Part I Display Technologies

1
Part I Display Technologies Image Formation
2
Cathode Ray Tube (CRT)
Hearn Baker , 1994

• Phosphor glows briefly when electrons strike it
  hard enough.

3
Vector Display vs. Raster Display
Hearn Baker , 1994

• Raster displays have dominated (since early 70s).
• Disadvantages of raster display
• Scan conversion or rasterization is needed
  Graphics commands specifying straight lines and
  other geometric objects are scan-converted into a
  set of discrete intensity points.
• Approximation of smooth lines Jaggies/staircasing
  is inevitable. See Part II Line Drawing.

4
Raster Display Terminologies

• pixel picture element
• scan line horizontal line of pixels
• resolution the maximum of pixels displayed on
  a screen
• e.g. 640?480, 1024?768, 1280?1024, 1400?1050,
  etc.
• Typical display hardware has an aspect ratio of
  43. (In contrast, HDTV is of 169.)
• frame buffer a memory area where the picture
  definition for the entire screen is stored
• Both the frame buffer and video controller are
  typically located on a graphics card.
• refresh rate the of times per second the
  image is redrawn
• not only for raster display, but also for vector
  display
• typically 60 frames/sec for raster display

frame buffer
drawing engine
video/graphics controller
screen
5
Bilevel System

• bilevel (monochrome) system one bit per pixel
• bit 1 ? gun on
• bit 0 ? gun off
• Lets suppose 1024?1024 resolution.
• 1024?10241M pixels.
• One bit per pixel. So, the frame buffer size is
  1M bits 125KB.

1024
gun on
0 0 1 0 1 0

1024
1024 x1024 frame buffer
6
Color Monitor

• All colors can be created with the primary colors
  of red, green and blue (RGB).
• Three types of phosphor dots
• The 1st type glows red when struck by
  high-density electrons.
• The 2nd type glows green and the 3rd type glows
  blue.
• Such an RGB triplet makes a pixel.

Hearn Baker , 1994
7
Color System

• For RGB color systems, we need at least 3
  bits/pixel.
• The first frame buffer was developed by Bell Labs
  in 1969.
• One bit for each R, G, and B.
• Each gun can get on/off.
• 8 colors possible!
• Suppose 8 bits for each R, G, and B.
• Each of R, G, and B has 28256 levels of
  intensities.
• 24 bits/pixel, and so 224?16.4M colors possible!
• Its called a true color system.
• e.g. for 1024?1024 resolution, 3MB frame buffer
• cf. 125KB frame buffer for monochrome system
• The number of bits per pixel is often called the
  depth of the frame buffer.

8
Frame Buffer to Monitor
1024
(0,0,60) (34,0,255)
(222,19,1) (0,51,0) (25,41,99) (0,255,0)

1024
1024x1024 frame buffer with each of RGB in the
0,255 range
255 maximum voltage 0 no voltage
9
RGB Color Model

• In general, 0,255 and 0,1 ranges are used
  interchangeably.
• In OpenGL (which is a general-purpose graphics
  library), each RGB component is specified as a
  number between 0.0 and 1.0. e.g. glColor3f(1.0,
  0.5, 0.7)

10
An Image Processing Example Negative Images

• Negative image 255,255,255 - R,G,B
• for(i0 iltymax i) // row
• for(j0 jltxmax j)
• Bij.r 255 - Aij.r
• Bij.g 255 - Aij.g
• Bij.b 255 - Aij.b
• DrawPixel(Bij)

Jihoon 980226-1171227
11
Another Image Processing Example Morphing

• Morphing (short for metamorphosis) is the
  visual transformation of one object into another,
  usually using image warping and cross-dissolve
  simultaneously.
• Image warping transforms the shape of the images.
• Cross-dissolve deals with the colors.
• Image warping is beyond the scope of this class.
  Take a related graduate class.

12
Cross-Dissolve

• for(t0.0 tlt1.1 tt0.1)
• for(i0 ilt100 i)
• for(j0 jlt100 j)
• Cij.r (1-t)Aij.r tBij.r
• Cij.g (1-t)Aij.g tBij.g
• Cij.b (1-t)Aij.b tBij.b
• DrawPixel(Cij)
• sleep(0.5)

13
HLS Color Model

• RGB model is hardware-oriented!
• Human-oriented is HLS model.
• 3 quantities in color
• Hue(H) is the tint, or what most of us would call
  color (red, yellow, etc.) such that
  complementary colors are 180o apart.
• Lightness(L) or darkness usually measured as a
  percentage from 0 (black) to 1 (white).
• Saturation(S) is purity of the color, and
  represents the amount of gray in proportion to
  the hue, measured as a percentage from 0 (gray)
  to 1 (fully saturated).
• Examples
• Red (0, 0.5, 1) Pink (0, 0.75, 0.5)
• Yellow (1/6, 0.5, 1) where 360o1
• White (, 1, ) Gray (, 0.5, 0)
• Its straightforward to convert HLS into RGB, and
  vice versa.

PHIGS Programming Manual, 1992
14
HLS Color Model An Example

• Color editor in Paintbrush.

Hue
Saturation
Lightness
15
CMY Color Model

• RGB model is good when light comes directly from
  the light sources to our eyes. How about
  hard-copies where light is reflected from the
  hard-copies?
• Suppose we have a cyan-colored paper. Why does it
  look cyan?
• If an object lit by the white light appears cyan,
  it means that the object absorbs the R component
  and reflects G and B.
• C subtracts R from the white light WRGBRC
  and so CW-R.
• Using C, we can control how much R comes to our
  eyes.
• If C1, 100 of R is subtracted, and 0 of R
  comes to our eyes.
• If C0, 0 of R is subtracted, and 100 of R
  comes to our eyes.
• If C0.4, 40 of R is subtracted, and 60(0.6) of
  R to our eyes.
• In general, if Rn is wanted, set C to 1-n.

whiteR,G,B
cyanG,B
cyan-colored
R
16
CMY Color Model (contd)

• Similarly, Magenta controls/subtracts Green, and
  Yellow does Blue.
• Note that C, M and Y are complementary colors of
  R, G and B respectively, which are located
  opposite in the RGB diagonal and the HLS disk.
• In order to control the RGB colors coming to our
  eyes, lets use RGBs complementary colors C, M
  and Y. New 3 primary colors!
• Its the CMY color model.

whiteR,G,B
whiteR,G,B
yellowR,G
magentaR,B
magenta-colored
yellow-colored
G
B
G
Y
C
R
M
B
17
CMY Color Model (contd)

• Suppose that we want to have a blue-colored
  paper, i.e. we want only B to come to our eyes.
• Then, lets subtract R and G.
• C subtracts R.
• M subtracts G.
• So, put C and M on the paper.
• Recall that, if Rn is wanted, set C to 1-n. Same
  for GM and BY.
• So, given (R,G,B), (C,M,Y)(1,1,1)-(R,G,B).

blue(R,G,B)(0,0,1)
R,G,B
G
Y
C
B
R
R,G
cyanmagenta-colored
B
M
18
CMYK Color Model

• RGB model is an additive color model in the sense
  that RGB primaries add light to an initially
  black display.
• On the other hand, CMY model is a subtractive
  color model where colors are specified by what
  is subtracted from while light.
• Popular in hard-copy devices is the CMYK model.
• CMYK is an extension of CMY model, where K stands
  for blacK and specifies how much gray is coated.
• For example, suppose that C,M,Y0.4,0.5,0.3
  is specified. That means all of R, G and B should
  be absorbed by 0.4, 0.5 and 0.3 respectively.
  Then, if gray is coated on the paper by 0.3, it
  absorbs all of RGB by 0.3. Then, C, M and Y can
  now be coated only by 0.1, 0.2 and 0
  respectively. CMYK saves cost!!

K min(C,M,Y) C C K M M K Y Y K