Page 138 - Enhancing CAD Drawings with Photoshop

4386.book Page 121 Monday, November 15, 2004 3:27 PM

EXTRACTING ENTOURAGE 121

Figure 4.23
Altering tone in the
Levels dialog box

Saving the Extraction as an Alpha Channel
Now that you are nearing the end of the tutorial, it is time to create an alpha channel so that you can
use the tree as entourage in 3D programs. (You will use the alpha channel in the “Entourage in
Autodesk VIZ” section later in this chapter.)
Use the Calculations dialog box to save the image’s transparent areas as an alpha channel, as I
described earlier. For specific instructions on how to do that for this particular image, follow these
steps:
1. Choose Image Calculations to open the Calculations dialog box. Select Transparency from
the Channel drop-down in the Source 1 group. If you leave everything else with the default set-
tings as shown in Figure 4.24, you can click OK to create the alpha channel.
Figure 4.24
Calculating an
alpha channel from
transparency

2. Figure 4.25 shows the grayscale alpha channel image that was generated in the previous step.
This image will tell 3D programs where to make the entourage object opaque (white), trans-
lucent (shades of gray), or transparent (black). To return the color image, click the Channels
palette and click the RGB channel.