Working with Alpha Channels for Compositing

The film industry uses compositing to enhance the visual effects of a story. In short, compositing is the layering of images or other video footage over the main story line for effects purposes. Essentially this is exactly what you do in Photoshop with layers when you blend two or more images together to portray a concept.

In this section, you are going to get a little experience applying alpha channels to isolate a subject from its background.

The standard image format that all video-editing programs can read and apply an alpha mask to is Targa. Targa files are associated with a .tga extension, and Photoshop CS will save to this extension.

I created the image shown in Figure 27.10 in a 3-D program called LightWave by Newtek (www. newtek .com). When I exported into a bitmap, Photoshop placed the object on its own layer. You can take advantage of this to create a selection of your object. Try this:

Figure 27.10: My 3-D spaceship

1. From the Photoshop CS Savvy CD, open the file starship.psd from the ch27 folder.

2. Open your Layers palette and /Ctrl-click the Star Ship layer. Photoshop has now made a selection based on the existing pixels. In this case, it s the ship itself.

3. Now you will create your alpha channel from this selection. Choose Selection ’ Save Selection. Click OK to save it as an alpha channel. The additional channel will designate transparency in your image file (see Figure 27.11) that any video-editing program will read and honor . Take note that the black areas in the channel designate transparency.

   
   Figure 27.11: Saving a selection adds an alpha channel to the file.

4. Now you are ready to save the file to the universal Targa format. Choose File ’ Save As and save it under a new name . From the Format list, choose the Targa format (see Figure 27.12). Your file will be given a .tga extension.

   
   Figure 27.12: Format dialog box

5. Choose Select ’ Save Selection and click OK.

In another page or so, you ll use this starship file, with the saved selection, to add a flying spaceship to a still image.

ALPHA CHANNELS

Alpha channels are nothing more than imagery made up of 256 shades of gray in Photoshop. What does this mean to us as artists ? It simply means that we now have 256 levels of transparency. The black areas designate transparency, whereas the white areas will leave the image unaffected. The grays in between give us various levels of transparencies . This alpha channel can be edited with any of Photoshop s painting tools to alter how it will appear in the video-editing program.