|
The second and third parts of this series looked
at the standard blending modes available for layers and painting
tools (and a number of commands). We'll close out the series
with a discussion of the "other" blending modes, Behind, Clear,
Add, and Subtract.
In addition to the blending modes used with
layers, the painting tools also offer a mode called Behind.
Painting tools blending modes are selected in the Options Bar.
Behind is not available when the active layer is
a background layer or when the active layer's transparency has
been locked.
Simply put, when Behind is selected, the
painting tool will work only on pixels' transparency. Opaque
pixels (100% opacity) are not affected. Partially transparent
pixels are partially affected.
Using our buddy the bear (from the stock art supplied with
Photoshop 6), we've made an elliptical selection with a feather
of 25 pixels, inverted the selection, and deleted. That's left
us with a an image containing a large range of opacity, from
100% in the center, to 0% at the corners.
Using the Paint Bucket, set to Behind and with a
tolerance of 255 (to ensure that we fill every available pixel),
we can add the foreground color "behind" the image.
The effect is the same as adding another layer,
filled with black, below the bear's layer.
Similarly, we can use the Paintbrush on a layer that contains
transparency. Here's a bunch of type that, as you can see from
the Layers palette, has been rasterized (Layer> Rasterize>
Type). We have to rasterize in order to be able to paint on the
layer; you cannot use painting tools on a type layer.
We'll opt for the Behind blending mode so that
we can add a highlight behind the letters without changing their
color or having to work on a separate layer. The Airbrush does
just the job we need.
When working with the Paint Bucket, you have the
option of using the blending mode Clear. (It's also available
for the Fill and Stroke commands in the Edit menu.) Clear
creates transparent pixels where colored pixels had been. (Of
course it's not available for background layers or layers with
locked transparency.) In this case, we'll click once in a letter
with the Magic Wand to select all of the text. (The Wand is set
to any tolerance and Contiguous is unchecked.)
We can now use the Stroke command, set to
blending mode Clear, and a width of 2 pixels. We'll set it to
Inside. The command deletes the stroked pixels, leaving
transparency.
In conjunction with the Clear blending mode, we
can revert back to Behind. Using it with the Gradient tool adds
some color to our transparent areas.
The final two blending modes are Add and
Subtract. You'll find them only with two commands, Image> Apply
Image and Image> Calculations. They can be used with either an
image's composite channel (all three or four channels) or with
an individual color channel. They are only available with images
that have exactly the same pixel dimensions.
When you combine two channels or images, the Add
blending mode quite literally adds together the color values of
the pixels. Because the images or channels have identical pixel
dimensions, each pixel in one channel has a corresponding pixel
in the other. If, for example, the pixel in the upper-left
corner of the first image has an RGB value of 25/157/212 and the
upper-left pixel of the second image has an RGB value of
75/90/30, then the resulting pixel will have an RGB value of
100/247/242. Black (0/0/0) remains unchanged in either image or
channel, while white in either channel produces white. No color
value can be higher than 255.
While adding color values can be fun, it doesn't
come in handy very often. Where the Add blending mode earns its
keep is with the other variables in the dialog box.
Scale can range from 1.000 to 2. Think of it as
averaging. If Scale is set to 2, the color values are added
together then divided by two, giving you their average. Using a
Scale value of 1 simply adds the colors together (dividing by 1
leaves the original number). Using any value in between produces
a weighted averaging.
Another way to affect the result color is
Offset. You can lighten or darken the pixels in the destination
channel by any value from 255 to –255. (Positive lightens,
negative darkens; using 255 produces white, -255 produces
black.) This allows you to change the brightness values of all
pixels in the resulting image.
Working with two versions of the same image, we
can use the Add blending mode to keep the best features of each.
In one exposure the details of the tree look good, but the snow
on the ground is blown out – virtually none of the detail is
visible. In the second version, we've got lots of detail on the
ground, but the tree is too dark.
Using Image> Apply Image, we chose the Add
blending mode, a Scale of 1.75, and an Offset of 10. This left
us with the best features of both images, and even brought out
the small clouds near the horizon.
The blending mode Subtract, also available only
with Apply Image and Calculations, is the opposite of Add.
Rather than combining the color values for corresponding pixels,
it subtracts the values. It can produce wonderful special
effects. (To use Apply Image and the blending mode Subtract to
average channels, start with these setting: Invert checked,
Scale 2, Offset 128.) |