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The contrast ranges from black at the weakest intensity to white at the strongest. [1] Grayscale images are distinct from one-bit bi-tonal black-and-white images, which, in the context of computer imaging, are images with only two colors: black and white (also called bilevel or binary images). Grayscale images have many shades of gray in between.
White is at the extreme upper end of the achromatic value scale and black is at the extreme lower end of the achromatic value scale, with all the colors normally considered tones of achromatic gray colors in between. Since achromatic colors have no hue, the hue code (h code) is left blank for achromatic colors (usually marked as a dash).
The Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer. [1] Adams described the Zone System as "[...] not an invention of mine; it is a codification of the principles of sensitometry, worked out by Fred Archer and myself at the Art Center School in Los Angeles, around 1939–40."
The scale between dark (black) and light (white) values. Value refers to the degree of perceivable lightness of tones within an image. [ 2 ] The element of value is compatible with the term luminosity , and can be "measured in various units designating electromagnetic radiation ". [ 6 ]
As the value increases above 50%, it has the effect of tinting, and full lightness produces white. At zero saturation, lightness controls the resulting shade of gray. A value of zero still produces black, and full lightness still produces white. The midpoint value results in the "middle" shade of gray, with an RGB value of (128,128,128).
In photography, painting, and other visual arts, middle gray or middle grey is a tone that is perceptually about halfway between black and white on a lightness scale; [1] in photography and printing, it is typically defined as 18% reflectance in visible light. [2] [3]
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In principle, one wants to scale all relative luminances in an image so that objects which are believed to be neutral appear so. If, say, a surface with = was believed to be a white object, and if 255 is the count which corresponds to white, one could multiply all red values by 255/240.