Bob Reese 4/23/01Memory Issues in Graphics Hardware 14/23/01 1Display Technology■ Display Technology Tree:◆ CRT◆ Flat Panel✦ Active• Fluorescent• Gas Discharge (plasma)• Electroluminescent• LEDs• Incadescent✦ Passive• Liquid Crystal Displays (LCDs)• Electromechanical4/23/01 2Cathode Ray Tubes■ Most common display type Vertical DeflectionHorizontal DeflectionElectron BeamPhosperGridCathode4/23/01 3Raster ScanActive linesRetrace linesBeam directed in lines (scan lines) across screen. Dots are created by turning the beam on & off. During retrace, the beam is off.Horizontal retrace is when beam returns to left edgeVertical retrace is when beam returns to upper left corner.Bob Reese 4/23/01Memory Issues in Graphics Hardware 24/23/01 4Horizontal, Vertical SyncsHorizontal oscillatorHorizontal amplifierCRTHorizontalSyncVertical oscillatorVertical amplifierVerticalSyncVideo amplifierVideoInput60 hz15.75 Khz4/23/01 5The horizontal sweep controls the number of lines on the screenSweep across screen (slow), drives horizontal deflectorHorizontal Retrace (fast), moves beam back to left edge of screen. The vertical sweep controls how fast each screen is displayed.Sweep down the screen, (drives vertical deflector)Vertical Retrace moves beam back to top left corner 4/23/01 6How many lines on screen?■ Let Horizontal sweep = 15.75Khz■ Let Vertical sweep = 60 hz◆ Num lines = horizontal/vertical = 15750/60 = 262.5■ These numbers are for standard North America broadcast television◆ Half line allows next field (next 262.5 lines) to be offset by one half line (even & old fields). Two fields make one frame◆ Gives an interlaced display of 524 lines refreshed at 30 hz✦ Human eye can detect flicker at 45 Hz - don’t notice flicker on TV because of image typesBob Reese 4/23/01Memory Issues in Graphics Hardware 34/23/01 7How many Dots on screen? ■ Video input controls whether beam on or not■ How fast we can turn beam on/off during horizontal trace time ■ Monitors use internal clock to sample video signal◆ Monochrome - only one line for video signal◆ Digital RGB - three digital lines (Red, Green, Blue)✦ Gives 8 colors (a 4th line, an intensity signal, can be used to give 16 colors)◆Analog RGB - three analog lines, each driven by 8-bit DAC - gives 256 * 256 * 256 = 2 24colors4/23/01 8VGA Timing (640 dots x 480 lines) Horizontal Sync = 31.5 Khz, Vertical Sync = 60 HzInternal Monitor clock (Dot Clock) for latching video signal is 25.175 Mhz#max dots per line = Dot Clock Freq/ Horizontal Sync= 25.175 Mhz / 31.5 Khz = 800 DotsOnly can use 640 dot times out of possible 800 for display because we need black areas on left/right edges and time for horizontal retrace. #max lines per screen = Horizontal Sync/Vertical Sync = 31.5 Khz / 60 hz = 525 linesOnly 480 lines usable, need blank areas on top/bottom, time for vertical retrace4/23/01 9Horizontal Sync Timing 659 755756 7990 639 640659 75531.75 us (31.5 Khz)Horizontal retrace Left BorderDisplayRight BorderNext Line800 dot times per lineCounter can be used to keep track of horizontal screen position.Bob Reese 4/23/01Memory Issues in Graphics Hardware 44/23/01 10Vertical Sync Timing493 494495 5240 479 480493 49416.67 ms ( 60 hz)Vertical retrace Top BorderDisplayBottom BorderNext Screen525 line times per screenCounter can be used to keep track of vertical screen position.4/23/01 11Other resolutions■ 800 x 600◆ Dot clock 36 Mhz◆ Horizontal Sync 35.15 Khz◆ Vertical Sync 56 hz■ 1024 x 768◆ Dot clock 64.142 Mhz◆ Horizontal Sync 48.3 Khz◆ Vertical Sync 60 hz■ Allow about 20% of horizontal trace time for borders, retrace■ 6% to 8% of vertical trace time for borders, retrace4/23/01 12What drives video signal? ■ Every dot clock time for visible pixels, need to determine value of video signal◆ For monochrome, 1 bit per pixel (black or white, on or off)◆ For Digital RGB, 4 bits per pixel (R,G,B, Intensity)◆ For Analog RGB, N bits per R,G,B value where each RGB value can have 2Ndistinct values■ The memory that defines the screen contents is called the display memory or frame buffer.Bob Reese 4/23/01Memory Issues in Graphics Hardware 54/23/01 13Memory Calculations■ 800x 600 x 24 bit color – how much memory?800 x 600 x 24 bits/pixel = 11520000 bits= 1440000 bytes = 1406 Kbytes = 1.4 Mbytes2 Mb video card fine for this resolution/color1280 x 1024 x 24 bits = 3.75 Mbytes Need 4 Mbyte Video card4/23/01 14Display Memory Characteristics ■ Accessed at high data rates■ Will need to accessed by two sources◆ CPU which will be doing read/writes to random locations◆ Video signal driver which will be reading memory locations in a fixed pattern (the scan pattern)■ Can use either SRAM, DRAM, SDRAM or specialty graphics memory to implement the graphics memory.■ Graphics memory usually on same board as rest of video logic4/23/01 15Memory AccessVideo MemoryVideo ControllerRead Only,Sequential AccessCPUWrite Mostly,Random AccessVideo CardCRTVideo controller must arbitrate between its accesses and CPU accesses. CPU connection to card can either be via system bus (I.e. PCI bus) or dedicated connection (Advanced Graphics Port).Bob Reese 4/23/01Memory Issues in Graphics Hardware 64/23/01 16What type of Memory for Video?■ Early cards used SRAM because of simple interface (no refresh, no DRAM controller needed), and because of low memory size requirements ( a few Kbytes)■ As colors/video memory size increased, began using DRAM for density/cost reasons◆ Some special DRAMs appeared (called Video DRAMs) that had dual port access – parallel input/output for CPU, serial output for CRT. Died out because of cost reasons.■ Memory choice today is Synchronous DRAM since 3-D graphics cards need 16 Mbytes and up ◆ Some 3D cards performance is limited by speed of DRAM –even DDR-DRAM not fast enough.4/23/01 172D Graphics■ 2D graphics operations are very simple◆ 2D graphics operations required by ‘window’ type Graphic User Interfaces■ Block moves – memory to memory copy of rectangular areas on screen◆ Implements window movement■ Block Fills (fill rectangular area on screen)■ Hardware support for cursor operation ◆ As cursor moves over screen pixels under cursor are not damaged■ Hardware support for Sprite movement/collision detection◆ Sprite is rectangular group of pixels representing an object◆ Used by 2-D games4/23/01 18Animation■ To produce animation on a
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