How-to: Get started with Instant Alpha and Smart Lasso in Preview for OS X

As you may know, Instant Alpha is a feature that allows you to remove an object in an image from its background. You can find this feature in apps like Pages and Keynote, as well as the Preview app that comes built-in on Macs. As we’ve written recently in articles about signatures and marking up images, Preview has a lot of capabilities that don’t get much press.

In this article, you’ll learm how to use Instant Alpha and Smart Lasso to create custom images, remove an object from its background, and some advantages of using Preview to create custom images.

Images created in this way could be used for anything, including fun social media posts. They can be imported into iPhoto, cropped and used in books, calendars, or especially to customize a birthday or greeting card. Depending on what kind of image you want to create, you can really do quite a bit using Preview, Pages and a few other tips and tricks.

Removing an object from its background with Instant Alpha
This can be done in Preview, Pages, or Keynote. If you are making a Pages document or Keynote presentation and only plan to use the image in that document, you can just use Pages or Keynote. The image you create will be part of the Pages document or Keynote project you created.

However if you use Preview to perform Instant Alpha on the image, it can be saved in many formats for use in other projects, including websites, social media, iPhoto cards or calendars, email, and a lot more.

Another advantage of using Preview is the option to use selection tools called “Lasso” and “Smart Lasso.” Lasso lets you draw a circle (or any other shape) around and object to isolate it from the rest of the photo. Smart Lasso lets you draw a line around an object to select it while intelligently excluding the background. Using either tool, the object can then be saved as a new file or copied and pasted into a project.

To get started, open the image in Preview. The easiest way to do this is to drag the image to the desktop from iPhoto or Safari and double click to open it. If the image is already in Finder, simply double click on it. From here, click on the annotation button, a pen icon on the top right.

In this example, I will isolate and remove the red bell pepper. This can be done with relative ease because the background and surrounding objects are roughly the same colors. To do this, click on the Instant Alpha button, which looks like a magic wand icon. Then click, hold, and drag over the background of the image. As you drag, areas with similar colors are highlighted. The highlighted area will turn pink, and when you let go, a dotted line will appear around it. Press the delete key to remove the highlighted areas. Repeat this process as necessary, but only in the area surrounding the object you want to isolate.

To remove the image from the background, use the Smart Lasso tool to isolate the object. To do this, choose Smart Lasso from the Selection Tools menu at the top left. Next, click and drag a line around the border of the object. A wide pink line will appear as you drag. When you’ve completely circled the object and stopped clicking, a dotted line will appear around the border of the object.

To save the isolated object as a new file, choose Copy from the Edit menu, or press Command-C. Then choose “New from Clipboard” from the File menu or press Command-N. This will open the isolated object as a new image.

Then just choose Save from the file menu or press Command-S to save the new file.

Isolating the image using both Instant Alpha and the Smart Lasso tool often results in an image that has clean edges. It can be placed close to other images on another background because there is no border around the object. Depending on the nature of the project you might need to use Instant Alpha, or Smart Lasso, or both.

Using Pages to place the image on a different background
Now that the object has been removed from its background, it can be placed on a different background in Pages. To do this, drag both the image you created using Instant Alpha above and the image you want to use as the background into a Pages document. Click on the image you want to be the background and from the Arrange menu choose Send to Back. Then rearrange and resize the images until they are laid out the way you want.

Saving the final custom image can be done a few different ways. The quickest way is using Command-Shift-4 on the keyboard and dragging across the image to take a screen shot of it. The resulting screen shot is saved to the desktop. Although this is convenient, it results in a low quality image that may not be suitable for your project.

To create a higher quality image, you can use a two-step process. First, save the Pages document as a PDF. In the dialog box that appears, change the Image Quality to Best.

Then double click on the PDF, which opens it in Preview.  From the File menu, choose export and change the format to JPG. When doing this, slide the quality slider to “best” and change the DPI to 300 or higher. The resulting JPG will be higher quality. If necessary, crop the photo in iPhoto to maintain its quality.

Being able to create custom images can also be helpful for artistic and business needs.  The ability to do this simple but effective image manipulation is built into OS X with no external tools necessary.