graphics device principles b.sc. (hons) multimedia computingmedia technologies

Graphics Device Principles

B.Sc. (Hons) Multimedia Computing

Media Technologies

Agenda

Basic CRT Principles Vector Graphic Displays Raster Graphics Displays Aliasing


Media Technologies

Cathode Ray Tube (CRT)

Basic Principle of the CRT

Electrons emitted here

Focused here

x axis deflection plate

y axis deflection plate

Phosphor coatingPhosphor coating emits light from energy provided by electron beam but only lasts for short period and must be ‘refreshed’ to remove flicker of screen image. Typical refresh rates are 60Hz - 80 Hz the higher the more stable is the resultant display.

Cheaper CRTs would not always support higher refresh rates.

Display Devices


Media Technologies

Vector Displays (Random Scan) Raster Displays (Raster Scan)

Vector Scan Displays

Vector Displays

Vector Displays Often referred to as Random Scan Devices, stroke-

writing or calligraphic displays Picture definition stored as a set of line drawing

commands Draws each component line in turn Originally designed for architectural and engineering

layouts High resolutions with smooth point-to-point line

drawing functions Not able to display photo-realistic images

Raster Scan Displays

Raster Scan Displays Based on TV Technology Electron beam sweeps scan lines row by row down

the phosphor screen based on information stored in the Frame Buffer ‘painting’ one line at a time

Frame buffer holds total screen area Screen area comprised of picture elements (pixels, or

pel) Often characterised by their resolution, aspect ratio,

and colour depth.

Raster Scan

Raster Scan Properties Resolution = number of pixels available on display,

640 x 480 ( VGA ), 800 x 600, 1024 x 768 ( SVGA ) Aspect ratio = number of horizontal pixels / vertical

pixels so 800 / 600 = 1.3. or 4 / 3 = 1024 / 768 etc 4/3 artistically (visually) pleasing? Try wide-screen! Bit depth determined by Frame Buffer - colours

available for each pixel - determined by number of bits stored for each pixel - hence bits per pixel.

1 bit per pixel = monochrome (black and white) Typically 24 or 32 bits per pixel gives ‘true colour’

Raster Scan Aliasing

Raster Scan

Aliasing


Media Technologies

Bitmap images will exhibit aliasing of lines due to the underlying method of their storage and display.

Algorithms applied to final image to smooth jagged edges and give more gradation between edge colours to provide antialising

Aliasing


Media Technologies

Antialiasing Lines


Media Technologies

Aliased

Anitaliased

Aliased

Antialiased

graphics device principles b.sc. (hons) multimedia computingmedia technologies

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