Graphics Device Principles B.Sc. (Hons) Multimedia ComputingMedia Technologies

Agenda Basic CRT Principles Vector Graphic Displays Raster Graphics Displays Aliasing B.Sc. (Hons) Multimedia ComputingMedia Technologies

Cathode Ray Tube (CRT)

Basic Principle of the CRT Electrons emitted here Focused here x axis deflection plate y axis deflection plate Phosphor coating Phosphor 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 B.Sc. (Hons) Multimedia ComputingMedia Technologies Vector Displays (Random Scan) Raster Displays (Raster Scan)

Vector Scan 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

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 B.Sc. (Hons) Multimedia ComputingMedia Technologies Bitmap images will exhibit aliasing of lines dueto 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 B.Sc. (Hons) Multimedia ComputingMedia Technologies

Antialiasing Lines B.Sc. (Hons) Multimedia ComputingMedia Technologies AliasedAnitaliased

Aliased

Antialiased