SELECTIONS BY CALCULATION
> PHOTOSHOP USER>^ APRIL 2020[ 88 ]
In this issue, we’ll discuss features that are lesser known in Photoshop. Thanks to Dave
Williams, our resident travel photography guru, for pointing out that I haven’t shown Calculations
for making Selections up to this point. Calculations have been around for a really long time and were
a well-kept secret by professional editors for many years.Like so many selection techniques in Photoshop, Calcula-
tions is a way to build a selection or mask through a focus
on channels. Found under Image>Calculations, it allows you
to choose how to blend together multiple channels, while
using blend modes to influence the result of the blend. Let’s
break down the dialog to help you get started.
In the dialog, you can see there are two sources to blend
with. If you want to reference another document as the
source, it must be the same pixel dimensions as the file in
which you’re applying Calculations or it won’t be available
as an option. But if you’re only referencing your working
document, you can choose different layers as your source, or
the same layer, and at the bottom, you can choose a blend
mode to help process the blending result.Another important detail is that you can choose to invert
the source from a specific channel (Gray, Red, Green, or
Blue) of the layer from which you’re sourcing. If you’re
familiar with loading a channel as a selection, this means
you could use a positive source selection from a layer’s
Red channel and also have an inverted reference from
its Blue channel to help produce the selection. Flipping
these choices, inverting them, and adding a blend mode
or Opacity value gives you a wide range of options to try
to generate a good starting point for a high-contrast mask
that captures an area you want to select.AN EXAMPLE
For our example, let’s select and dye our subject’s hair.
(If you’d like to follow along with the image here, c lick
this link to download it for free.) This image has a lot of
very similar hues in the picture. Although the Quick Selec-
tion tool, mixed with the Select and Mask controls, such
as the Refine Edge Brush, could do a decent job with this,
it might not actually be the fastest way, because it would
involve a lot of back-and-forth refinement.In the screenshot here, we’re showing the Subtract blend
mode. Unique to Calculations, we have access to two addi-
tional, less common blend modes in Photoshop: Add and
Subtract. These are the powerful hidden gems of this tech-
nique because, when you choose one of them, you can refine
the result by adjusting the Offset and Scale values (more
about this later in the article). These options aren’t visible in
the dialog unless you’re using these two blend modes. You
don’t have to use these two options and can rely on more
common blend modes, such as Multiply, Color Burn, Screen,
or Overlay, but you should definitely try Subtract or Add as
an option because of the extra refinement options.Our goal with any starting selection is to get 80% of
the way to a good mask as fast as possible. If we review
the channels (Window>Channels) for this image, we can see
that tonally, there’s a good difference between the Red andMARKHEAPS
The Perfect Selection
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