The Calculations tool is similar,
but operates on any channel from
any layer in any open (compatible)
document. The output is either a
selection, a new alpha channel, or
a new document. Calculations is
aimed at creating masks and selec-
tions and doesn’t return a blended
layer to your document. Those
familiar with making luminosity
selections directly from channels
should see a lot of benefit from
this dialog, because it gives you a
preview of the operations before
committing them to an alpha
channel. Calculations are also far
more flexible than Boolean chan-
nel operations, but not an exact
replacement; both techniques are
important to know.
To lighten the apartments in this
image, I duplicated the Background
layer and set the duplicate layer to
Exclusion blend mode, then used
Calculations (Image>Calculations)
to take the difference between the
Blue channel of the Merged result
(that is, what’s visible on the can-
vas) and the inverted Blue channel
of the Background layer.
The Blue channels had the least
amount of spill from the various
light sources, giving me the best
chance at not introducing halos in
the mask. The result was returned
as a new Alpha 1 channel. I loaded
that channel as a selection and
added a Curves adjustment layer
to turn it into a Curves mask. Then,
I turned off the Exclusion layer and
adjusted the Curves settings. [For
more on Calculations, see “The Per-
fect Selection on page 88 .—Ed.]
There are untold riches to be
had in both of these features, but
they do require a little planning
and patience. With experience,
you can develop recipes that will
quickly get you some amazing
results and effects.> PHOTOSHOP USER>^ APRIL 2020[ 58 ]