Display Technologies

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Display Technologies
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Types of video display

• Cathode Ray Tubes (CRTs)
• TVs, RGB monitors, o-scopes
• Flat-Panel Displays
• PDAs, laptops, calculators, digital watches

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CRTs
Electrons are fired from a filament, focused,
accelerated, then deflected to a point on the
phosphor coating on the inside of the display
screen
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Random-scan CRTs
Electron beam is scanned along each line
segment Capable of displaying continuous lines
and very high resolution curves High-end
displays capable of 100k lines per refresh
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Random-scan CRTs

• Pros
• Excellent for line drawings
• Generally high resolution
• Cons
• Can not display realistic shaded images
• Not capable of color
• Common Example
• Oscilloscopes

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Raster-scan CRTs
Electron beam is scanned left-to-right,
top-to-bottom Beam retraces to top-left after
reaching bottom-right (vertical retrace) Capable
of displaying continuous range of intensities at
discrete positions High-end displays capable of
4k x 4k _at_ 120 Hz
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Raster-scan CRTs
Three electron guns are used, one for each
color The guns are aimed through a mask and onto
colored phosphors Colored phosphors are arranged
in RGB triples dots (delta) RGB
monitors stripes (inline) TVs, Sony Trinitron
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Raster-scan CRTs

• Pros
• Excellent for varying intensity
• Can display shaded images
• Color
• Cons
• Jaggies
• Common Example
• Televisions

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Color Models
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Human Visual System

• The human retina is covered in 2 kinds of
  photoreceptor, rods and cones
• The fovea, densely packed with cones, is
  responsible for detailed color vision

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RGB color cube

• Coordinate system with R, G, B as axes
• Grayscale axis runs from (0,0,0) to (1,1,1)

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CMY color model
Coordinate system with C, M, Y as axes useful
for describing color output to hard-copy
devices. Grayscale axis runs from (0,0,0) to
(1,1,1). Color - substractive process.
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The Framebuffer
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Raster-scan review

• Display composed of discrete, addressable points
• picture elements or pixels
• Can control intensity of each pixel
• Pixels can be composed of RGB triples

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True-color

• 3 channels, 8 bits per channel 24 bits per
  pixel
• Often includes a 4th, non-display, channel
  (alpha) used for image composition 32 bpp
• 256 intensity levels per channel
• 224 total colors
• Sometimes combined with a LUT per channel (gamma
  correction)

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Indexed-color

• 8 bpp
• Each byte is an index to a LUT (colormap)
• All 224 colors are available to the colormap, but
  only 28 colors are available to the framebuffer
• Can do animation by swapping colormap entries
• Multiple apps can cause flashing if they try to
  use different colormaps at the same time

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High-color

• 16 bpp, 5 bits per primary color
• Sometimes the extra bit is given to green
• Limited number of bits per color can lead to
  noticeable quantization effects (color banding
  artifacts) and can be worse than index color in
  certain circumstances

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Color quantization
Indexed-Color

• True-Color

High-Color
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Single-buffered

• Single-buffered mode writes pixels directly into
  active framebuffer memory
• Partial results are therefore visible
• This is especially noticeable when trying to do
  animation

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Double-buffered

• Double-buffered mode writes pixels into a
  secondary buffer (back buffer), different from
  the buffer currently on display (front buffer)
• When all pixels are written to the secondary
  buffer, an explicit call is made to swap the
  front and back buffers
• The swap is typically done during the displays
  vertical retrace period
• This technique is preferred for interactive
  graphics

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Framebuffer math

• How much memory is needed for a 1024 x 768
  true-color (32 bit) framebuffer
• Single-buffered?

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Framebuffer math

• How much memory is needed for a 800 x 600
  index-color framebuffer
• Single-buffered?
• Please calculate yourself

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Coordinate systems

• Most windowing systems
• OpenGL framebuffer

x
(0,0)
y
y
x
(0,0)
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Coordinate systems

• Does it matter? No, we just need to be aware of
  the difference
• Where a pixel in the framebuffer will show up on
  screen?
• How do we get the pixel address under the mouse
  pointer?
• Could some other display library have its
  framebuffer lay-out match your windowing system?
  Absolutely. Many do.
• What if all we never directly displayed our
  framebuffer, but wrote it out as an image for
  later display?
• Virtually all image formats use screen-space
  coordinates.
• What if we want to support both?
• Then we have to know when to invert the y-axis.
  When would you do it?

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Framebuffer coordinates

• Well pick OpenGLs coordinate system.
• Where will these points appear on the screen?
• (0, 0)
• (5, 7)
• (8, 3)

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