You modify the performance of the magnetic lasso tool by adjusting the values on the Options bar. (If you don't see the Options bar, press Enter to display it.) The first two options — Feather and Anti-aliased — define the softness of the final selection outline, just as they do for the standard lasso tool. The others control how the magnetic lasso positions lines and lays down points:
♦ Width: I might have named this option Sloppiness Factor. It determines how close to an edge you have to move the cursor for Photoshop to accurately see the image element. Large values are great for smooth elements that stand out clearly from their backgrounds. If I raise the Width to 20 when selecting the top of the skull, for example, I can move the cursor 20 pixels away from the skull and Photoshop still shrinks the selection tight around the skull's edge. That's a lot of wiggle room and makes my life easier. But when you're selecting narrow passageways, you need a low value to keep Photoshop from veering off to the wrong edge. The spot where the pointy jaw meets with the snout is a good example of a place where I need to set a small Width and move very carefully around the edge.
Tip The great advantage to the Width value is that you can change it on the fly by pressing a bracket key. Press the [ key to lower the Width value; press the ] key to raise the value. Shift+[ lowers the value to its minimum, 1, and Shift+] raises it to the maximum, 40.
If you have a pressure-sensitive tablet and select the Stylus Pressure check box, you can control the sloppiness factor dynamically according to how hard you press on the pen. Bear down to be careful; let up to be sloppy. Because this is the way you probably work naturally, you'll be able to adjust the width as needed without even thinking much about it.
♦ Frequency: This option tells the magnetic lasso when to lay down points. As you drag with the tool, the line around the image changes to keep up with your movements. When some point in the line stays still for a few moments, Photoshop decides it must be on target and anchors it down with a point. If you want Photoshop to anchor points more frequently, raise the value. For less frequent anchoring, lower the option. High values tend to be better for rough edges; lower values are better for smooth edges.
* Edge Contrast: This is the simplest of the options. It tells Photoshop how much contrast there has to be between the element you're trying to select and its background to even be recognized. If the foreground element stands out clearly, you may want to raise the Edge Contrast value to avoid selecting random flack around the edges. If the contrast between foreground and background is subtle, lower the value.
Most of the time, you can rely on the bracket keys to adjust the Lasso Width and leave the Frequency and Edge Contrast values set to their defaults. When dealing with a low-contrast image, lower the Edge Contrast value to 5 percent or so. And when selecting unusually rough edges, raise the Frequency to 70 or more. But careful movements with the magnetic lasso tool go farther than adjusting any of these settings.
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