Light, Color, and Displays Joshua Barczak* CMSC 435 UMBC * Numerous slides stolen verbatim from Dr. Marc Olano

Light Electromagnetic radiation Photon wavelength l, frequency f = c/l Visibile l ≈ 380 nm (blue) to 720 nm (red) Photon energy q = h f = h c/l (in J) h = Planck’s constant Power (flux)  = J/s (in Watts)

Spectral Quantities Spectral Energy Ql = J/nm Spectral Power l = /nm Graphics literature drops l by convention Density of Q/  in a l band

Spectral Power Distribution Make the bins infinitely small… Spectral power as a continuous function of wavelength: spectrum(l) 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 light’s 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 Cone 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 9

Tristimulus Theory “Colors” can be represented by 3 numbers Points in a “color space” Different spectra can produce the same color Metamerism

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 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 16

CIE XYZ Color Space Describes all perceptible colors using non-negative coordinates Imaginary Primaries Y coordinate is “Luminance” Wikipedia

Luminance Photometric Luminance Relative Luminance How bright does it look to us 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) 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 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 25

Random/Vector Display 26 26

Examples of Random Scan 27 27

Examples of Random Scan 28 28

Raster Display 29 29

Vector vs. Raster Vector is good for crisp, clean lines, bad for color or detailed fills 30 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 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 32

Liquid Crystal Display (LCD)‏ Light enters polarizer Nematic crystals twist based on voltage Allowing light to pass through to other polarizer 33 33

Color LCD 34 34

DLP Projector

Ink Jet Printer 36 36

Image Storage 2D Array of RGB pixel values

Image Storage Common pixel formats: 32bit (RGBA8)(R10G10B10A2) 16bit (RGB565)(RGBA4)(RGBA5551) Greyscale (8-16bits) Paletted 16 or 256 reference colors 4 or 8 bit index per pixel 1bit (black and white)

Gamma Displays are non-linear WRT input I = Imaxag Apply a correction curve before display: a = a 1/g Why are displays like this?

Gamma Human beings have non-linear brightness perception Approximately x0.45 Displays treat input as “apparent brightness” “50%” is ~20% of white point Our silly eyes think this is half as bright Blue line is perceived brightness Pink line is its derivative 40 40

Images are… Rendered/Captured in “linear space” Stored in “gamma space” Converted as needed Degamma: x2.2 Gamma: x0.45 These are approximations… Reason we bother with this is to get better precision in the darks. Blue line is perceived brightness Pink line is its derivative 41 41

No Really, Why do we do this? We want our bit allocation to be “Perceptually Uniform” More precision in the darks where we’re more sensitive to change Reason we bother with this is to get better precision in the darks. Blue line is perceived brightness Pink line is its derivative 42 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 Cable No Synchronization  Ugly flickering!

Double Buffering Scan Previous Frame Draw Next Frame SIMULTANEOUSLY Back Buffer Front Buffer Cable “Flip” (Swap pointers)

Tearing Swapping during scan Possible Solutions: Don’t swap until VSync period Input lag Lost time while waiting Triple buffering Wikipedia

Triple Buffering Swap on VSync Back Buffer 1 Back Buffer 2 Front Buffer Swap on Finished Frames

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 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 51

Problems with Interlacing 52 52