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ADOBE PHOTOSHOP CS5 DESIGN 13

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					      Chapter 1: Using Channels
      In This Chapter
      ✓ Understanding what a channel is
      ✓ Working with the Channels panel
      ✓ Editing channels
      ✓ Creating alpha channels
      ✓ Using the Channel Mixer



      I  f you’re reading this, it probably means that you didn’t quickly thumb
         through this chapter, say “Yuck, boorrrrringgg!” and move on to sexier
      topics such as blending, filtering, and retouching. You knew that would be a
      huge mistake.

      The wonderful thing about channels is that they offer you greater control
      and selectivity when doing those very things — blending, filtering, and
      retouching. Channels give you one more level of control when editing your
      images. You can use individual channels for layer-blending options and fil-
      ters, as well as starting points for masks.

      Channels also come into play when saving selections for
      later use or for adding spot (custom) colors to your
      image. You can also use channels to turn color images
      into nicely contrasted grayscale images. And finally,
      you can play around with the colors in an image by
      mixing up the channels. So bear with me. The topic
      of channels may be a bit dry and technical, but in
      the end, channels enable you to hold the envious
      title of Master Editor.

      To understand how channels work, you’ll find it help-
      ful to know a few things about colors — specifically,
      the various color modes, which I cover in Book II,
      Chapter 2.



Understanding Channels
      When you look at a color image, you see one big, 24-bit, composite collection
      of colored pixels. Technically speaking, however, Photoshop doesn’t see that
      at all. Photoshop perceives a color image as individual bands of 8-bit, gray-
      scale images. RGB images have three bands; CMYK images have four bands.
460   Understanding Channels


         I know it’s strange to think of a color image as being composed of several
         grayscale images, but it’s true. Each one of these bands, or grayscale images,
         is a channel. Specifically, they’re color channels. If you just can’t get past the
         fact that a color image is the sum of several grayscale channels, as shown in
         Figure 1-1, then just think of channels as holding tanks for color data.




                                                                           Purestock
         Figure 1-1: A color image is composed of 8-bit grayscale images
         referred to as channels.


         Another way of relating channels to the real world is in terms of hardware.
         Here’s how the most common hardware handles color:

          ✓ When you offset-print a CMYK image, the process separates the colors
            (see Book IX, Chapter 1) into four colors — cyan, magenta, yellow, and
            black. Paper passes through four individual rollers on the printing press,
            and each roller contains one of those four colored inks.
          ✓ Scanners scan in RGB via a pass of red, green, and blue sensors over
            your image.
          ✓ Digital cameras capture images in RGB.
          ✓ CRT screens display images via red, green, and blue tubes.

         In addition to color channels, there are channels called alpha channels (cov-
         ered in the section “Introducing Alpha Channels,” later in this chapter) and
         others called spot channels (discussed in Book IX, Chapter 1). Photoshop
         now supports up to 56 channels per file. So knock yourself out! Just remem-
         ber that each channel you add increases your file size.
                                                           Working with Channels               461



                         A little bit about bit depth
 When you’re standing around the water cooler      ✓ A 24-bit image can contain about 16 million
 or the color printer and hear people talking        colors (224). RGB images are 24 bit (3 chan-
 about a 1-bit or an 8-bit image, they’re refer-     nels × 8 bits).
 ring to something called bit depth. Bit depth
                                                   ✓ CMYK images are 32 bit (4 channels × 8 bits).
 measures how much color information is avail-
                                                     CMYK images, however, are limited to the
 able to display and print each pixel. A higher
                                                     number of colors that are physically repro-
 bit depth means the image can display more                                                           Book VI
                                                     ducible on paper, which is around 55,000.
 information — specifically, more colors. For                                                         Chapter 1
 example, a 1-bit image can display two color      ✓ High Dynamic Range (HDR) images are 96
 values — black and white. That’s why a purely       bit (3 channels [RGB] × 32 bits). Theoreti-




                                                                                                           Using Channels
 black-and-white image is called a bitmap            cally, they’re capable of having about 65
 image. Likewise                                     million colors, but in reality, the maximum
                                                     range is determined by what colors were
 ✓ An 8-bit image can contain up to 256 gray-
                                                     captured by the camera.
   scale levels (28). Grayscale images are 8
   bit (1 channel, 8 bits).                        Bit depths typically range from 1 to 64 bits.



           Briefly, you can use alpha channels to create, store, and edit selections,
           defining them not by a selection outline, but by black, white, and varying
           shades of gray pixels — in other words, a grayscale image. Black pixels rep-
           resent unselected areas of the image, white pixels represent selected areas,
           and gray pixels represent partially selected pixels.

           You can create spot channels when you want to add a spot (or custom)
           color to your image. Spot colors are premixed inks often used in addition to
           or in lieu of CMYK colors.

           All images, no matter what their color mode, have at least one channel.
           Grayscale, Bitmap, Duotone, and Indexed Color (for GIF Web images) modes
           have only one channel. RGB and CMYK images have three and four channels,
           respectively. They also contain a composite channel, which reflects the com-
           bination of the individual color channels and gives you the full color display.



Working with Channels
           Like with layers (which I discuss in Book V), channels have their own panel
           that acts as command central for viewing, creating, and managing tasks.
           The first step is accessing channels by choosing Window➪Channels. The
           Channels panel appears, as shown in Figure 1-2.
462   Working with Channels


         Viewing channels
         without a remote
         Selecting a channel in the Channels
         panel automatically makes it appear in
         the image window. To select a channel,
         click the channel thumbnail or name in
         the panel. To select more than one
         channel, Shift-click. To show or hide a
         channel, click in the eye column in the
         far left of the panel. You can also drag
         through the column to hide or show
         the channels quickly.

         CMYK, RGB, and Lab images have a
         composite channel, in addition to their
         individual channels. This composite
         channel is the combination of all the
         channels in the image and is named
         after the color mode. For example, the
         composite channel in Figure 1-2 is the
                                                                                  Corbis Digital Stock
         first one, called CMYK.
                                                    Figure 1-2: The Channels panel stores all
                                                    the image’s channels, from spot to alpha
         Changing the default                       channels.
         channel view
         The default setting is to view your channels in grayscale. You can, however,
         view them in color. To do so, choose Edit➪Preferences➪Interface (Photoshop➪
         Preferences➪Interface on the Mac) and select Show Channels in Color.

         Although this option graphically exemplifies the way an image comprises
         separate color channels, it really does you no good if you want to work with
         your channels for editing. That’s because the color view obscures details
         and makes measuring the impact of adjustments and filters more difficult.
         You need to see the channels in their true grayscale form for that.

         If you select or show more than one channel, even in the default grayscale
         view, the channels always appear in color.

         To change the size of the thumbnail that appears, select Panel Options from
         the Channels panel pop-up menu. Select your desired thumbnail size. If
         you’re working with several channels and you have a dinosaur of a com-
         puter, you can also choose None to turn off the thumbnails — which
         improves performance.
                                          Working with Channels          463

Duplicating and deleting channels
Duplicating channels is something you may do quite often. I know I do. And
deleting channels isn’t just for neat freaks: Channels take up a lot of memory,
so getting rid of the ones you no longer need is always good.

Here are some instances when duplicating channels is a good idea:

 ✓ When you want to create a channel mask: First, you find a suitable
   channel and then make a duplicate. (For more on this technique, see
   Book VI, Chapter 3.) You can use channel masks to select difficult ele-
   ments involving fine details, such as hair, fur, smoke, and so on.
                                                                                    Book VI
 ✓ When you want to make a backup copy of the channel before doing                 Chapter 1
   some editing: Having a backup, just to be on the safe side, is always a
   good idea. For example, you may want to apply an Unsharp Mask filter




                                                                                         Using Channels
   to one or two channels to improve the focus of the image. For more on
   the Unsharp Mask filter, see Book VII, Chapter 1.
 ✓ To insert a copy of an alpha channel into another image: For example,
   maybe you spent an hour creating elaborate alpha channels for shadows
   and highlights on a product photographed in flat lighting.
    You may have 12 products, all the same shape but different colors,
    that you need to apply those highlights and shadows to. Instead of re-
    creating the wheel each time, you could simply duplicate the alpha
    channels into each file.

Duplicating channels
To duplicate a channel, follow these short steps:

 1. Select your desired channel in the Channels panel.
 2. Select Duplicate Channel from the panel pop-up menu.
    The Duplicate Channel dialog box
    appears, as shown in Figure 1-3.
 3. In the Duplicate section, in the As
    field, name the channel.
    You can also drag the channel
    to the New Channel icon at the
    bottom of the panel. If you do this,
    Photoshop provides a default name Figure 1-3: Name and provide a destination
    and bypasses Steps 3 through 6.      for your duplicate channel.

    You can also duplicate a channel to another image by dragging the
    channel. Open your destination image and drag the desired channel
    from your current image into the destination image window. The dupli-
    cated channel appears in the Channels panel.
464   Working with Channels


          4. In the Destination section, select a file from the Document drop-
             down list.
             Or choose New to create a new image.
             You can choose your current image
             or any open image with the same
             pixel dimensions as your current
             image. (For more on pixel dimen-
             sions, see Book II, Chapter 1.)
             If you choose New, Photoshop cre-
             ates a new image that has a single
             channel. Provide a name for the file.
          5. Select Invert if you want to reverse
             the selected and unselected areas
             of the duplicate channel.
             You use the Invert option primarily
             when you duplicate an existing
             alpha channel. For more on
             alpha channels, see the section
             “Introducing Alpha Channels,”
             later in this chapter.
          6. Click OK.
             The dialog box closes. Your
             duplicate channel appears in the                                          Digital Vision

             Channels panel, as shown in             Figure 1-4: You can duplicate channels for
             Figure 1-4.                             masking, as well as for backup purposes.

         Replacing one channel over another
         To replace the contents of one channel with another, you can use the copy-
         and-paste method. In your current image, select your desired channel in the
         Channels panel. Choose Select➪All and then Edit➪Copy. Select the channel in
         the destination image that you want to replace and choose Edit➪Paste. The
         pasted content of the channel replaces the original content.

         Deleting unwanted channels
         To delete an unwanted channel — something you definitely want to do
         because channels can eat up a lot of space — select the channel in the
         Channels panel and do one of the following:

          ✓ Drag the channel to the trash at the bottom of the panel.
          ✓ Select Delete Channel from the panel pop-up menu.
          ✓ Click the trash and then click Yes in the dialog box.
          ✓ Alt-click (Option-click on the Mac) the trash.
                                                  Working with Channels                  465

Rearranging and renaming channels
Although you can’t shuffle or rename color channels, you can do so with
spot and alpha channels. To move a spot or alpha channel, simply drag it up
or down in the Channels panel. When you see a dark line appear where you
want the channel to go, release your mouse button. You can move a spot or
alpha channel above a color channel only in a multichannel image. In short,
in a multichannel image, each channel becomes an independent spot chan-
nel, and the channels no longer have a relationship with each other.
Multichannel images don’t support layers. For more details, see Book II,
Chapter 2.

To rename a spot or alpha channel, double-click the name in the Channel                                 Book VI
panel and type a new name. You can also select Channel Options from the                                Chapter 1
panel pop-up menu.




                                                                                                             Using Channels
Splitting channels
You can split the channels of your image into separate images in separate
files. For example, in Figure 1-5, you see the Red, Green, Blue, Alpha, and
Spot channels split into individual channels. Choose Split Channels from the
panel pop-up menu. When you do so, your original image closes. The chan-
nel files have the name of your original image plus the channel name. You
can split channels only on a flattened image — in other words, an image that
has no individual layers.




                                                                                Corbis Digital Stock
Figure 1-5: Be sure to save all changes in your original image before you split it because
Photoshop closes your file.


You might want to split channels if you need to save your original file in a for-
mat that doesn’t preserve channels — such as EPS, which doesn’t support
alpha channels. Or you may want to split channels to merge them later on.
466   Working with Channels


         Merging channels
         You can merge channels into a single image. The channels must be opened
         as separate images, in grayscale mode, and have the same pixel dimensions.
         You can merge channels only when they’re flattened images and have no
         layers.

         Merging color channels can create some unique special effects. For example,
         by mismatching your channels when you merge them, you can create
         bizarre, and sometimes beautiful, color shifts.

         To merge channels, follow these steps:

          1. Open your split channel files and activate any one of them.
          2. Choose Merge Channels from the Channels panel pop-up menu.
          3. In the Merge Channels dialog
             box, choose your desired color
             mode, as shown in Figure 1-6.
             Any modes that are unavailable
             are grayed out. You may not have
             enough channels for the grayed-
             out modes.                           Figure 1-6: Choose the color mode and
                                                  number of channels in this dialog box.
          4. Enter the number of channels you
             want.
             When you choose your mode in Step 3, Photoshop automatically fills in
             the number of channels for the mode. If you deviate and enter some-
             thing different, the file becomes a multichannel file.
          5. Click OK.
          6. Select your channels in the dialog box that appears.
             In my example, the Merge CMYK Channels dialog box appears. If you want
             to merge the channels normally, make sure that each channel matches
             (Red for Red, and so on). If you want to rearrange the channels, you can
             mix them, as I did in Figure 1-7, so
             that the Cyan channel is mixed with
             the Magenta channel, and so on.
          7. If you’re merging into a multi-
             channel image, click Next.
             Repeat this step for each channel.
          8. Click OK.
             You’ve now merged your files into    Figure 1-7: You can mismatch your
             a single image, which appears in     channels when merging them to create a
             your Photoshop window.               unique effect.
                      Using Painting and Editing Tools with Channels                  467

           Photoshop closes individual channel files and                Original
           merges any spot channels as alpha channels.
           Check out Figure 1-8 to see how my sunflower
           went from yellow to magenta just by merging
           the layers a little differently.

       If you have an image that includes alpha or spot
       channels, select Multichannel from the Mode drop-
       down list in Step 3; otherwise, Photoshop doesn’t
       include those channels in the merged image. After
       you merge the image, Photoshop gives all the chan-
       nels the names Alpha 1, Alpha 2, Alpha 3, and so                                              Book VI
       on. To get back to a color composite, choose                                                 Chapter 1
                                                                    Split and merged
       Image➪Mode➪RGB Color or CMYK Color.




                                                                                                          Using Channels
Using Painting and Editing
Tools with Channels
       Sometimes, it’s better to edit individual channels
       rather than the composite image. Mediocre flat-
       bed scanners often reproduce, and digital cam-
       eras sometimes capture, an image that’s slightly
       soft or out of focus. You may want to counteract                    Photodisc/Getty Images
       that effect by applying an Unsharp Mask or Smart        Figure 1-8: When you split
       Sharpen filter. Before you do, you should examine       and merge channels, you can
       each channel separately. You may find that the          create botanical specimens
       Blue channel contains a lot of garbage — artifacts,     from another world.
       dithering, and other nasty crud.

       Blue channels are notorious for acquiring this junk, so try to avoid sharpen-
       ing this channel unless you really want to accentuate what’s already ugly.

       Instead of applying the Unsharp Mask or Smart Sharpen filter on Blue chan-
       nels, select the Red and Green channels in the Channels panel and then
       choose Filter➪Sharpen➪Unsharp Mask or Smart Sharpen. Similarly, you can
       apply a Gaussian Blur filter to a channel to soften the unsightly pattern (called
       a moiré pattern) caused by scanning a halftone. (See Book VII, Chapter 1 for
       more on moiré patterns, and filters are covered throughout Book VII.)

       Although Unsharp Mask, Smart Sharpen, and Gaussian Blur are a few correc-
       tive filters that you’ll use frequently, I also find it useful to apply a special-
       effect filter to individual channels. Sometimes, applying a filter to the
       composite image produces an effect that’s, well, overdone. Applying the
       filter to one or two channels can produce an effect that’s subtler and less
       in-your-face. For example, in Figure 1-9, I applied a filter to just the Blue
       channel for the flower on the top. Using individual channels can
468   Introducing Alpha Channels


         also be useful for applying filters that produce
         monochromatic images, such as the Graphic Pen
         or Photocopy filters. (Be sure your foreground
         and background colors are black and white.) If
         you apply the filter to the entire image, you get a
         black-and-white image. If you apply it to an indi-
         vidual channel, you retain some color.

         You can select a color channel and then edit that
         channel by using a painting or editing tool in the
         image. Keep these facts in mind:

          ✓ Painting with white adds the color channel’s
            color at full intensity in the composite image.
          ✓ Painting with black removes the color in the
            composite image.
          ✓ Painting with a value of gray adds color at
                                                                              Photodisc/Getty Images
            varying levels of intensity in the composite
            image.                                             Figure 1-9: By editing
                                                               individual color channels,
         For example, if you paint with white on the Blue      you can selectively and
         channel in an existing image, Photoshop adds          subtly apply filters (top) or
         more blue to the color composite image. But if        adjust color (bottom).
         you paint with black, Photoshop adds yellow to
         the image because when you remove blue, you’re left with the opposite (or
         complementary) color — yellow. To perform this channel magic, select the
         Brush tool and then select your desired brush size from the Options bar.
         Select your desired color in the Color panel. Select the channel you want to
         edit in the Channels panel and paint on the image. You can see the results
         by selecting the composite channel in the Channels panel. Refer to Figure
         1-9 to see how I gave a flower a channel-color makeover.

         The results are a little different if you try this technique on a blank CMYK
         canvas. For example, when you paint with black on the Cyan channel, your
         composite color image displays cyan. When you paint with white, you get no
         change.



Introducing Alpha Channels
         You use alpha channels for selections that are incredibly detailed or that
         you want to save and reuse. To make that selection, an alpha channel uses
         black, white, and shades of gray to create a mask. The selected pixels are
         white, and unselected pixels are black. For example, in the alpha channel of
         the image shown in Figure 1-10, the selection includes the lanterns and trees;

				
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