color2-13may10-150dpi-med

# color2-13may10-150dpi-med - Color II: applications in...

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Color II: applications in photography Marc Levoy Computer Science Department Stanford University CS 178, Spring 2010 Begun 5/13/10, finished 5/18, and recap slides added.

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! Marc Levoy Outline ! spectral power distributions ! color response in animals and humans ! 3D colorspace of the human visual system and color Flter arrays in cameras ! reproducing colors using three primaries ! additive versus subtractive color mixing ! cylindrical color systems used by artists (and Photoshop) ! chromaticity diagrams color temperature and white balancing standardized color spaces and gamut mapping 2
! Marc Levoy Newton’s color circle ! previous authors could not move beyond linear scales, because they felt compelled to include black and white as endpoints ! Newton closed the circle by removing black and white, then adding extra-spectral purples not found in the rainbow by mixing red at one end with violet at the other end 3 Peter Paul Rubens and F rançois d'Aguilon (1613) Isaac Newton (1708) (http://www.handprint.com/HP/WCL/color6.html)

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! Marc Levoy Cylindrical color spaces (contents of whiteboard) ! given one circular scale and two linear scales, i.e. one angle and two lengths, the logical coordinate system is a cylindrical one ! selection of colors within such a system is easily done using 1D scales for H, S, and L, or 2D surfaces of constant H, S, or L 4
! Marc Levoy Cylindrical color spaces ! a cylinder is easy to understand, but colors near the top and bottom are indistinguishable single cone solves this by compressing top & bottom to a point ! when artists mix “complementary” lights, they expect to get white, but halfway from red to cyan in HSL space is gray HSV pushes the white point down onto the max-S plane painters might prefer an inverted cone, with black on this plane 5 HSL cylinder (wikipedia) HSL double cone HSV single cone

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! Marc Levoy Munsell color system ! spacing of colors is perceptually uniform (by experiment) ! outer envelope of solid determined by available inks 6 Albert Munsell (1858-1918) 3-axis colorspace 1905 book CG rendering of 1929 measurements (wikipedia)
! Marc Levoy A menagerie of color selectors 7

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! Marc Levoy Color selection in Photoshop 8
! Marc Levoy Photoshop’s color selector in HSL space (contents of whiteboard) ! the main rectangle in Photoshop’s color selector is a 2D surface of constant hue in cylindrical color space, hence varying saturation and lightness ! the vertical rainbow to its right (in the dialog box) is a circumference along the outside surface of the cylinder, hence a 1D scale of varying hue and constant lightness and saturation 9

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! Marc Levoy Color selection in Photoshop 10 brightness saturation hue
! Marc Levoy Color selection in Photoshop 11 Cartesian to cylindrical coordinate conversion HSV ! HSB

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! Marc Levoy Color selection in Photoshop 12 3 x 3 matrix conversion
! Marc Levoy Color selection in Photoshop 13 we’ll cover this later in the lecture

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! Marc Levoy Recap !
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color2-13may10-150dpi-med - Color II: applications in...

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