue Channel Mask
Blue Channel Mask.

At first glance, this looks like it would be an ideal solution, but the result looks fake!

Looks Fake
Combined version looks fake – too washed out… somethings not right.

A close up shows that the bare tree branches have a subtle washed out look – manually editing the mask just makes everything worse!

Washed out tree branches!
A Close up of the file shows weirdly washed out tree branches against the sky.

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:

Landscape Final
The final result has just the right sunset mood with the bright fall colors and red brick of the house enhanced.

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:



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