Computer Graphics Raster Scan Display System, Rasterization, Refresh Rate, Video Basics and Scan Conversion
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Refresh and Raster Scan Display System Used in Television Screens. Refresh CRT is point plotting device. It use refresh buffer to store display primitives(line, characters, shaded, patterned areas). Refresh buffer store the primitives in terms of points and pixels components. Display screen is a matrix of pixels. And each pixel brightness can be controlled. 3
Refresh and Raster Scan Display System Each point is an addressable point in screen and memory. Line can not be drawn directly from one point to another. This causes the effect of aliasing, jagging, staircase effect. Refresh buffer is also called bit plane. 4
Rasterization General Line Special Line B D B A A C (a) General Line (b) Special Line 5
Raster is stored as a matrix of pixels representing the entire screen area. Entire screen is scanned out sequentially by the video controller (One raster line at a time) Raster lines are scanned from top to bottom and then back to the top. The intensity of the beam decide the brightness of the pixel. At least on memory bit for each pixel is required.(bit - plane). 6
Basic Video Controller Refresh Operations Raster-Scan Generator Horizontal and Vertical Deflection Voltages X Register Y Register Memory Address Pixel Register INTENSITY Frame Buffer 7
Architecture of Raster Display 8
Raster Scan With outline primitives With filled primitives B 9
A Typical Example Refresh Rate If one uses a 512 X 512 element raster display, then 2 18 bits are required in a single bit plane. Memory size required 32 KB A DAC is used to convert the bit values (0,1) to analog signals for refreshing the screen. Memory size required for N- bit gray level frame buffers: N Size in KB 3 96 8 256 24 768 10
Refresh Rate Refresh Rate to avoid flickering 60 Hz If one uses a 1024 X 1024 high resolution CRT: N Display Color Memory Size 1 Black & White 128KB 8 256 color 1MB 24 16 million color 3MB 32 16 million color 4MB 11
Refresh Rate Refresh Rate of CRT is the number of times the image is drawn on the screen per second. Reducing refresh rate increases flicker. Horizontal Scan Rate is the number of scan lines the circuit drives a CRT display per second. HSR= Refresh Rate X number of scan lines Resolution of screen is depend on spot size. CRT resolution is not a function of bitmap resolution. For large spot size, resolution decreases. Horizontal Resolution depends on spot size and beam switching (ON/OFF) speed. 12
Resolution Resolution is the number of pointes per inch or centimeter that can be plotted horizontally & vertically. The smaller the spot size, the higher the resolution. The higher the resolution, the better is the graphics system High quality resolution is 1280x1024 The intensity distribution of spots on the screen have Gaussian shape. Intensity distribution Adjacent points will appear distinct as long as their separation is greater than the diameter at which each spot has intensity of about 60% of that at the center of the spot. 13
Persistence How long small spots continue to emit light after the beam is moved. How long it takes to the emitted light from the screen to decay to one-tenth of its original intensity. Lower persistence requires high refresh rate & it is good for animation High persistence is useful for displaying highly complex static picture. Graphics monitors are usually constructed with 10 to 60 microseconds. 14
Bandwidth Of Display The rate at which the beam can be turned ON to OFF and vice-versa. For N pixel per scan line, it is necessary to turn the electron gun at maximum rate of N/2 times ON and N/2 times OFF. This will create alternative black and white lines on the screen. 15
1-Bit Memory, Monochrome Display (Bitmap Display) 1 bit 2 levels Electron Gun 16
3-Bit Color Display 3 red green blue COLOR: black red green blue yellow cyan magenta white R G B 0 0 0 1 0 0 0 1 0 0 0 1 1 1 0 0 1 1 1 0 1 1 1 1 17
True Color Display 8 24 bitplanes, 8 bits per color gun. 2 24 = 16,777,216 8 8 Red Green Blue 18
Horizontal Retrace : As the electron bean reached at right edge of the screen, it is made invisible and rapidly return to left edge. Time taken for horizontal retrace is 17% of allotted to scan horizontal line. Vertical Retrace: Return the beam from bottom left edge to top right edge. Odd field Vertical Retrace Even field Vertical Retrace 19
Raster Scan Horizontal Retrace Vertical Retrace Scan Line 20
Video Basic Standards NTFS: (Amarican Standerd Video) has 525 horizontal lines with a frame rate of 30 fps. Viewing aspect ratio 4:3 Each frame has two fields, each containing half the picture. Fields are interlaced or interwoven. Fields are presented alternatively every other 1/60 th of second. One field contain odd scan line(1,3,5, ). Other field contain even scan line (0,2,4,6,.). 21
Computer colour system let s try to we think this way: BLACK No light is reflected All the light is absorbed YELLOW Yellow light is reflected The rest of light is absorbed WHITE All the light is reflected No light is absorbed Conclusion: If the model is based on reflection, then black is 0 (RGB). If the model is based on absorption, then black is 1 (CMY). 22
RGB R G B RGB (Red, Green, Blue) Red (255,0,0) Green (0,255,0) Blue (0,0,255) Each colour has a range from 0 to 255, i.e. 256 levels (255,255,0) (215,185,229) There are 256 x 256 x 256 = 16,777,216 possible colors. 23
The colors may also be written as range of: binary numbers from 00000000 to 11111111 hexadecimal from 00 to FF (255,255,255) (#FFFFFF) (0,0,0) (#000000) (255,255,0) (#FFFF00) (56,108,98) (#386C62) CMY system 24
How colours and pigments combine RGB describes colours: R red G green B blue CMYK describes pigments: Y yellow M magenta (pink) C cyano (aqua) K black (key colour) 25
Hue, Saturation, and Brightness/Lightness = HSL/HSB Saturation change RGB (255,255,0) HSL (42,255,128) HSL (x,x,255) = full light HSL (x,x,0) = no light HSL (42,155,255) HSL (42,114,255) HSL (42,0,255) HSL (42,255,226) HSL (42,255,114) Brightness change HSL scheme is an alternative to RGB to express colours 26
Colour depth 1 pixel = 1 bit, i.e. it can be 0 or 1 to express only black or white colors The image consisting of 1-bit pixels is called: 1-bit image or bi level image or black-and-white image If we have a two-megapixel image (1,920,000 pixels), then such an image will have 1,920,000 bits, i.e. 240,000 bytes (240 KB) 27
Colour depth 1 pixel = 8 bits (= 1 byte), i.e. we can express a palette of 256 different colors Then the above two-megapixel image will have 1.92 MB 1 pixel = 24 bits (= 3 bytes), i.e. each byte can express 256 colors, i.e. altogether 16,777,216 possible colors Then the above two-megapixel image will have 5.76 MB 28
Colour depth The color depth says in how deep the pixel is, i.e. it informs about the number of possible colors. 24-bit 1-bit 4-bit 8-bit 12-bit 29
However, if the color is special If the R, G, and B components have equal values, e.g. (57,57,57) then the pixel will be grey Such an image will not have 16 million colors, but only 256 shades of grey As we need only one byte to express this, the image will be three times smaller than the full (true) color image The image in shades of grey is a color image, we should not talk about it as about black-and-white image (as we do in the field of non-color photographs) 30