When you are building a presentation or designing a website on your Mac, you frequently need to overlay a logo or a product photograph onto a colored background. If the original image was saved as a standard JPEG, it will have an ugly, solid white background box surrounding it, completely ruining the aesthetic of your design. You do not need to purchase expensive graphic design software like Adobe Photoshop to remove it. The built-in macOS Preview application contains a highly advanced “Instant Alpha” tool that can digitally erase solid backgrounds in seconds.
How to Access the Instant Alpha Tool
The Instant Alpha engine uses edge-detection algorithms to identify exactly where the background ends and the main subject begins.
- Double-click your image file to open it in Preview.
- Click the Show Markup Toolbar button at the top-right corner of the window (the icon shaped like the tip of a pencil).
- In the new toolbar that appears, look at the far left side. Click the Instant Alpha icon (it looks like a magic wand with stars around it).
Once you click the icon, it will turn dark grey to indicate it is active, and your mouse cursor will transform into a precision crosshair.
How to Remove the Background
Using the tool requires a specific click-and-drag motion to dictate the tolerance level of the edge-detection algorithm.
- Position your crosshair directly over the solid white background (away from your primary subject).
- Click and hold down your mouse button.
- Slowly drag your mouse slightly to the left or right. As you drag, you will see a red, semi-transparent mask begin to aggressively expand across the white background.
- Continue dragging until the red mask perfectly touches the edges of your primary subject without bleeding inside of it.
- Release the mouse button. The area you selected will now be highlighted with a moving, dotted line (the “marching ants”).
Press the Delete key on your Mac keyboard. The entire white background will instantly vanish, replaced by a grey-and-white checkerboard pattern. This checkerboard is the universal symbol for digital transparency.
Understanding PNG Conversion
The standard JPEG file format is mathematically incapable of supporting transparency. If you attempt to delete the background of a JPEG image, Preview will instantly display a warning box stating that the document must be converted to a PNG file.
Simply click the Convert button on this warning prompt. Preview will automatically rewrite the file header and save it as a high-quality PNG, permanently locking in your new, perfectly transparent background. You can now drag and drop this image into Keynote, Pages, or any web design software without the ugly white box.