Photoshop Selection: Winning Against the Marching Ants
Grab the Lasso tool in Photoshop and trace around something. Or use one of the other selection tools if you prefer. Regardless of how you do it, the boundary of your selection will be displayed as a dotted line that is commonly referred to as the "marching ants" because of how the dots seem to be moving around the outline of the selection. But while it can be nice to see what is selected and what isn't, the "ants" can get in your way sometimes by making it hard to see details near the boundary. Here's how to win against the ants.
The most straightforward solution, but still one overlooked by many users is to simply turn off the selection edge display. No more marching ants, no more problem seeing what you are doing. The pixels are still selected just the same as they were, but the visual cues that let you know where the edge is aren't there anymore. If you didn't know Photoshop would let you do this, it will. And all you have to do is to hit Control-H (or Command-H on Mac OS). This will turn off display of all "extras" including the marching ants selection edge, the grid, slices, smart guides and so on. You can turn them back on by hitting Control-H a second time. If you want to turn off just the selection edge without affecting any other "extras" you can use the View >> Show >> Selection Edges menu option.
My favorite way to deal with the problem though through the use of layer masks. If the adjustment you want to make can be done on an adjustment layer, simply make your initial selection via the Lasso or whatever your preference is, then create your adjustment layer. The display of your selection edge will be gone but your selection itself will now be contained within the layer mask for your new adjustment layer. From there you can use the paintbrush or other tools on your new mask to fine tune your selection. This works for Curves, Levels and anything else you can do on an adjustment layer. And you get the added bonus of being able to tweak both your selection and the effect at any point in the future, and without any added adverse consequence from having not done it that way in the first place.
By the way, since Photoshop allows you fade the edges of a selection and "partially" select pixels you may be wondering on just what the marching ants boundary represents. Adobe engineers had a decision to make. What they decided to do was to make the boundary represent an average. Pixels within the outline are at least fifty percent selected while those less than fifty percent selected will be outside.
You may also be wondering where the idea of the marching ants themselves came from. Folklore.com has it that early Apple pioneer Bill Atkinson was waiting for his dinner at a local burger joint in Los Gatos while thinking about how to represent a selection edge in a way that would be easy to see no matter what the underlying image looked like. As he did so, he noticed a waterfall on a sign in the bar for Hamm's Beer. The waterfall seemed to be animated in a way that it appeared to flow over some rocks into a lake below. This was in the early eighties when true computer animation would have been out of the question so he was fascinated by how the illusion was achieved. A moving mask layer within the sign selectively blocked portions of the falls and as each part came into and went out of view it conveyed a sense of motion. Perhaps he had already had a few beers himself, or perhaps not. Either way, Atkinson realized he could do something similar by animating the selection outline. It was Rod Perkins of the Apple Lisa applications team who decided that the effect looked like "marching ants." And the name stuck.