Since both axes use logarithmic units, the slope of the linear section of the curve is called the gamma of the film. For a given film formulation and processing method, this curve is its characteristic or Hurter–Driffield curve. When a photographic film is exposed to light, the result of the exposure can be represented on a graph showing log of exposure on the horizontal axis, and density, or negative log of transmittance, on the vertical axis. For a power-law curve, this slope is constant, but the idea can be extended to any type of curve, in which case gamma (strictly speaking, "point gamma" ) is defined as the slope of the curve in any particular region. That is, gamma can be visualized as the slope of the input–output curve when plotted on logarithmic axes. Gamma correction is, in the simplest cases, defined by the following power-law expression: Gamma correction or gamma is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems. The effect of gamma correction on an image: The original image was taken to varying powers, showing that powers larger than 1 make the shadows darker, while powers smaller than 1 make dark regions lighter.
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