DISPLAY TECHNOLOGY

• CRTs - Cathode Ray Tubes

• Electron gun
• Deflection plates
• B&W: Screen coated evenly with phosphors
• Color: 3 different phosphors, 3 different guns shadow mask, triads

- can't be used for vector displays, only raster
 
• Phosphor persistence - time from removal of excitation till the phosphorescence has decayed to less than 10% of the initial light output.

Typically 10 -60 µs.

• Critical fusion frequency - the refresh rate above which a picture stops flickering and "fuses" into a steady image.

• Aspect ratio: display height to the display width. (number of rows to number of columns)

American TV is 3:4

-Seen as jaggies (spatial), jittering(temporal), or flashing/crawling (temporal)

-Arises from not sampling the signal enough (undersampling).

-This is why wagon-wheels spin backwards in movies and TV.
    (you never see this in person -- unless you blink rapidly)

-Not a feature, want a system with anti-aliasing, not aliasing

-Fundamental problem in many aspects of computer graphics
    We are approximating a continuous world with discrete samples!

-Need to sample at twice the highest frequency in the signal (Nyquist limit)

-Example:

DISPLAY TYPES

• Vector Displays - spots

• System Components:
• Display Processor

- processes the commands for plotting points, lines and characters Refresh buffer
- stores the display list from the CPU normally:
• move (x,y),
• draw (x,y),  line (x,y), and
• character (a,x,y) commands.
• CRT & video-controller
• How it works (general)
• Display processor cycles through the commands in the refresh buffer and sends the corresponding vector commands to the video-controller.
• The video-controller then causes the deflection of the electron-beam to produce the line on the screen.
• Must refresh the display rapidly so the image doesn't flicker (cycle through the display list 30 times per second)
• Diagram:
   
• Advantages of Vector Displays:
• No Scan conversion necessary. DPU just processes the lines.
• No aliasing of the lines.
• Disadvantages:
• Scene complexity limited because of refresh rate

- DPU must be able to process the entire display list in 1/30th of a second or have flicker.
• Can't do filled objects -only line drawings
• More expensive that raster-based systems.
• Basic Results:


 

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• Raster Displays -(pixels)
• 

• System Components:
• Display Processor

- processes the commands for plotting points, lines, characters, polygons, and filled areas.
- May be absent in cheaper systems - Expensive hardware component ($500-$100,000)
• Frame (refresh) buffer

- stores the screen pattern
• Scanline

- horizontal row of pixels in the frame buffer
• Pixel - picture element

- smallest addressable unit in a raster system (both the screen and frame buffer)
• Typical resolutions: 640x480 (video), 1024x768, 1280x1024 (most high quality color displays)
• CRT & video-controller
 
• How it works (general)
• Display processor processes the commands in the display list and produces the raster image in the frame buffer.
• The video-controller then scans out the raster image one scan-line at a time.
• May use a color lookup table(LUT) or palette between frame-buffer and video-controller.
• Must refresh the display rapidly so the image doesn't flicker (redisplay the raster image 30-60 times per second.)
• Diagram:

• Advantages:
• can display solid areas
• cheaper
• scene complexity not a problem for refreshing
• Disadvantages: Rasterization
• Aliasing
• must rasterize everything. (can't send line & character primitives)

Frame-buffers

• Monochrome : 1 bit per pixel (bitmap)
• Full color: 24 bits/pixel - 8 for each or r,g,b
• Others: if a LUT is not used, can have as many colors or shades of grey as specified by the number of bits/pixel

8 bits => 256 colors (normally 3,3,2) or shades of grey.
• Color LUTs (palettes)
• Each entry in the frame buffer is an index into the LUT.

- if n bits/pixel => 2ⁿ entries in the LUT
• LUT entry then determines the color sent to the screen.
• If each LUT entry is p bits, then can display 2^p possible colors (example p=24 => 16 million colors in the palette)
• Can only display 2ⁿ colors simultaneously.
• Diagram:

• Example (typical) :
• frame-buffer: 8 bits/pixel
• LUT: 24 bits/entry
• Therefore, can display 256 colors at any one time out of a possible 16 million
 
• Why use an LUT?

- cheaper than full color (3 bytes*1280*1024 = almost 4MB of memory)
- allows more displayable colors than without one.
- Can do color table animation (later in course)