Alpha 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 do not need Photoshop to work with the alpha channel, you can do it from Preview in just a few easy steps.

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 is rare that one or two selections will clear a whole background so you may need to keep selecting areas and deleting them manually.