In the vast world of Adobe Photoshop, alpha channels play a pivotal role in managing and manipulating image transparency. While many users might be familiar with layers and layer masks, diving into alpha channels can unlock additional advanced capabilities. Here’s a deep dive into creating and working with alpha channels in Photoshop.
1. What are Alpha Channels?
In essence, alpha channels store selections as grayscale images, where white represents selected areas, black signifies unselected regions, and shades of gray indicate levels of transparency. They are used by masks to store data, can be accessed by tools like InDesign for wrapping text or hiding backgrounds and a whole lot more.
2. Why Use Alpha Channels?
- Precision: They’re perfect for preserving complex selections like wisps of hair, fur, or feather details.
- Flexibility: Store multiple selections without creating additional layers.
- Combining Selections: Easily combine multiple saved selections.
3. Creating an Alpha Channel
3.1. From a Selection
- Create a selection using any of Photoshop’s selection tools, from the Rectangle Marquee tool right through to the Object Selection tool.
- Switch to the Channels panel. If it’s not visible, you can open it via Window > Channels.
- Click on the Save selection as channel icon (a circle inside a rectangle) at the bottom of the Channels panel (marked A in Fig 1.1).
- A new channel, usually named Alpha 1, will appear, representing your selection.
3.2. Alternative method from a Selection
- Create a selection using Photoshop’s tools, as discussed in step 3.1.
- Go to the Select menu and towards the bottom of the menu choose Save Selection.
- The Save Selection dialog box contains a number of options as follows:
Document: Select the document you want to save your channel to.
Channel: New will create a new channel, while selecting an existing channel will allow you to add to it.
Name: Here you can give your new channel a name.
Operation: Depending on your selection under the Channel dropdown you can now choose between New Channel, Add to Channel, Subtract from Channel and Intersect with Channel.
- Click OK and your selection will now show as a new channel in the Channels panel.
3.3. From Scratch
- In the Channels panel, click on the Create new channel icon (next to the trash bin icon).
- This creates a new alpha channel. You can now use brushes or other tools to paint areas of white (selected) or black (unselected) onto the channel.
Figure 1.1: New Alpha Channel in Photoshop
4. Editing Alpha Channels
Editing an alpha channel is similar to editing a grayscale image:
- Paint Brush: You can use the Paint Brush tool with black, white, or shades of grey to paint on the channel.
- Gradients: You can apply a wide range of gradients to the channel using the Gradient tool.
- Modify: You can apply filters or adjustments to your channel. For example, Filter > Blur > Gaussian Blur can soften the edges of an area within your channel.
- Load: To load an alpha channel as a selection, you can Ctrl (pc) / Cmd (Mac) + click on the channel’s thumbnail. to create a selection from it.
5. Combining Channels
If you’ve stored multiple selections as separate alpha channels, they can be combined:
- Ctrl-click (Cmd-click on Mac) on one channel’s thumbnail to load the selection.
- Ctrl+Shift+click on another channel’s thumbnail to add to the selection or Ctrl+Alt+click to subtract from it.
6. Using Alpha Channels with Layers
Alpha channels can be easily converted to layer masks:
- Load the selection from your alpha channel.
- In the Layers panel, select the layer you want to mask.
- Click on the Add a mask icon at the bottom of the layers panel.
Your alpha channel selection is now a layer mask.
7. Exporting with Alpha Channels
When saving images with transparency (e.g., for web use), alpha channels determine transparent areas:
- Ensure your file format supports transparency, a number of different file types support alpha channels, but not all alpha channels are created equal. Some support only black and white alpha channels while other file types can store thousands of shades of grey in an alpha channel. I typically save my files as a PSD containing alpha channels, however other formats that also support alpha channels to some degree include TIFF, PNG and GIF (more limited support).
- Save the image, in my case as a PSD file. The alpha channel ensures that transparent areas are maintained.