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Gamma : The values produced by a monitor from black to white are nonlinear. If you graph the values, they form a curve, not a straight line. Gamma defines the slope of that curve at halfway between black and white. Gamma adjustment compensates for the nonlinear tonal reproduction of output devices such as monitor tubes.
Gamut : The total range of colors produced by a device. A color is said to be "out of gamut" when its position in one device's color space cannot be directly translated into another device's color space.
GIF : Graphic Interchange Format. This file format is commonly used on the internet.
Gradient Fill : In PhotoShop, a graduated blend between the Foreground and Background colors that is produced using the Gradient tool.
Grayscale : A color space where colors are represented by their luminance values only, i.e. saturation and hue are zero. An image that contains black, white, and up to 256 shades of gray, but no color In PhotoShop, Grayscale is a one-channel image mode.