When applied to processing images, masking refers to hiding (technically, making transparent) parts of an image, so that only the unmasked part remain visible. The result of masking is that the unmasked part looks like it was cut out of the original image, and the rest was discarded.

How does one apply a mask to an image? Both Keynote and Powerpoint offer tools.


If you haven’t masked with Keynote before, you will first want to add some icons to your default toolbar. You only need to do this once, and these icons will make all later manipulations quicker.

Adding the icons is simple. Just go to: View => Customize Toolbar, and then drag the “Mask” and “Instant Alpha” icons to this toolbar, then press “Done.”

This installation takes less than 20 seconds:

These two icons open up two possibilities for masking.

Instant Alpha

The Instant Alpha tool is useful for images like the bird photograph above, in which an object is surrounded a background of a uniform color. To use it, just:

  1. click on the image to select it
  2. click on the Instant Alpha icon
  3. click on the background, and drag to expand the area masked

Instant alpha can be used to liberate objects from their frames. This juxtaposition of pictures

can thus be transformed into a juxtaposition of the objects inside the pictures:

Masking with Shapes

The Mask icon is used together with the Shapes icon. With this combination you can cut out a portion of a given shape out the image. The simple process involves these steps:

  1. Select a shape and adjust its contours and placement
  2. Use ⌘ + click to select BOTH the background image and the foreground shape
  3. Click on the Mask icon, then press Done

By selecting the Pen tool within the Shape menu, you can also customize the shape that you cut out.


Masking in PowerPoint can be confusing since PowerPoint does not have a tool named “Mask” like Keynote does. Worry not! You can achieve the same effect with these simple steps:

  1. Click the image and select CROP in the Picture Format tab
  2. Choose a shape and adjust it to mask the desired segment
  3. Press CROP again

What if you want to mask a shape that PowerPoint default crop tool does not support? Again, worry not! You can solve it with Toolbar, a default image editing tool on Mac.

  1. Duplicate the image file to prevent any permanent alternation to it
  2. Click “Toolbar” icon on the top right
  3. Choose “Lasso Selection” and mark the desired shape
  4. Press “Crop” and “Convert”
  5. Open the duplicate file (cropped and converted) and lay it over the original image slide

Next Steps: Crafting More Complex Masks

Keynote and Powerpoint make it easy to mask simple shapes. Eventually, however, the usefulness of masking for effects like highlighting may inspire you to try to cut out more complex shapes. This is when you may want to begin exploring Photoshop.

Photoshop is extremely powerful, but its correspondingly large array of tools can make learning it seem daunting. Fortunately, there are some excellent tutorials. Some of the best of these tutorials can be found at LinkedIn Learning (formerly and Harvard affiliates can use their Harvard logins to access these tutorials for free.

Once you have learned how to mask in Photoshop, you can create shifting highlight animations such as this:

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