Adobe Photoshop CC
About selecting pixels
A selection isolates one or more parts of your image. By selecting specific areas, you can edit and apply effects and filters to portions of your image while leaving the unselected areas untouched.
The easiest way to select pixels in your image is to use the quick selection tools. You can also select areas of a certain shape with the marquee tools or use the lasso tools to make a selection by tracing an element in your image. It's also possible to make selections based on a range of colors in an image. There are commands in the Select menu to select, deselect, or reselect all pixels.
In addition to pixels, vector data can be used to make selections. Use the pen or shape tools to produce precise outlines called paths. Paths can be converted to selections.
Selections can be copied, moved, and pasted, or saved and stored in an alpha channel. Alpha channels store selections as grayscale images called masks. A mask is like the inverse of a selection: it covers the unselected part of the image and protects it from any editing or manipulations you apply. You can convert a stored mask back into a selection by loading the alpha channel into an image.
While paths can be converted to selections, selections can also be converted to paths.
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InfiniteSkills expert Andy Anderson explains the basics of how to make selections.
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Select, deselect, and reselect pixels
If a tool is not working as expected, you may have a hidden selection. Use the Deselect command and try the tool again.