Luminosity masks are nothing more than selections based on the brightness values of the image. The basic luminosity mask, called "Lights", selects pixels in proportion to their brightness in the image. There are three different ways to create this basic luminosity mask (Figure 1). I'll list all three alternatives, but you only need to do one of them to create the selection.
The marching ants appear on the image to define the selection (Figure 2). Since Photoshop only puts marching ants around pixels that are at least 50% selected, and since this selection is based on the brightness of the image, the marching ants only appear around pixels that are equivalent to 50% gray or brighter. I'll discuss this in more detail later. For now it should be obvious that the marching ants are enclosing the lighter areas of the image.
Once you've created the selection of the light areas of the image, it's a good idea to save it. To do this:
That's it! This is all that's needed to create the luminosity mask for the light areas of the image. It took a bit of writing to explain, but once you try it you'll see how easy it really is.
Technically, what was actually created in the above steps was an alpha channel of the selection. It won't actually be a mask until it's applied to a layer in the Layers palette. However, masks and alpha channels look entirely the same, they just reside on different palettes, so I'll be using the term "mask" to describe both from here on.
|YOU ARE HERE—The Basic Mask—"Lights"|
|Using the "Lights" Mask|
|Different Masks for Different Tones|
|Experimentation is the Key|
|Getting the Actions|