masking is only an on or off proposal for certain areas:
Any content below 50% opacity isn’t included in the oper-
ations, and layer opacity doesn’t mean anything until it
gets below the halfway mark.
You may have seen stack modes used for noise
reduction, object removal, and star trails. In this month’s
“Photo shop Proving Ground” (p. 92 ), I put together a
synthetic galaxy using a star trails technique in which you
choose Maximum from the Layer>Smart Object>Stack
Mode menu. For noise reduction, the two popular mode
choices are Mean and Median. Since noise is considered
random, if you take several frames of the same stationary
subject (say, a night sky), the noise should be different on
each frame.
Mean mode averages the channel values of each pixel
on each layer, so if the noise is relatively dark compared to
the subject areas, it will become a dark, neutral color and
become easier to remove with contrast enhancement. If you
remember the old-school method of looking at each chan-
nel to choose the least noisy, and then using Channel Mixer
to blend the others back in, you’re familiar with the basics
of this mode.
Median, on the other hand, takes the middle value of
the range of pixel colors and brightness from each layer.
This is also good for noise removal, but is often used to
remove unwanted, transitory objects from an image.
Let’s say you’re taking photos of a public place and there
are always a couple of people in your frame. So long as
you can get the same areas unobstructed a few times, the
Median mode will use the information common to those
areas and reject the “differences” introduced by people,
cars, or other stuff. For a fantastic example of this, check
out David Williams’s
“Photo Effects” article
from August 2019 where
he uses the Median stack
mode in Statistics.
In each of the stack
modes, the layers repre-
sent information that’s
best understood in the
context of the entire
stack. Such tools are used
to compare changes over
time or other variable val-
ues (or to look to see if
changes exist). An exam-
ple might be examining
the change in color of a
bacterial sample as somechemical is added, or reflectance (as seen through changes
in brightness) of a plastic surface as temperature is varied.ZOOMIFY
Zoomify is a third-party plug-in included with Photoshop that
allows very large images to be effectively tiled so that they
can be displayed quickly on a webpage and then zoomed/
panned once loaded. Your main image is saved in progres-
sively smaller sizes, and then those sizes are split into tiles that
are loaded in the background. This trick allows extremely
large images to be displayed much more quickly as smaller
versions, then zoomed in and panned for more detailed
viewing. NASA uses this technique to display its huge pho-
tographs stitched from hundreds of smaller images.
It’s pretty easy to use, too. Simply choose File>Export>
Zoomify and fill out the options. Select a Template and
Folder, set the Base Name, Quality, etc., then c li ck OK.
When the files are created, you can open them up in a
browser to preview the results. The output is HTML 5 for> PHOTOSHOP USER>^ APRIL 2020[ 56 ]