PHOTOSHOP CS3
User Guide
275
Subtract From Selection
Subtracts the loaded selection from existing selections in the image.
Intersect With Selection
Saves a selection from an area intersected by the loaded selection and existing selections in
the image.
You can drag a selection from one open Photoshop image into another.
Channel calculations
Blending layers and channels
You can use the blending effects associated with layers to combine channels within and between images into new
images. You can use either the Apply Image command (on single and composite channels) or the Calculations
command (on single channels). These commands offer two additional blending modes not available in the Layers
palette—Add and Subtract. Although it’s possible to create new combinations of channels by copying channels to
layers in the Layers palette, you may find it quicker to use the calculation commands to blend channel information.
The calculation commands perform mathematical operations on the corresponding pixels of two channels (the
pixels with identical locations in the image) and then combine the results in a single channel. Two concepts are
fundamental to understanding how the calculation commands work.
•
Each pixel in a channel has a brightness value. The Calculations and Apply Image commands manipulate these
values to produce the resulting composite pixels.
•
These commands overlay the pixels in two or more channels. Thus, the images used for calculations must have the
same pixel dimensions.
Blend channels with the Apply Image command
The Apply Image command lets you blend one image’s layer and channel (the
source
) with a layer and channel of the
active image (the
destination
).
1
Open the source and destination images, and select the desired layer and channel in the destination image. The
pixel dimensions of the images must match for image names to appear in the Apply Image dialog box.
Note:
If the color modes of the two images differ (for example, one image is RGB and the other is CMYK), you can apply
a single channel (but not the source’s composite) to the destination layer’s composite channel.
2
Choose Image > Apply Image.
3
Choose the source image, layer, and channel you want to combine with the destination. To use all layers in the
source image, select Merged For Layer.
4
To preview the results in the image window, select Preview.
5
To use the negative of the channel contents in the calculation, select Invert.
6
For Blending, choose a blending option.
For information on the Add and Subtract options, see “Add and Subtract blending modes” on page 276. For infor-
mation on other blending options, see “List of blending modes” on page 344.
7
Enter an Opacity value to specify the effect’s strength.
8
To apply the results only to opaque areas in the result layer, select Preserve Transparency.
