Page 59 - CC2019_PS_Look_Inside

Understanding Channels

You need a bit of background about channels to understand what’s happening in the Quick Mask you will use in the next
exercise. (You will use channels extensively in later projects.)
Every image has one channel for each component color. Each channel
contains the information for the amount of that component color in
any given pixel. An RGB image has three channels: Red, Green, and Blue
(right top). A CMYK image has four channels: Cyan, Magenta, Yellow, and
Black (right bottom).

In RGB images, the three additive primaries can have a value of 0 (none of
that color) to 255 (full intensity of that color). Combining a value of 255 for
each primary results in white. A value of 0 for each primary results in black.
In CMYK images, the three subtractive primaries plus black are combined in
percentages from 0 (none of that color) to 100 (full intensity of that color) to
create the range of printable colors. Channels in a CMYK image represent the
printing plates or separations required to output the job.

Understanding Alpha Channels
An Alpha channel is a special type of channel, in which the Channel button at the bottom of the Channels panel. This
value determines the degree of transparency of a pixel. In adds a channel named “Quick Mask copy,” which becomes a
other words, a 50% value in the Alpha channel means that permanent part of the fi le even if you exit Quick Mask mode.
area of the image will be 50% transparent. You can then double-click the Alpha channel name in the
panel to rename it, as we did in the following image (naming
When working in Quick Mask mode, a temporary Quick Mask the channel “Baby Face”).
channel stores the degree of transparency based on the
current selection. A semi-transparent red overlay shows areas
being masked (i.e., the areas that are not included in the
current selection).
Permanent Alpha channel
Temporary Quick Mask channel

New Channel button

You can change the appearance of an Alpha channel mask by
double-clicking a channel thumbnail in the Channels panel. In
the top half of the resulting dialog box, you can change the
overlay to show selected areas
instead of the default masked areas.
Quick Masks are useful when you need to work with a
temporary selection, or if you are still defi ning the exact Clicking the Color swatch opens a
selection area. As long as you stay in Quick Mask mode, the Color Picker, where you can change
temporary Alpha channel remains in the Channels panel the color of the Quick Mask overlay.
(listed as “Quick Mask”). If you return to Standard mode, the You can also use the Opacity fi eld to change the transparency
Quick Mask disappears from the window and the panel. of the overlay (the default is 50%). Keep in mind that these
settings only affect the appearance of the mask in Photoshop;
You can save a Quick Mask channel as a permanent Alpha the density of the selection is not affected by changing the
channel by dragging the Quick Mask channel onto the New overlay opacity.

Project 1: Music CD Artwork 71