Magnetic Lasso Tool
Whereas the Lasso and Polygonal Lasso tools are relatively straightforward, the Magnetic Lasso tool has a bunch of neat tricks up its sleeve. This tool can be a huge timesaver in that it allows you to trace around the edge of an object without having to be overly precise. You don't have to break a sweat making all of those tiny, painstaking movements with your mouse. Instead, you can make big sloppy selections, and the Magnetic Lasso will do the fine-tuning for you (you don't even have to hold down the mouse button). What's more, if it doesn't do a great job in certain areas, you can hold down Option (Mac) or Alt (Windows) to temporarily access the freeform Lasso tool. However, before using the Magnetic Lasso tool, you'll want to experiment with its settings in the options bar (Figures 2.30 to 2.32). If you do nothing else, pay close attention to the Edge Contrast setting.
Figure 2.31. A Magnetic Lasso selection with a Frequency setting of 5. (©2005 Stockbyte, www.stockbyte.com)
Figure 2.32. A Frequency setting of 99.
Edge Contrast
I think this setting is the most important of the bunch. It determines how much contrast there must be between the object and the background for Photoshop to select the object. If the object you're attempting to select has well-defined edges, you should use a high setting (Figure 2.33). You can also use a large Lasso tool width. On the other hand, if the edges are not well defined, you should use a low setting and try to be very precise when dragging (Figure 2.34).
Figure 2.33. High edge contrast (20%). (©2005 Stockbyte, www.stockbyte.com)
Figure 2.34. Low edge contrast (7%).
If the Magnetic Lasso tool is not behaving itself, you can temporarily switch to the freeform Lasso tool by holding down Option (Mac) or Alt (Windows) as you drag (with the mouse button held down). You can also periodically click to manually add anchor points to the selection edge. If you want to use the Polygonal Lasso tool, hold down Option (Mac) or Alt (Windows) and click in multiple areas of the image (instead of dragging). If you don't like the shape of the selection, you can press the Delete key to remove the last anchor point. (Pressing Delete multiple times deletes multiple points.) Once you have a satisfactory shape, finish the selection by pressing Return or Enter or by double-clicking. Remember, if you don't create a closed shape, Photoshop will finish it for you with a straight-line segment.
If you really get used to the features available with the Magnetic Lasso tool, you'll be able to create most of your basic selections with this tool alone. This will take some time, and you'll sometimes have to supplement its use by holding down Option (Mac) or Alt (Windows) to access the other Lasso tools for areas the magnetic one has trouble selecting. And if it ever gets completely out of hand, you can always press the Escape key to abort your selection and then start from scratch again.