Use Instant Alpha to quickly cut out images in Preview

Preview is the application that opens by default when you double-click on a JPG, PNG or other image file on your Mac. As OS X has matured, Preview has taken on some interesting features that are not immediately obvious but that provide some of the functionality of bigger image editing programs. The ability to identify areas of an image that are of similar tone and colour is one such tool, and can be used to cut parts of a picture out of their background. It works with the alpha channel, which is a part of a digital image that stores information about transparency and the way that existing colours overlap. You don’t need Photoshop to work with the alpha channel – you can do it from Preview.

Step 1: Open your image
Take an image where you want to cut something out of its background or remove an element from the whole picture, leaving only the background.

Step 2: Choose the tool
Go to the Select button in the Toolbar in Preview and click on it to reveal the additional tools. Select the tool called Instant Alpha.

Step 3: Select the background
Click somewhere in the background and drag the mouse to choose a larger area. The detection works best where the background colour is quite uniform.

Step 4: Redo if necessary
If Preview has included some incorrect areas, click once to undo the selection. Drag the mouse while holding to increase or decrease the sensitivity of the tool.

Step 5: Hit backspace
With the areas selected, hit backspace on your keyboard. The selected content is deleted. With complex backgrounds it’s better to work in stages.

Step 6: Keep removing background
It’s rare that one or two selections will clear a whole background so you may need to keep selecting areas and deleting them manually.

Step 7: Use the other tools
If you’re left with lots of small, fiddly areas of background, switch to the Rectangle or Lasso tool and draw around larger areas to delete them quickly.

Step 8: Stick with it
When you have lots of tiny areas to remove, such as areas between strands of hair, you will have to spend a while deleting but you will end up with a cut out image.

Step 9: Reverse the process
To remove the subject rather than background, select using the Instant Alpha tool then choose Edit>Invert Selection. Hitting backspace will remove the subject.

(Click on the image below to zoom in and view the annotations)