› ›kelbyone.com047Background: ©Adobe Stock/picsfiveWhile this looks like a trivial example, it’s the basis for
understanding more complex techniques. Here are two key
elements to think about:
- Photoshop moves information up. Whatever is at the
bottom is processed first, then the output is moved up
as input to the next layer. - Each layer represents a composite of everything below
it. This is pretty important because it says functions
don’t combine across layers.
The second point can be confusing, so think about it this
way: Each layer behaves as if it’s the top layer in the stack,
so if you turn off every layer above it, what you see on the
canvas is what Photoshop uses as input from that layer to the
layer immediately above it. There are no leftovers, only vari-
ables that weren’t affected due to transparency. Put another
way, 100% transparency on a layer is a unique set of instruc-
tions that say, “Do nothing.”
What do we get from these two somewhat academic
observations? Mostly this: order matters. That is, if you
change the order of layers, it’s likely that your results will
change. The output of any given layer is the input to the next
layer up, so changing the order of the layer stack will change
the total output.
The other idea we get is that of a virtual image. As I men-
tioned, the image you see on Photoshop’s canvas is the result
of performing functions on variables—content and instruc-
tions being combined. The image doesn’t exist until you
render it in some way. Even when you save and reload the
PSD or TIFF file, the canvas is the representation of all those
layers, not a single image by itself, so that means you can
change the output by changing something on any of the lay-
ers. When you flatten the layers or print it, then it becomes a
real image. This is less important to understand, but I felt the
need to include it for completeness—and because I’m a geek.
Alpha Instructions
Now that we have these fundamental ideas out of the way,
let’s describe some features of content and instructions. I
want to define “content” as actual pixels on a layer. In the
example above, the text content is only the letters you can
see. All the areas where you can see the photograph are not
considered content on the text layer. Why should you care?
Because layer styles such as Drop Shadows, Bevel & Emboss,
and Stroke all depend on that definition. They apply to the
edges of the content on the layer.
Content can have opacity. Content Opacity, or how
much of the current content you can see through, is a fea-
ture of each individual pixel. Check out this gradient. The
white area (100% Opacity) is content that gradually fades
to transparent (0% Opacity) where you can see the check-
erboard pattern. The pixels themselves carry transparency
information. Even if a pixel has 0% Opacity, it’s still content
on the layer.There is a twist, however. Opacity is a special type of
instruction channel called “alpha.” Photoshop applies this
instruction in different ways depending on the content and
any other instructions that are being used. (Note: Alpha is
actually a channel, but would take more space than I have to
explain properly. Calling alpha a type of instruction is a neces-
sary simplification.)
For now, you can think of alpha as a step in the stack of
instructions that gets shuffled around in different situations.
The practical use for this is that you can get different trans-
parency effects by choosing different alpha tools.
Now that we’re clear (get it?), let’s look at other alpha
instructions:Layer Opacity: Layer Opacity applies to all content and layer
styles. As you drag the Opacity slider toward zero, every-
thing on that layer becomes more transparent, including
layer styles.Fill Opacity: A special version of Opacity is called Fill, which
does the same thing as Opacity, but it’s calculated only on
the content itself; it ignores layer styles. That means you can
apply a Drop Shadow to some layer content, then reduce
the Fill to zero and be left with only the Drop Shadow. The
content vanishes. If you did that with Opacity, both the con-
tent and the Drop Shadow would vanish. Fill also applies to››PHOTOSHOP PROVING GROUND