lecture17 - Color and Texture The Appearance of Colors...

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1 Color and Texture The Appearance of Colors Color appearance is strongly affected by (at least): other nearby colors, adaptation to previous views “state of mind” We show several demonstrations in what follows.
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2 The Appearance of Colors Hering, Helmholtz: Color appearance is strongly affected by other nearby colors, by adaptation to previous views, and by “state of mind” Film color mode: View a colored surface through a hole in a sheet, so that the colour looks like a film in space; controls for nearby colors, and state of mind. Other modes: Surface colour Volume colour Mirror colour Illuminant colour By experience, it is possible to match almost all colors, viewed in film mode using only three primary sources - the principle of trichromacy . Other modes may have more dimensions Glossy-matte Rough-smooth Most of what follows discusses film mode.
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4 Why specify color numerically? Accurate color reproduction is commercially valuable Few color names are widely recognized by English speakers - About 10; other languages have fewer/more, but not many more. It’s common to disagree on appropriate color names. Color reproduction problems increased by prevalence of digital imaging - eg. digital libraries of art. How do we ensure that everyone sees the same color? Color Matching Experiments - I Show a split field to subjects; one side shows the light whose color one wants to measure, the other a weighted mixture of primaries (fixed lights). Each light is seen in film color mode.
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5 Color matching experiments - II Many colors can be represented as a mixture of A, B, C write M=a A + b B + c C where the = sign should be read as “matches” This is additive matching. Gives a color description system - two people who agree on A, B, C need only supply (a, b, c) to describe a color. Subtractive matching Some colors can’t be matched like this: instead, must write M+a A = b B+c C This is subtractive matching. Interpret this as (-a, b, c) Problem for building monitors: Choose R, G, B such that positive linear combinations match a large set of colors
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6 The principle of trichromacy Experimental facts: Three primaries will work for most people if we allow subtractive matching Exceptional people can match with two or only one primary. This could be caused by a variety of deficiencies. Most people make the same matches. There are some anomalous trichromats, who use three primaries but make different combinations to match. Grassman’s Laws For colour matches made in film colour mode: symmetry: U=V <=>V=U transitivity: U=V and V=W => U=W proportionality: U=V <=> tU=tV additivity: if any two (or more) of the statements U=V, W=X, (U+W)=(V+X) are true, then so is the third These statements are as true as any biological law. They mean that color matching in film color mode is linear.
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7 Linear color spaces
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lecture17 - Color and Texture The Appearance of Colors...

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