Using the Quick Mask Option
Photoshop represents selection using an animated single-pixel wide marquee, sometimes referred to as a crawling or marching ant marquee. Typically the enclosed or "marquee" area represents the working area of the document. Unfortunately, when selections become complicated you wind up with crawling ants marching all over the screen. While complicated selections are a part of the Photoshop designer's life, they shouldn't have to be hard to visualize or modify. Photoshop knows this and created the Quick Mask option. When you're using Quick Mask, Photoshop displays the selected areas with a user-defined color and opacity. Then by using your painting tools, you can make quick work of modifying the selection.

Modify Selections with Quick Mask
Create a selection using any of Photoshop's selection tools.
Click the Default Colors button to default your foreground and background painting colors to black and white.
Click the Quick Mask button to enter Quick Mask mode.By default the selected area remains clear and the unselected area becomes masked with a 50 percent red.
Select the Paintbrush tool.
Add to the selection by painting the Quick Mask with white and black. In Quick Mask mode, painting with black produces 50 percent red, and painting with white opens up the original image.
Click the Edit In Standard Mode button to revert the image back to a normal selection marquee.
Continue using the Edit In Quick Mask and Edit In Standard Mode buttons until you achieve the desired selection.
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