Channel-Based Compositing

Channel-Based Compositing

Adobe once counted channel-based compositing among Photoshop s most powerful features. The extensive layer compositing techniques introduced in recent versions of Photoshop have made this capability redundant. It still, however, enables you to apply additional modes that, even in Photoshop CS, have not been added to the Layers palette. Photoshop s two channel-based compositing techniques are the Apply Image command and the Calculations command.

Apply Image

The Apply Image command applies a source image to the target image. To use Apply Image, both images have to be open on the desktop. The images must be exactly the same physical size and resolution.

First, look at the Apply Image dialog box (see Figure 24.6) to understand its commands. To access it, choose Image ’ Apply Image.

Figure 24.6: You can use the Apply Image dialog box for channel-based compositing.

You can set these options in the dialog box:

Source This is the image that is applied, or overlaid. Choose the desired image from the pop-up list.

Layer The pop-up lists all of the layers and the Background in the source image. You can overlay all the layers if you choose Merged.

Channel Select the channels you apply to the image from this list. If you choose the composite channel (RGB, CMYK, and so forth), the entire channel will be mixed.

Invert Check this box to invert the contents of the selected channel. Pixels that are black will be applied as if they were white, and vice versa.

Target This is the image that is active. The target lists the channel(s) and layers that the image will be applied to.

Blending The pop-up list displays the blending modes in which the color will be applied (see Appendix C, Blending Modes, for a description of each mode).

Opacity Enter the opacity of the mode.

Preserve Transparency This check box leaves transparent areas on a layer unaffected.

Mask Checking this option masks off a part of the source image. When you choose Mask, the Apply Image dialog box expands. From the lists, choose an image and a layer. Choose a color channel, an alpha channel, or an active selection on the source image to isolate the application of the effect.

Scale and Offset If you choose either the Add or Subtract modes, you can enter values for scale and offset. These values are used in the calculation to determine the brightness values of the superimposed pixels. (See Appendix C.)

The Apply Image command can produce some beautiful artistic effects. You can vary the results by choosing different settings. In this example, you ll start with a sepia watercolor, and then apply a broadly painted version of the image and mitigate the effect by adjusting the blending modes and the opacity.

1. Open the Apples.psd file in the ch24 folder on the CD (see Figure 24.7). For this exercise, refer to the full-color versions of this image included in the color section (Figures C39a “e).

   
   Figure 24.7: Use this sepia-toned watercolor to experiment with the Apply Image command.

2. Choose Image ’ Duplicate to make a copy of the image. Name the new image Paint .

3. With the painting tools, color the image with broad strokes as in Figure C39b.

4. Click the Apples.psd image to make it active.

   Note  

   The image that is active will become the target image.

5. Choose Image ’ Apply Image. Enter the following and then click OK:

   Source	Paint Image
   Layer	Background
   Channel	RGB
   Blending	Subtract
   Opacity	50%

6. Try different combinations of modes, channels, and opacities to produce vastly different results.

Overlaying Graphics

Apply Image can also be used to produce interesting overlay graphic effects. In Chapter 17, Modifying and Mapping Color, you created a posterization and a halftone from the same image. You can combine these two types of images to produce a strong posterized halftone effect. You can vary the effect by applying different values, modes, and opacities. Follow these steps:

1. Open the images Listerine_poster.psd and Listerine_halftone.psd from the ch24 folder on the CD.

2. Click on Listerine_halftone.psd to activate it. Choose Image ’ Mode ’ Grayscale to convert the bitmapped image to a grayscale.

   Note  

   Channel-based compositing techniques do not work on Bitmap images. It is therefore necessary to convert their mode.

3. Click Listerine_poster.psd to activate it. It will be the target document.

4. Choose Image ’ Apply Image. Enter the following specifications and click OK:

   Source	Listerine_halftone.psd
   Layer	Background
   Channel	Gray
   Blending	Add
   Opacity	40%

   The halftone image is applied to the target. (Figure 24.8 shows the before and after results.)

   
   Figure 24.8: The posterized image (a), the halftone (b), and the halftone superimposed on the posterized image by using the Apply Image feature (c)

5. Try different combinations of modes, channels, and opacities to produce different results. (a) (b) (c)

Calculations

Despite the complexity of its dialog box, the Calculations command performs a rather simple operation ” it creates only one channel. Image ’ Calculations works exclusively to combine two source channels into a new channel. Like the Apply Image command, it calculates the numerical values of pixels and applies a mode or mathematical formula to produce results. The difference is that it uses the information on individual channels to produce a new selection, a new alpha channel, or a new document. The Calculations command has three purposes: to create a new grayscale image, or to combine two masks into either a single alpha channel or an active selection.

Like Apply Image, this command requires the images to be exactly the same height, width, and resolution. When you choose Image ’ Calculations, you see the Calculations dialog box (see Figure 24.9). It s quite similar to the Apply Image dialog box; the only difference is the additional source. The two source areas let you choose two channels to combine.

Figure 24.9: The Calculations dialog box

Here you ll combine a couple of alpha channels to see how the Calculations command works:

1. From the ch24 folder on the CD, open the document Cowpoke.psd (see Figure 24.10).

   
   Figure 24.10: The cowpoke image and its Channels palette

2. Look at the Channels palette. Alpha 1 is a channel of the circle surrounding the cowboy; Alpha 2 is the outline of the cowboy. You are going to combine the two channels into a third.

3. Choose Image ’ Calculations. Enter the following:

   Source 1	Cowpoke.psd
   Channel	Alpha 1
   Source 2	Cowpoke.psd
   Channel	Alpha 2
   Blending	Difference
   Result	New Channel

4. Click OK, and Photoshop creates the Alpha 3 channel, which combines Alpha 1 and Alpha 2 (see Figure 24.11).

   
   Figure 24.11: The new combined alpha channel

5. You can also use the Calculations command to blend channels from two source documents and create a new grayscale document. Open the file Cowboy_Sky.psd from the ch24 folder on the CD. Then click Cowpoke.psd to reactivate it.

6. Choose Image ’ Calculations. Enter the following:

   Source 1	Cowpoke.psd
   Channel	Gray
   Source 2	Cowboy_Sky.psd
   Channel	Blue
   Blending	Darken
   Result	New Document

7. Click OK. A new document is created, combining the composite Gray channel of the Cowpoke.psd document and the Blue channel from the Cowboy_Sky.psd document (see Figure 24.12).

Figure 24.12: This image was created by using Calculations to combine the Gray channel of the cowpoke with the Blue channel of the cowboy sky.

The Calculations feature is complex, to say the least, and as always there are several workarounds that perform the same operations. Mastering it, however, gives you the skills you need to quickly combine channels, and that can extend your selection-making capabilities.