Light, Color, and Displays Joshua Barczak* CMSC 435
Light, Color, and Displays Joshua Barczak* CMSC 435 UMBC * Numerous slides stolen verbatim from Dr. Marc Olano Light Electromagnetic radiation Photon wavelength , frequency f = c/ Visibile 380 nm (blue) to 720 nm (red)
Photon energy q = h f = h c/ (in J) h = Plancks constant Power (flux) = J/s (in Watts) Spectral Quantities Spectral Energy Q = J/nm Spectral Power = /nm Graphics literature drops by convention Density of Q/ in a band
Spectral Power Distribution Make the bins infinitely small Spectral power as a continuous function of wavelength: spectrum( Wikipedia Color Perception
Quote from the book (pg 533): "Color is the aspect of visual perception by which an observer may distinguish differences between two structure-free fields of view of the same size and shape, such as may be caused by differences in the spectral composition of the radiant energy concerned in the observation. Wyszecki & Stiles, 2000. In other, less verbose words Color Perception
Color is NOT an intrinsic property of light It is our brains trying to tell us about the lights SPD It is all in our heads ... the Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour. Isaac Newton, Opticks, 1704. Color Perception
Rod cells Only active at low light levels Little color sensitivity Cone cells Three types (S,M,L) Sensitivity Functions at different wavelengths Normalized SML Sensitivity Wikipedia
SML Sensitivity 9 Tristimulus Theory Colors can be represented by 3 numbers Points in a color space
Different spectra can produce the same color Metamerism s SPD( ) S ( )d m SPD( ) M ( )d l SPD( ) L( )d RGB Color Space
Additive color mixing Three light sources Origin is black Coordinates: How high to set each source Colors are points in space 11 CMY(K) Color Space
Subtractive color mixing Three kinds of ink Origin is white Coordinates: How much ink to use Colors are points in space Origin is here
now! HSV HSV: Cylindrical Coordinates Hue = angle Saturation = distance from central axis Value = distance along axis Intuitive space for
color picking Wright/Guild Experiments Pick Three Monochromatic Lamps Red one, Green one, Blue one Hook them up to knobs Display a test color Have subject tweak knobs until they match CIE RGB Tristimulus Curves
Wait, Negative?? Wikipedia Gamut Spectral Locus 16
CIE XYZ Color Space Describes all perceptible colors using non-negative coordinates Imaginary Primaries Y coordinate is Luminance Wikipedia
Luminance Photometric Luminance How bright does it look to us L 683.002lm / W y ( ) s( )d Unit: Lumen Relative Luminance Normalized to
reference spectrum White point Y color match curve is also the photopic luminous efficiency function Yuv Space Brightness and color are independent Same Y as XYZ, derived u and v Also called Luminance/Chrominance
Chromaticity Coordinates (Chrominance) X u X Y Z Y v X Y Z
Book and others use xyY instead, for added confusion Dynamic Range Your monitor is nowhere near as bright as a sunny day Tone Mapping: Converting real dynamic range to displayable dynamic range Best done in Yuv space Rescale luminance while preserving chroma
What We Should Do Specify SPD of light source, in watts Multiply by spectral reflectance of surface Integrate against XYZ curves Convert XYZ to Yuv, tonemap
Convert to RGB, clamp any negatives, gamma, display What We Get Away With Specify lights using arbitrary RGB colors Multiply by other arbitrary RGB colors Clamp or rescale as needed, gamma, display Artists tweak the above until it looks right
Display Questions?? CRT Electrons fire off of heated cathode and shoot in direction of anode Electrons strike phosphors and create light Fluorescence (fraction of usec) Phosphorescence (10-60 usec) 24
Phosphors Have different color based on makeup Red: europium yttrium vanadate Green: zinc calcium sulfide Blue: zinc sulfide May also have different persistence Image mush be refreshed to avoid flicker Below 60Hz can be unpleasant
25 Random/Vector Display 26 Examples of Random Scan 27 Examples of Random Scan
28 Raster Display 29 Vector vs. Raster 30
Raster Display Each left-to-right trace is called a scan line Each spot on the screen is called a pixel Beam turned off to swipe back and up screen Called a retrace or blanking interval 31 Color CRT Uses triads of red, green & blue at each pixel
Uses 3 electron guns one for each color Shadow mask used to make each kind of phosphor only visible from one gun 32 Liquid Crystal Display (LCD) Light enters polarizer Nematic crystals twist based on voltage Allowing light to pass through to other polarizer
7 4 1 8 0. 0.7 0.8 0.9 0.9 Relative Luminance Blue line is perceived brightness Pink line is its derivative
42 Gamma Correction Uncorrected Just Right Double-corrected Display Protocols HDMI/DVI/VGA are all designed around
CRT displays Image transmitted line by line Control periods between lines for HSync/VSync Display decodes the signal and updates itself Display Scan-Out Pixels in Frame Buffer (DRAM)
Cable Scanout HW (In there someplace) Display Scan-Out Frame Buffer No Synchronization Ugly flickering!
Cable Double Buffering Draw Next Frame SIMULTANEOUSLY Back Buffer
Scan Previous Frame Front Buffer Cable Flip (Swap pointers) Tearing
Swapping during scan Possible Solutions: Dont swap until VSync period Input lag Lost time while waiting Triple buffering Wikipedia
Triple Buffering Swap on VSync Back Buffer 1 Back Buffer 2 Swap on Finished Frames
Front Buffer Interlacing Interlacing is a crude way to transmit two frames for the price of one 480i (standard TV) is 480 scan lines, interlaced 1080i HD is 1080 scan lines, interlaced 1080p HD is 1080 scan lines, non-interlaced Odd Fields (frame i)
Even Fields (frame i+1) 50 Interlacing Interlaced Non-Interlaced (progressive)
Problem: CRTs can display interlaced images correctly. Modern LCDs CANNOT. If you are watching Interlaced video, your TV is doing image processing to in a desperate attempt to not look terrible. 51 Problems with Interlacing 52
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