Join Jan Kabili for an in-depth discussion in this video Snapping to edges with the Magnetic Lasso tool, part of Photoshop CS5: Selections in Depth.
The Magnetic Lasso tool is useful for selecting objects that have well-defined high-contrast edges. This tool does a lot of the work for you: it locates the edge of an object based on edge contrast and lays down anchor points to fasten a selection boundary to that edge. All you have to do is guide the tool near the edge of the object. You don't even have to keep your finger pressed down on the mouse as you move your cursor along the edge. This is an ideal image on which to practice using the magnetic lasso, because there's a lot of contrast between the light skull and the dark background.
I'll select the magnetic lasso from the toolbox here behind the Lasso tool. I'll get the tool started by clicking to set the first anchor point. Then, without pressing down on the mouse, I'll just move the mouse along the edge, floating it there, as it automatically sets down anchor points and lines that represent the selection border. It's doing a great job for me, until I get here, and now it's set down some anchor point that I don't want.
So, my tip for you is how to undo anchor points that you don't want. I am going to zoom in, so you see this little better. As you can see, there are a couple of anchor points on this inner line on the skull. So, to delete those, without pressing down on the mouse, I'm going to move my cursor back, and I'm going to press the Delete key, and each time I press the Delete key, I'm undoing another anchor point, as if I were unthreading. When I get back to what I consider a good anchor point, I'll try moving forward again to allow the tool to set some anchor points.
Now, I know that I am going to getting trouble here because this is a really low-contrast edge with a high-contrast dark line nearby, and it's just fooling the tool. So, there are a couple of things that I can do to perhaps help the tool along. One is that I can change the width of the brush tip. I'm going to hold the Caps Lock key and move my mouse a bit, so that you can see the width of the brush tip. This width is determining the circumference within which the magnetic lasso is going to set down points.
So, if I move along here, you can see that this black line is within the circumference of the brush, and that's why it's laying down anchor points there. So again, I'm going to back up and press the Delete or Backspace key until I get to a good anchor point. Now, I'm going to help the tool by narrowing the width of the brush tip. The best way to do that is on-the-fly, using the Left Bracket key on the keyboard. If you keep your eye in the Width field up there in the Options for this tool, you'll see that as I press the Left Bracket key, the width is being reduced.
So, now with that smaller brush circumference, let's see what happens as I move along the edge. Now, I am getting a better result. So that's one thing I can do. Another thing I can do is try to set my own anchor points along this difficult part of the edge by just clicking as I move along. So, I am clicking and clicking, and each time I do that I'm manually creating an anchor point. I am going to press the spacebar and then click and drag to move up a bit. I'll release the spacebar.
I'll show you one more thing that you can do to get past the difficult part of an edge, and that is to temporarily switch the magnetic lasso to a regular lasso and draw free-form. To do that, I am going to press down on the Option key on the Mac or the Alt key on the PC and then click and drag. You can see that my cursor tip changes, and I'm just making a free-form drawing along the edge here. When I get to an area of high contrast, again, I'll release the Option or Alt key and release my mouse.
I'm also going to make the width of the brush tip larger, so that I can be a little looser about where I'm floating the cursor; it's just easier to work with a larger brush tip. So, I'll press the Right Bracket key a few times, and now I'll continue moving along this edge. Here's another place where the brush is having trouble, so I'll back up, I'll press the Delete key, and I'll try manually creating some anchor points by clicking. I am going to zoom out, and I'll just continue along this edge without pressing down on the cursor, allowing the Magnetic Lasso tool to find the edge and set down these anchor points.
Where necessary, I'll click to set my own anchor points until I get all the way up to the top of the image. Then I'm going to hold down the Option key, the Alt key on the PC, and move my mouse to change back to the Lasso tool temporarily, so that I can just draw along this top edge. When I get back to the beginning, I'll release my Option or Alt key, and I'll release the mouse, and that completes the selection. Now, I can treat this like any other selection.
I could fill the selection with color, I could filter the selection, or use the selection for any of the other purposes that I've shown you throughout this course. Admittedly, the magnetic lasso can take a little getting used to, but if you try using it with the Caps Lock key on so that you can judge the width of the brush tip and use the Left and Right Bracket keys to change the width where necessary and set your own anchor points if need be by clicking along the edge, I think you'll come to like using this tool to select objects with high- contrast edges, like this.
- Understanding the when and why of making selections
- Combining and transforming selections
- Selecting fine detail with Refine Edge
- Capturing soft and hard edges in one selection
- Understanding the relationship of selections to masks
- Removing color fringe around selections
- Using the Marquee and Lasso tools
- Working with the Color Range command
- Selecting with the Pen tool and paths
- Making easy selections with the Quick Selection tool
- Working with Refine Mask
- Sharing selections between images