Luminosity Masks in GIMP

I had previously written about using Luminosity Masks in GIMP, but had generated the masks in a cumbersome way.  Recently, I revisited the creation of these luminosity masks using channels in GIMP to more closely mimic the original tutorial by Tony Kuyper for Photoshop.

The basic idea behind luminosity masking is to generate layer masks for an image based on the luminosity of the scene.  In this way, you can easily obtain masks that correspond to specific tonal levels in the image, and the mask is self-feathering to preclude any problems with hard transitions.

For instance, here is the succession of lighter/darker masks as generated from an image:

Light, Lighter, and Lightest masks generated from an image. Dark, Darker, and Darkest masks generated from the same file.

These masks allow a user to isolate particular portions of the image for modifications independent of the other tonal regions if needed.  This is a very powerful and simple method for approaching steps such as split-toning, or color/exposure modifications to specific areas.

Of particular interest to photographers are the mid-tone masks.  This channel mask is the intersection of both the light and dark masks, so that emphasis for this mask is on the mid-tones of the image (the lightest and darkest portions of the image are excluded).

For example, here is the base mid-tone mask as generated from the same image above:

 Basic Mid-Tone mask

The effect of color modifications on a layer with this mask can be quite striking.  To illustrate, here is the base image these masks were generated from, and a version with a very aggressive curve applied to a layer with the base mid-tone mask:

The base image with no modifications.
Base image with a heavily curved layer, masked with a Mid-Tone mask.

As you can see, the aggressive curve on the Mid-Tone mask layer provided a boost to contrast and saturation across the entire image, without crushing dark tones too far or blowing light tones out (as the mask doesn't allow those tonal regions to be modified very heavily if at all).

For the complete tutorial walkthrough on creating and using these masks see:

Getting Around in GIMP - Luminosity Masks Revisited

The full tutorial also includes a GIMP Script to automate the process of creating these channels (making them trivial to create and use)!

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