Adobe 13101332 - Photoshop - Mac User Manual page 277

Adobe Photoshop Help
Using Help | Contents
9 Select Mask if you want to apply the blending through a mask. Then choose the image
and layer containing the mask. For Channel, you can choose any color or alpha channel to
use as the mask. You can also use a mask based on the active selection or the boundaries
of the chosen layer (Transparency). Select Invert to reverse the masked and unmasked
areas of the channel.
10 For Result, specify whether to place the blending results in a new document, or in a
new channel or selection in the active image.
About the Add and Subtract blending modes
The Add and Subtract blending modes are available only for the Apply Image and Calcula-
tions commands.
Add Adds the pixel values in two channels. This is a good way to combine nonover-
lapping images in two channels.
Because higher pixel values represent lighter colors, adding channels with overlapping
pixels lightens the image. Black areas in both channels remain black (0 + 0 = 0). White in
either channel results in white (255 + any value = 255 or greater).
Add mode divides the sum of the pixel values by the Scale amount and then adds the
Offset value to the sum. For example, if you wanted to find the average of the pixels in two
channels, you would add them, divide by 2, and enter no Offset value.
The Scale factor may be any number between 1.000 and 2.000. Entering a higher Scale
value darkens the image.
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.
Subtract Subtracts the pixel values in the source channel from the corresponding pixels
in the target channel. As with Add mode, the result is then divided by the Scale factor and
added to the Offset value.
The Scale factor may be any number between 1.000 and 2.000. The Offset value lets you
lighten or darken the pixels in the destination channel by any brightness value between
+255 and –255.
About masks (Photoshop)
Masks let you isolate and protect areas of an image as you apply color changes, filters,
or other effects to the rest of the image. When you select part of an image, the area that is
not selected is "masked" or protected from editing. You can also use masks for complex
image editing such as gradually applying color or filter effects to an image.
In addition, masks let you save and reuse time-consuming selections as alpha channels.
(Alpha channels can be converted to selections and then used for image editing.) Because
masks are stored as 8-bit grayscale channels, you can refine and edit them using the full
array of painting and editing tools.
Using Help | Contents
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Using Channels and Masks
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