Light, Color, and Displays Joshua Barczak* CMSC 435

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

33 Color LCD 34 DLP Projector Ink Jet Printer

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 nonlinear WRT input I = Imaxa Apply a correction curve before display:

a = a 1/ 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 1.4 1.2 1 0.8

0.6 0.4 0.2 0 0 07 14 2 1 28 35 4 2 49 56 6 3 0 . 0. 0. 0. 0 . 0. 0. 0. 0. 7 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 40

Images are Rendered/Captured in linear space Stored in gamma space Converted as needed Degamma: x Gamma: x0.45 2.2

These are approximations 1.4 1.2 1 0.8 0.6 0.4 0.2

0 0 07 14 2 1 28 35 4 2 49 56 6 3 0 . 0. 0. 0. 0 . 0. 0. 0. 0. 7 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 41 No Really, Why do we do this? We want our bit

allocation to be Perceptually Uniform More precision in the darks where were more sensitive to change 1.4 1.2 1

0.8 0.6 0.4 0.2 0 0 07 14 2 1 28 35 4 2 49 56 6 3 0 . 0. 0. 0. 0 . 0. 0. 0. 0. 7

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

Recently Viewed Presentations

  • Azure Provides a Secure, Scalable Platform Partner Logo

    Azure Provides a Secure, Scalable Platform Partner Logo

    Azure Provides a Secure, Scalable Platform for ScheduleMe, an App That Enables Easy Meeting Scheduling with People Outside of Your Company. MICROSOFT AZURE APP BUILDER PROFILE: ScheduleMe ScheduleMe is a cloud-based solution that solves the problem of setting up meetings...
  • Target setting: Is it feasible in the Early years foundation ...

    Target setting: Is it feasible in the Early years foundation ...

    ELG: children count reliably with numbers from 1 - 20, place them in order and say which one is one more or less than the other. I can count sets of different objects to 20. I can identify numbers to...
  • Study Guide for Chapter 21 Physics 2 - PC\|MAC

    Study Guide for Chapter 21 Physics 2 - PC\|MAC

    Study Guide for Chapter 21 Physics 2. Chapter Summary 1. An electric field exists around any charged object. The field produces forces on other charged objects. The electric field is the force per unit charge. Creating and Measuring Electric Fields....
  • Computer Security in Higher Education

    Computer Security in Higher Education

    Example: GRC.COM Example: GRC.COM hi, its me, wicked, im the one nailing the server with udp and icmp packets, nice sisco router, btw im 13, its a new addition, nothin tracert cant handle, and ur on a t3.....so up ur...
  • Logitrank Based Measures of Joint Income and Wealth ...

    Logitrank Based Measures of Joint Income and Wealth ...

    Source CPS IPUMS 1965-2017 each 5th year - male population . Dependent variable. Log family income (equivalized) gap between BA holders and the others (many variants processed). (log wages "medianized" by year) Independent variables.
  • Supporting the less able 17% of students leaving

    Supporting the less able 17% of students leaving

    Women should make a decision about abortion based on what will make them most happy (taking into consideration economic, social and emotional well-being.) Peter Singer, a Humanist philosopher argues that life does not begin at conception (a direct contradiction of...
  • General

    General

    The application can be retrieved online at the noted link. [Q C10 ANSWER:] Once a selection is made and at the appropriate time, our Primary POC, Tricia Halpin, Airport Safety Certification Inspector would be available to support the selected operator...
  • LAFS.8.RL.1.1: Cite the textual evidence that most strongly ...

    LAFS.8.RL.1.1: Cite the textual evidence that most strongly ...

    Grade 8 Reading Standards for Literature LAFS.8.RL.1.1: Cite the textual evidence that most strongly supports an analysis of what the text says explicitly as well as inferences drawn from the text.