When I talk about an unfamiliar processing construct such as a mask, I ask people to consider the simplest element of an image — a pixel. A pixel has two attributes: color (chrominance) and brightness (luminance). However, in Photoshop, pixels have an additional attribute called opacity, which determines the visibility of the pixel and how easily you can see a pixel in an underlying layer. Grayscale images called masks manage the opacity of a pixel.
Every pixel has an associated percentage that defines its opacity. A 100 percent opacity means the pixel is fully visible and a 0 percent opacity means it is invisible.
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