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the Blue channel. Clicking on the Red channel will show you
a light grayscale version of the image, as shown here.

This is because there are so many warm tones in the
image. So, the brighter the channel is, the more of that color
frequency is used in the mix of those pixels. Because this
image is mostly warm sienna hues, it makes sense that the
Red channel would be very light. There are lots of red values
in there. (Shameless plug here: If you want to learn more
about channels, check out my new course on KelbyOne,
A Guide to Commanding Color.) This means that the Blue
channel shouldn’t be as bright, because there isn’t much
data from that color frequency in the mix of these pixels.
You can see the Blue channel for this image here, and you’ll
notice how much darker the shades are.

Neither of these channels on their own give us a great
starting point for a mask without doing a lot of work to adjust
the contrast and then converting it into a mask. But, if we
use Calculations, we can experiment with those two chan-
nels as a source, and explore the options available to produce
a good, 80% done, starting mask. One major advantage is
that it previews the mask while you make adjustments, giv-
ing a clear indicator of the quality of the mask.

We’re using the same layer for both sources from our image
file, but we’re choosing the Red channel for Source 1 and
the Blue channel for Source 2. The nuance here is that we’re
using the Invert option on the Blue channel.

This, mixed with the Subtract Blending mode gives us
a negative effect from the two channels that also carries
a high degree of contrast. We can then modify or adapt
this by adjusting the Offset and Scale settings to increase or
reduce the contrast and blending.
Note: The Offset value lets you lighten or darken the
pixels in the destination channel by any brightness value
between +255 and –255. Negative values darken the image;
positive values lighten the image. The Scale factor may be
any number between 1.000 and 2.000. Entering a higher
Scale value darkens the image. Learn more about the
math of this here.
The choices for the Result option at the bottom of the
Calculations dialog are to make this a new document, new
channel, or a selection. We’ll choose New Channel and click
OK, which means that Photoshop will make it an Alpha
channel, a saved selection that you can find in the Channels
panel (shown here).
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