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

2. 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)

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

4. Spectral Power Distribution
Make the bins infinitely small…
Spectral power as a continuous function of wavelength:
spectrum(l)
Wikipedia

5. 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…

6. 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.

7. 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

8. Normalized SML Sensitivity
Wikipedia

9. SML Sensitivity 9 9

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

11. 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

12. 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!

13. HSV HSV: Cylindrical Coordinates Hue = angle
Saturation = distance from central axis
Value = distance along axis
Intuitive space for color picking

14. 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

15. CIE RGB Tristimulus Curves
Wait, Negative??
Wikipedia

16. Gamut
Spectral Locus 16 16

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

18. 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

19. 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…

20. 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

21. 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

22. 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”…

23. Display
Questions??

24. 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

25. 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

26. Random/Vector Display26 26

27. Examples of Random Scan27 27

28. Examples of Random Scan28 28

29. Raster Display 29 29

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

31. 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

32. 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

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

34. Color LCD 34 34

35. DLP Projector

36. Ink Jet Printer 36 36

37. Image Storage
2D Array of RGB pixel values

38. 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)

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

40. 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

41. 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

42. 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

43. Gamma Correction
Uncorrected
Just Right
Double-corrected

44. 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

45. Display Scan-Out Pixels in Frame Buffer (DRAM) Cable Scanout HW
(In there someplace…)

46. Display Scan-Out Frame Buffer Cable
No Synchronization  Ugly flickering!

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

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

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

50. 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

51. 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

52. Problems with Interlacing52 52