OS X: Instant Alpha Using Preview

Use Preview’s Instant Alpha feature to erase backgrounds in images or make certain parts transparent.

Start with a base image.  In this example, I use a picture of the Minneapolis skyline and will try to remove the fog/clouds on top of the building.

Open your image in Preview.  Click the Edit button and then the Instant Alpha button.

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Drag and hold your cursor over the clouds.  As you move your mouse (without letting go), you will see the red area increase/decrease.  This colored area will be what will become transparent.

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Depending on your image, you might go too far and highlight something you don’t want.  In that case, just reverse your mouse.

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Once you are satisfied with your selection, let go and the affected area will become highlighted.

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Press Delete to remove the clouds.  You will be prompted to convert the file to a PNG since that format supports the alpha channel.

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Now the clouds will be deleted.  Depending on your image, the feature might not have grabbed everything.  For instance, on one of the buildings, I zoomed in and refined the part that was missed.

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Now the image will be transparent where the clouds used to be.  You can easily use this feature to customize logo on the OS X login screen, or use it with Photoshop to make custom images.

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