Graphics Displays

• Give Section 1.1 of your book a read.
• A pixel is a PIcture ELement.
  • This is the smallest addressable element of a raster display.
• Raster graphics are graphics based on stored pixels.
• Ok, that is circular.
• A bit of history
  • We started with screens covered with phosphor.
  • When an electron beam hit the phosphor, the screen would glow at that point.
  • Phosphor glows for a bit then fades
  • So it needs to be refreshed. (This will be important in a minute)
• We have some choices with phosphor
  • Originally it just glowed or not, monochrome.
  • Later we decided we could shoot different intensities of electrons (I hope there are no physics majors in the room) for different shades of gray. (Grayscale)
  • Finally, we could add three different types of phosphor and get colors (RGB)
• In a crt
  • The front is coated with phosphor or some other Florescent material.
  • An electron gun would produce a stream of electrons
  • Deflectors would change where the gun "hit" the screen.
  • (From https://www.learnersplanet.com/worksheets-printables/how-does-crt-television-work)
  • This could trace any pattern on the screen.
• Vector graphics vs Raster Graphics
  • Again, raster graphics are presented as a set of dots.
  • Vector graphics where we store the vectors, or strokes
  • (Hallmark)
  • Think lineTo(), arcTo() ...
• We work with vectors all the time,
  • You draw your letters that way.
  • You paint that way,
  • Unless you are Georges Seurat
    • (From wikipedia)
• But storage becomes an issue
  • A complicated image might involve many different primitives.
  • And storing these primitives will involve different data structures.
  • We would need to store these primitives to refresh the screen as it fades.
  • And we need to train the electron gun to perform each of these movements.
• The solution, Raster graphics
  • We have the gun trace out a regular pattern
  • (https://www.cs.uic.edu/~jbell/CourseNotes/ComputerGraphics/VectorVsRaster.html")
  • We put a screen so we only hit the phosphor at designated areas.
  • We have the gun read the values to "fire" from a table.
  • This table eventually becomes the frame buffer.
• This means that
  • We need to convert vectors to dots in the frame buffer.
  • This is known as rasterization.
  • We will study multiple rasterization techniques.
• The equivalent is storing images.
  • The NetPBM format is nice for demonstrating this
  • It is very old and probably not used very much, but it is simple.
  • A pgm demo.
  • Another item might be this file, a map of the US saved as a scalable vector graphics (svg) file.
  • I have a program that edits this file to produce This map
• Vector vs Raster
  • Vector is continuous
    • This means you can scale at will.
    • But irregular memory/storage
    • Display/redisplay time related to number of objects and not uniform.
    • Filled areas problematic.
  • Raster is discrete
    • Known storage and redisplay time.
    • No problem filling in areas.
    • Jaggies,
• a raster vs vector scaling demo
• By the way
  • Memory was/is expensive
  • And originally would not fit onto the "chip"
  • So a frame buffer is part of a graphics card. (Or other device)
  • This is a giant array, the size of the screen (or resolution) supported
  • One bit for monochrome
  • Multiple bits for greyscale and color.
  • The first commercial frame buffer
    • had resolution 512x512
    • 8 bits per pixel (greyscale)
    • $15,000 (and you thought you paid too much for your graphics card)
  • A 32 bit color frame buffer
    • Generally supports 8 bit color for red, green and blue
    • And 8 bits for color.
    • So a 1280x1024x32 bit display has about 6MB of memory in the frame buffer.
  • Eventually memory became inexpensive enough that we could
    • Double the size of the frame buffer.
    • Use 1/2 for displaying the current image
    • Use 1/2 for drawing the next image.
    • This is called double buffering.
• Simple graphics cards had algorithms to rasterize simple 2d primitives.
• Later we added the ability to rasterize 3d primitives.
  • This is much harder
• We will discuss these techniques soon.
• Most modern displays are still based on these ideas
  • There is a grid of
    • Liquid crystal "elements"
    • leds
    • Some other method of producing various intensitis of red, green and blue.