Take advantage of one of Keynote’s most powerful features to adjust photos

Many people hail Photoshop as the king of all photo editors, but you don’t need such an expensive piece of software to perform one of its best tricks: background removal. While Adobe’s professional tool does provide some excellent ways to cut out unwanted sections of an image, Keynote has a powerful feature of its own – known as Instant Alpha – that will do the same job in most instances.

You don’t need an expensive graphics app to replace a background in Keynote.

This nifty feature is available from Keynote’s Format menu and enables you to quickly cut out sections of an image by dragging your cursor over the area you want to remove. The process isn’t the same as cloning, where the software attempts to fill in the offending area. Instant Alpha looks at the colors you are selecting and removes them from the image, leaving a transparent space and keeping the rest of the image intact. Because of this, it’s best to work with large clean areas of background color when cutting out images, otherwise you might end up inadvertently removing parts of your foreground image too.

In this tutorial, we’re going to remove a rather dull background from behind a row of houses in one of Keynote’s default images and replace it with a more interesting skyline of our own. You can follow the guide and use one of your own photos as the background, but also make sure to experiment with other uses of Instant Alpha such as removing a background from an image so that it can sit above your slide without blocking text and any other elements.

Remember to save your project frequently; should you make a mistake, you can jump back quickly and fix things, but also remember that you can hit command+Z at any time during your Instant Alpha work to undo the last action you performed.

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Change a Photo’s Background with Instant Alpha

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  Select Your Image

  For the purpose of this tutorial we’ve selected the Typeset Keynote theme and loaded the Photo - Horizontal slide template from the Masters menu at the top of the Keynote interface. This slide has a row of houses with a simple clean background for us to remove.

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  Enter Instant Alpha

  In order to swap backgrounds we need to first remove the existing background. Click on the image then select Instant Alpha under the Format menu. Drag the cursor across the background, making sure the blue area only covers the background and not the houses.

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  Fine-Tuning

  Instant Alpha may not have made some areas close to the foreground transparent. Zoom in using shift+option and apply small amounts of Instant Alpha to these areas until the background has gone completely, making sure not to touch the foreground image.

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  Add a New Background

  Locate an image with a background you would like to add from the Finder or use the Media Browser in Keynote. Drag the image into your project, making sure that it’s outside the field for your existing image and select Mask with Shape from the Format menu.

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  Mask Your Image

  The next thing to do is position your background image above the original and use the mask to crop it down to size. It’s best to use a rectangle as a mask so that you can fit it exactly over the original image and replace the entire background.

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  Send it Back

  Now select your new background and click on Send to Back from the Arrange menu. This will move it behind your existing image and make it appear as a background. Then select it and edit the mask if need be to create the perfect fit for the image.

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  Final Adjustments

  You may notice areas that need a little more Instant Alpha work once the background is in place. If so, click on the original image to select it and apply Instant Alpha to these sections. With the original image selected, hold the ç key and click on the new background.

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  Lock it Down

  To reduce the chance of moving either image accidentally, select Lock from the Arrange menu, which will fix the position of both images on the slide to prevent them from moving until they are unlocked. You can also unlock an image from the Arrange menu.

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  Quick Tip

  You can make as many passes using Instant Alpha as you wish to remove a background. When a background has a more complex set of colors it’s worth zooming in and using Instant Alpha to remove the background piece by piece.

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