The Magnetic Pen is an option of the Freeform Pen tool that lets you draw a path that snaps to the edges of defined areas in your image. You can define the range and sensitivity of the snapping behavior, as well as the complexity of the resulting path. The Magnetic Pen and Magnetic Lasso tools share many of the same options.
For Width, enter a pixel value between 1 and 256. The Magnetic Pen detects edges only within the specified distance from the pointer.
For Contrast, enter a percentage value between 1 and 100 to specify the contrast required between pixels for that area to be considered an edge. Use a higher value for low-contrast images.
For Frequency, enter a value between 0 and 100 to specify the rate at which the Pen sets anchor points. A higher value anchors the path in place more quickly.
If you are working with a stylus tablet, select or deselect Pen Pressure. When this option is selected, an increase in pen pressure causes the width to decrease.
The most recent segment of the border remains active. As you move the pointer, the active segment snaps to the strongest edge in the image, connecting the pointer to the last fastening point. Periodically, the Magnetic Pen adds fastening points to the border to anchor previous sections.
Alt-drag (Windows) or Option-drag (Mac OS) to draw a freehand path.
Alt-click (Windows) or Option-click (Mac OS) to draw straight segments.
Press the open square bracket key ([) to decrease the Magnetic Pen width by 1 pixel; press the close square bracket key (]) to increase the pen width by 1 pixel.
Press Enter (Windows) or Return (Mac OS) to end an open path.
Double-click to close the path with a magnetic segment.
Hold down Alt (Windows) or Option (Mac OS), and double-click to close the path with a straight segment.