How to Create an Alpha in CS5

Many alpha channels look just like this soft-edged image.

When you're working in a bitmap editing program like Adobe Photoshop CS5, you use alpha channels to save selections and create layer masks. If you think these channels look like regular grayscale images, you're right, regardless of whether you're working in an RGB, CMYK or grayscale file. If you look at selections as the simple byproducts of the Marquee, Lasso or Magic Wand tools, broaden your horizons. You can use the entire Adobe Photoshop toolset to help you build and modify the selections you save as alpha channels.

Instructions

1. Make a selection. For simple geometric selections -- rectangular, elliptical or polygonal -- use the Marquee tools. For selections based on areas of similar color, use the Magic Wand tool or open the "Select" menu and try out the "Color Range" command. For more complex selections based on hand-drawn outlines enclosing specific image content, press the "Q" key to enter Quick Mask mode and paint your selection with the Brush tool.

2. Enter Quick Mask mode if you're not already working in it. Now you can see your selection as a colored overlay on top of your image detail. This view can help you spot flaws, such as areas that should be included but aren't, or vice versa. For an uncluttered solo view, press the tilde key to the left of the numeral 1, to see your selection as a grayscale image.

3. Blur your selection to soften its edges and improve its appearance if you use it as a layer mask. Open the "Filter" menu, navigate to its "Blur" submenu and choose "Gaussian Blur." Use a small blur radius for slightly softened edges or a high value to make your selection broadly feathered. These edge refinements can help you composite a masked-off image onto other subject matter.

4. Erase any selected areas you didn't intend to include. Use the Eraser tool to correct small flaws. Activate the Marquee or Lasso tools to click and drag around larger areas you want to remove from your selection. Press the "Backspace" key (Windows) or "Delete" key (Mac) to fill your selection with the background color shown in the toolbox. Use white to erase and black to broaden the selected area.

5. Save your selection without leaving Quick Mask mode. Open the "Window" menu and choose "Channels" to bring up the Channels panel if it isn't visible. Drag the "Quick Mask" entry in the Channels panel onto the "Create New Channel" icon at the bottom edge of the panel, to save an alpha channel automatically named "Quick Mask copy."

6. Press the "Q" key to exit Quick Mask mode. To save your selection, open the "Select" menu and choose "Save Selection." Leave the "Document" drop-down menu set at its default -- the name of your current file -- and set the "Channel" drop-down menu to "New." Enter a name for your alpha channel and set the "Operation" radio buttons to "New Channel." Click on the "OK" button to finish saving your selection.

7. Add your selection to an active layer as a layer mask. Open the "Window" menu and choose "Layers" to bring up the Layers panel. If your document only contains the default "Background" layer, double-click on its name to bring up the "New Layer" dialogue box so you can rename it, turning it into a regular document layer that can contain transparency. Once you click on the "OK" button to complete the "New Layer" operation, you can click on the "Add Layer Mask" button at the bottom of the Layers panel to turn your active selection into a layer mask that reveals only the image areas defined by your selection.

Tips & Warnings

• To avoid reinventing complex selections in which you've invested time and energy, save them as alpha channels if they've required more than 10 minutes to create.

• To remove a layer mask or alpha channel, drag its icon to the trash can at the bottom of the Layers or Channels panel.

• To activate a layer mask or alpha channel as a selection, open the "Select" menu and choose "Load Selection." Choose the channel you want to load and click on the "OK" button. You also can load the inverse of your mask or alpha channel using the "Invert" check box, and load your selection instead of, in addition to or in combination with an active selection.

• Use layer masks to hide image details instead of deleting them. You can de-activate these layer-specific alpha channels and save them without having to apply them to an image layer, but once you delete image detail, it's gone.

References

• Adobe Systems: Using Adobe Photoshop CS5

Resources

• The Photoshop CS3/CS4 WOW! Book; Linnea Dayton and Cristen Gillespie
• The Photoshop CS/CS2 WOW! Book; Linnea Dayton and Cristen Gillespie
• Photoshop Masking and Compositing; Katrin Eismann
• Photoshop Restoration and Retouching, Third Edition; Katrin Eismann and Wayne Palmer