Color II applications in photography CS 178 Spring 2011 Begun 5 12 11 finished 5 17 Marc Levoy Computer Science Department Stanford University Outline spectral power distributions color response in animals and humans 3D colorspace of the human visual system and color filter 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 The RGB cube choose three primaries R G B pure wavelengths or not adjust R 1 G 1 B 1 to obtain a desired reference white this yields an RGB cube Flash demo http graphics stanford edu courses cs178 applets locus html 3 programmers like RGB as a way of selecting colors but artists don t Marc Levoy Newton s color circle http www handprint com HP WCL color6 html Peter Paul Rubens and Fran ois d Aguilon 1613 4 Isaac Newton 1708 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 added extra spectral purples not found in the rainbow by mixing red at one end with violet at the other end Marc Levoy Cylindrical color spaces contents of whiteboard 5 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 Marc Levoy Cylindrical color spaces wikipedia HSL cylinder double 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 single cone pushes the white point down onto the max S plane painters might prefer an inverted cone with black on this plane Marc Levoy 6 HSV single cone a cylinder is easy to understand but colors near the top and bottom are indistinguishable HSL double cone Munsell color system wikipedia Albert Munsell 1858 1918 3 axis colorspace 1905 book CG rendering of 1929 measurements 7 spacing of colors is perceptually uniform by experiment outer envelope of solid determined by available inks Marc Levoy A menagerie of color selectors 8 Marc Levoy Photoshop s color selector in HSL space contents of whiteboard 9 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 Marc Levoy Color selection in Photoshop brightness saturation 10 hue Marc Levoy Color selection in Photoshop Cartesian to cylindrical coordinate conversion HSV Photoshop s HSB 11 Marc Levoy Color selection in Photoshop 3 x 3 matrix conversion 12 Marc Levoy Color selection in Photoshop we ll cover this later in the lecture 13 Marc Levoy Recap 14 hue is well represented by a color circle formed from the rainbow plus mixtures of the two ends to form purples saturation is well represented by a linear scale from neutral black gray or white to fully saturated single wavelength lightness is well represented by a linear scale either openended if representing the brightness of luminous objects or closed ended if representing the whiteness of reflective objects given one circular scale and two linear scales the logical coordinate system is cylindrical where H S L r y selection of colors within such a system is easily done using 1D scales for each of H S and L or that in combination with 2D surfaces of constant H S or L Que s t ions Marc Levoy Outline spectral power distributions color response in animals and humans 3D colorspace of the human visual system reproducing colors using three primaries additive versus subtractive color mixing cylindrical color systems used by artists and Photoshop 15 and color filter arrays in cameras chromaticity diagrams color temperature and white balancing standardized color spaces and gamut mapping Marc Levoy Chromaticity diagrams choose three primaries R G B pure wavelengths or not adjust R 1 G 1 B 1 to obtain a desired reference white this yields an RGB cube blue 16 R R G B g G R G B white one may factor the brightness out of any point in the cube by drawing a line to the origin and intersecting this line with the triangle made by corners Red Green Blue all points on this triangle which are black addressable by two coordinates have the same brightness but differing chromaticity r green red Marc Levoy Chromaticity diagrams choose three primaries R G B pure wavelengths or not adjust R 1 G 1 B 1 to obtain a desired reference white this yields an RGB cube Flash demo r R R G B g G R G B http graphics stanford edu courses cs178 applets threedgamut html 17 one may factor the brightness out of any point in the cube by drawing a line to the origin and intersecting this line with the triangle made by corners Red Green Blue all points on this triangle which are addressable by two coordinates have the same brightness but differing chromaticity g r Marc Levoy Chromaticity diagrams 18 g this triangle is called the rgb chromaticity diagram for the chosen RGB primaries mixtures of colors lie along straight lines neutral black to white lies at r 0 g 0 does not enclose spectral locus the same construction can be performed using any set of 3 vectors as primaries even impossible ones 0 or 0 or 0 the CIE has defined a set of primaries XYZ and the associated xyz chromaticity diagram x 0 y 0 does enclose spectral locus one can connect red and green on the locus with a line of extra spectral purples x y is a standardized way to denote colors rgb chromaticity diagram r y Hunt CIE xyz chromaticity diagram x Marc Levoy Application of chromaticity diagrams 1 color temperature and white balancing correlated color temperatures 3200 K 4000 K 5000 K 6000 K 6500 K 7500 K incandescent light cool white fluorescent equal energy white D50 E midday sun photo flash overcast television D65 northern blue sky wikipedia 19 the apparent colors emitted by a black body radiator heated to different temperatures fall on a curve in the chromaticity diagram for non blackbody sources the nearest point on the curve is called the correlated color temperature Marc Levoy White balancing in digital photography 1