4 2.1 Video Display Devices CRT basics: The screen output is stored in the frame buffer andis converted into voltages across the reflection platesvia a digital-to-analog converter (DAG)Light is emitted when electrons hit phosphorBut light output from the phosphor decaysexponentially with time, typically in 10 – 60microseconds– Thus the screen needs to be redrawn or refreshed– Refresh rate is typically 60 Hz to avoid flicker (“twinkling”)– Flicker: when the eye can no longer integrate individual light pulses from a point on screen, e.g., due to low refresh rate
5 2.1 Video Display Devices Shadow-mask color CRTs Three different colored phosphors (R, G, B)dots are arranged in very small groups (triads) on coatingWe see a mixture of threecolorsThree electron guns (R, G,B) emit electron beams in acontrolled fashion so thatonly phosphors of theproper colors are excited
7 2.1 Video Display Devices The Frame Buffer Stores per-pixel information– Depth of a frame buffer: number of bits per pixel– E.g. for color representation, 1 bit => 2 colors,8 bits => 256 colors,24 bits => true color (16 million colors)Color buffer is only one of many buffers, otherinformation, e.g., depth, can also be usedImplemented with special type of memory instandard PCs or on a graphics card for fastredisplayPart of standard memory in earlier systems
8 2.1 Video Display Devices Raster-scan basics: The screen is a rectangular arrayof picture elements, or pixelsResolution: determines thedetails you can seenumber of pixels in an image,e.g.,1024×768, 1280x1024, 1366 x 768, etc.also in ppi or dpi – pixel or dot per inch
9 2.1 Video Display Devices Raster-Scan Pattern: Horizontal scan rate: # scan lines per secondInterlaced (TV) vs. non-interlaced displays
10 2.1 Video Display Devices Random-Scan Display Also called Vector-scan displayPictures are generated as line drawings
11 2.1 Video Display Devices Flat-Panel Display A class of video devices that have reduced volume, weight and powerTwo categories:Emissive displays: convert electrical energy to light (e.g. Plasma panels and Light-emitting diode (LED)Nonemissive displays: use optical effect to convert light from a certain source into graphics patterns (e.g. Liquid-crystal display (LCD)
12 2.2 Raster-Scan Systems Simple raster-graphics system Video controller (display processor) controls operations of display device.
13 2.2 Raster-Scan SystemsRaster system with fixed portion of main memory reserved for the frame buffer.
15 2.2 Raster-Scan Systems Raster – scan display processor - Digitizes picture definition into a set of pixel values for storage in the frame buffer (scan conversion).
16 2.2 Raster-Scan SystemsAdditional functions for Raster – scan display processor- generating line styles- display color areas- transformations- interface with interactive devices.
17 2.4 Input Devices Input devices include Pointing/locator devices: indicate location on screen– Mouse/trackball/spaceball– Data tablet– Joystick– Touch pad and touch screenKeyboard device: send character inputChoice devices: mouse buttons, function keys