Photoshop Tutorial : What is an alpha channel and how do you use them?

May 10, 2007 by Jennifer    9 Comments    Posted under: Photoshop Tutorials

Today I gave the class an exam – a practical and a short written paper – that covered Photoshop and Illustrator. Everyone did fine on the practical but almost everyone got caught up on one particular question on the written paper – I asked them “What is an alpha channel?”.

The simple and correct answer is: An alpha channel lets you save and load selections.

How to save and load a selection in Photoshop

In this tutorial I’m going to show you how to save as selection as an alpha channel, edit it and the convert the channel back to a selection.

1. Open up the image that you want to make a selection in.
2. Use your favourite selection tools to create the selection. For this particular example I used the Magnetic Lasso Tool to select the outside of the boy’s head and body. To pick out the small section between his shoulder and arms I subtracted from the selection again using the Magnetic Lasso tool and holding down the Alt key (Windows) or Option key (Mac).

   
   

3. If you’ve spent more than 30 seconds making a selection, it’s a good idea to save it, in case you need to use it again or if you want to refine it further using an alpha channel. Choose Select > Save Selection. This lets you convert your selection to an alpha channel.

   
   

4. Choose New for Channel and give your selection a name. (Mine’s called laughingboy). Click OK.
5. Click on the Channels tab (usually in the Layers palette group) to view the new alpha channel.
6. (Optional) If the thumbnails in the Channels palette are very small, you might find it useful to increase their size by clicking on the Channels palette menu and choosing Palette Options. From there you can pick larger thumbnails.

   
   

7. Click on the new channel (laughingboy) in the Channels palette.
8. Even though you have converted the selection to what is now a mask, you’ll see that the dancing ants still appear around the original selection. Press Ctrl + D (Windows) or Cmd + D (Mac) to deselect.
   
   
9. At this point you can refine your selection even further. If you paint with a black Paintbrush, you are subtracting from the selection and if you paint with white you are adding to the selection. You can fill in any gaps this way.
10. To soften the outline selection use Filter > Blur > Gaussian Blur.
11. You may find that you have a few sharp corners in your selection, if you do use Filter > Noise > Median to round them off a bit.
12. To convert the mask back to a selection click the Load Selection button at the bottom of the Channels palette.

    
    

    Then click on the RGB channel at the top to switch back to a full colour composite.

13. Open up a second image that you want to use as a background for your selection then with the Move tool , drag your selection onto the new image.

    
    

14. To complete the image, I flipped the boy horizontally by choosing Edit > Transform > Flip Horizontal. And my final image looks like this :

    
    

The alpha channel in the original image is saved when the file is saved and can be reused at any time.

Got anything to say? Go ahead and leave a comment!