ColorSlide 2Slide 3Slide 4Slide 5SuperpositionAnnouncementsSlide 8Slide 9Perceptual color spaceColor MatchingSome colors can’t be matchedSlide 13Slide 14CIE ModelSlide 16Slide 17Slide 18Additive Color Model RGBSubtractive Color Model CMYKColor SpecificationPerceptual Color Model HSVLogarithmic PerceptionSlide 24Logarithmic DisplaySlide 26Gamma CorrectionHalftoningSlide 29Spatial versus Intensity ResolutionSlide 31Slide 32Slide 33Slide 34Slide 35Slide 36Opacity and BlendingBlending in OpenGLDepth Cueing and FogIllumination AttenuationColor•Physics–Light is E-M radiation of different frequencies.–Superposition principle•Perception–3 cones -> 3D color space. (Metamers).–Convex subset of 3D linear space.–Color matching: can’t represent w/ 3 primaries.•Color Spaces–CIE – a standard–RGB – a bit more intuitive, Monitors, OpenGL–CMYK – subtractive, what we learn in art class.–HSV – More intuitive•More Perception –Perceptual distance–Context•Refs: H&B Chapter 12; “The Foundations of Color Measurement and Color Perception”, by Brian Wandell: ftp://white.stanford.edu/users/brian/ise/sid-colornotes.pdf(Wandell)Newton’s drawing:(Varshney)(Image Removed)(Image Removed)Color is a function(Angelopoulou)Superposition•Light is linear.•Light from source A + light from source B = Light from sources A & B.–Any color is a combination of pure colors.•Doubling intensity of source doubles amount of light reaching us.Announcements•Practice midterm available.Human Color Perception•Cones allow color perception.•3 types of cones sensitive to different frequencies•Perceptual color depends on how these are stimulated.(Image Removed)Metamers(Wandell)Perceptual color space•3D•Convex subspace–Cones don’t have negative response•So why are there so many color spaces, instead of just one?–There isn’t a unique color for each cone.–“Green” light also excites “red” cones.–So to produce some greenish lights we need negative red light.Color Matching(Wandell)Some colors can’t be matched•But we can match that color + a primary color, using the other two primaries.•Adding red to our color is like matching it with negative red.•All colors can be matched like this–Shows perceptually color is 3D–But we can’t have negative light in a display.–Display space is convex too, but can’t match perceptual convex space.(Wandell)CIE Model•CIE: International Commission on Illumination (1931)•Describes any visible color with only positive primaries•Primaries are called: X, Y, Z•Color is described by chrominance x, y, and luminance YZYXXxZYXYyZYXZz“Apart from the very approximate relationship between Y and brightness, there is almost nothing intuitive about the XYZ color-matching functions. While they have served us quite well as a technical standard, they have served us quite poorly in explaining the discipline to new students and colleagues or as an intuition about color appearance.”- Wandell(Image Removed)(Image Removed)(Image Removed)Additive Color Model RGB•Mix Red, Green, Blue primaries to get colors•Cartesian Coordinate System with origin as black. •Used in display devices: CRTs, LCDs.(Image Removed)Subtractive Color Model CMYK•Start with white and subtract different subtractive primaries–Cyan ink: Absorbs red –Magenta ink: Absorbs green –Yellow ink: Absorbs blue •Used in color printing•C+M+Y = Black, but added fourth black ink for good black color and also to preserve CMY ink for black textColor Specification•Hue: Distinguishes among colors–red, yellow, blue•Saturation: Color Purity (difference from white)–blue and sky blue•Lightness: Perceived intensity of reflected light–blue and darker shades of blue•Brightness: Perceived intensity of self-luminous objects•Artists:–Tint: Add white (decrease saturation)–Shade: Add black (decrease lightness)Perceptual Color Model HSV•R = 0o, G = 120o, B = 240o•Complementary colors are 180o apart•S = 0: Gray levelsVSHRedYellowGreenCyanBlueMagentaWhiteBlackLogarithmic Perception•Constant ratios of intensities are perceived as being equally different•Example: intensities of 0.01 and 0.02 will be perceived to be have the same difference between them as intensities of .5 and 1.0•Other examples of logarithmic perception:–Decibel scale: sound–Richter scale: earthquakes(Wandell)Logarithmic Display•Uniformly partitioning the displayable intensity range into equidistant arithmetic intervals is wasteful•Each intensity should differ from the previous by a constant ratio.I0 = minimum non-zero displayable intensityIn = maximum displayable intensity = 1.0I1 = r I0, I2 = r I1 = r2 I0 , I3 = r I2 = r2 I1 = r3 I0In = rn I0 r = (1/ I0)1/n(Wandell)Gamma Correction•I = KV I = intensity, V = voltage K and are CRT dependent•Given an intensity I, find j as: j = round (logr (I/I0))•Given j, find Ij = rj I0 and then find the voltage Vj for a given pixel as: Vj = round (Ij /K)1/Halftoning•Halftoning: Smaller black disks for brighter and larger disks for darker areas•Eyes do the intensity averaging•Allows more displayable intensity levels at a cost of lower spatial resolutionHalftoning(Image Removed)Spatial versus Intensity Resolution•Halftone Approximation: Dither–n  n pixels encode n2 + 1 intensity levels•The distribution of intensities is randomized: dither noise, to avoid repeating visual artifactsIntensity depends on context(Adelson)But context is a subtle thing.(Knill and Kersten)Context can also create transparency(Adelson)(Adelson)Opacity and Blending•Alpha channel to allow different levels of opacity amongst objects: = 1  Perfectly opaque= 0 Perfectly transparent 0   Different levels of translucency•Blending is mixing colors of two sets of pixels: source and destination–source and destination each have relative weights or blending factors to control the operationBlending in OpenGL• glEnable (GL_BLEND)• glBlendFunc (source_factor, destination_factor)–GL_ONE, GL_ZERO, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_DST_ALPHA, GL_ONE_MINUS_DST_ALPHA•Eg: glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)DemoDepth Cueing and Fog•Depth Cueing: Draw objects farther from the viewer darker•Fog: Draw objects farther from the viewer whiter•Let the color