Introduction to computer graphics part 2
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This document discusses cathode ray tubes (CRTs) and how they work as display devices for computer graphics. It explains that CRTs contain an electron gun that emits a stream of electrons which are focused into a beam and directed to specific points on the phosphor-coated front of the picture tube. When the electron beam hits a phosphor dot, it glows proportionally to the beam strength. Color CRTs use three electron guns and a shadow mask to separately excite red, green, and blue phosphor dots, allowing for color displays. The document also covers other properties of CRTs like resolution, persistence, and aspect ratio.
Introduction to computer graphics part 2
- 1. Introduction To Computer Graphics By Ankit Garg Amity University, Haryana 1
- 2. 2 Video Display Devices Display Hardware • Video display devices Input devices • Locator Devices • Keyboard devices
- 3. 3 Display Technologies Cathode Ray Tubes (CRTs) – Most common display device today – Evacuated glass bottle – Extremely high voltage – Heating element (filament) – Electrons pulled towards anode focusing cylinder – Vertical and horizontal deflection plates – Beam strikes phosphor coating on front of tube
- 4. 4 Electron Gun • Contains a filament that, when heated, emits a stream of electrons • Electrons are focused with an electromagnet into a sharp beam and directed to a specific point of the face of the picture tube • The front surface of the picture tube is coated with small phosphor dots • When the beam hits a phosphor dot it glows with a brightness proportional to the strength of the beam and how often it is excited by the beam •The picture is repeatedly repainted (refresh CRT)
- 5. 5 CRT Monitor Electron Guns Red Input Green Input Blue Input Deflection Yoke Shadow Mask Red, Blue, and Green Phosphor Dots CRT
- 6. 6 List the properties of CRT 1. Persistence 2. Resolution 3. Addressability 4. Aspect ratio
- 7. 7 Persistence How long small spots continue to emit light after the beam is moved. How long it takes for 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 pictures. – Graphics monitors are usually constructed with 10 to 60 microseconds.
- 8. 8 Resolution • The maximum number of points that can be displayed without an overlap on a CRT is referred to as the resolution. • 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. • 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. Intensity distribution
- 9. 9 Addressability • Addressability is the number of individual dots per inch (d.p.i.) that can be created. If the address of the current dot is (x, y) then the next dot will be (x + y), (x + y + 1) • The picture on a screen consists of intensified points. • The smallest addressable point on the screen is called pixel or picture element
- 10. 10 Aspect ratio This number gives the ratio between vertical points and horizontal points necessary to produce equal length lines in both directions on the screen. Aspect ratio = ¾ means that a vertical line plotted with 3 points is equal in length to a horizontal line plotted with 4 points.
- 11. 11 Display Technologies: CRTs 1. Raster Scan Displays 2. Vector Displays 3. Liquid Crystal Displays (LCDs) 4. Plasma Panel 5. Organic LED Arrays
- 13. 13 Raster Scan Displays (1) – Raster: A rectangular array of points or dots – Pixel: One dot or picture element of the raster. Its intensity range for pixels depends on capability of the system – Scan line: A row of pixels – Picture elements are stored in a memory called frame buffer
- 14. 14 Raster Scan Displays (2) – Frame must be “refreshed” to draw new images – As new pixels are struck by electron beam, others are decaying – Electron beam must hit all pixels frequently to eliminate flicker – Critical fusion frequency • Typically 60 times/sec
- 15. 15 Raster Scan Displays (3) – Intensity of pixels depends on the system for example black and white screens each point can be on or off thus it needs one bit of memory to represent each pixel. – To paint color screen additional bits are needed. If three bits are used, then number of different colors are 2*2*2. – A special memory is used to store the image with scan-out synchronous to the raster. We call this the frame buffer.
- 16. 16 Raster Scan Displays (6) Raster CRT pros: – Allows solids, not just wire frames – Leverages low-cost CRT technology (i.e., TVs) – Bright! Display emits light Cons: – Requires screen-size memory array – Discrete sampling (pixels) – Practical limit on size
- 17. 17 Frame Buffers • A frame buffer may be thought of as computer memory organized as a two-dimensional array with each (x,y) addressable location corresponding to one pixel. • Bit Planes or Bit Depth is the number of bits corresponding to each pixel. • A typical frame buffer resolution might be – 640 x 480 x 8 – 1280 x 1024 x 8 – 1280 x 1024 x 24
- 18. 18 Monochrome Display (Bit-map Display) Electron Gun 1 bit 2 levels
- 19. 19 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
- 20. 20 True Color Display 24 bit planes, 8 bits per color gun. 224 = 16,777,216 Green Red Blue N N N
- 21. 21 Color Map Look-Up Tables Extends the number of colors that can be displayed by a given number of bit-planes. 0 1 0 0 0 0 1 1 67 100110100001 0 67 255 1001 1010 0001 R G B RED GREEN BLUE Pixel displayed at x', y' Pixel in bit map at x', y' 0 x 0 y x max max y Bit map Look-up table Display Video look-up table organization: each table entry is a 12 bit per entry. A pixel with value 67 is displayed on the screen with the red electron gun at 9/15 (binary 1001) of maximum, green at 10/15, and the blue is 1/15.
- 22. 2 Basic Techniques for Color Display Beam-Penetration Method Shadow-Mask Method
- 23. 23 Beam-Penetration Method • Used with random scan monitors • The screen has two layers of phosphor: usually red and green • The displayed color depends on how far the electron beam penetrates through the two layers. • A beam of slow electrons excites only the outer of the red layer, a beam of fast electrons penetrates through the red layer and excites the inner green layer, and at intermediate beam speeds, combinations of the two colors are emitted to show other colors (yellow & orange)
- 24. 24 Shadow-Mask Method Color CRTs have – Three electron guns – A metal shadow mask to differentiate the beams
- 25. • The Shadow mask in the previous image is known as the delta-delta shadow-mask. • The 3 electron beams are deflected and focused as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns. • The 3 beams pass through a hole in the shadow mask and activate a dot triangle, which appears as a small color spot on the screen. • A second arrangement of the 3 electron guns is in-line Where the corresponding red-green-blue color dots on the screen are aligned along one scan – line instead of a triangular pattern.
- 26. Controlling Colors in shadow mask • Different colors can be obtained by varying the intensity levels of the three electron beams. • Example: Simply turning off the red and green guns, we get only the color coming from the blue phosphor. • Yellow = Green + Red • Magenta = Blue + Red • Cyan = Blue + Green • White is produced when all the 3 guns possess equal amount of intensity.
- 27. Application: • Color CRT’s in graphics systems are designed as RGB monitors, which employ the shadow mask technique. • The color CRT takes the intensity level for each electron gun ( red, green and blue) directly from the computer system without any intermediate processing. • An RGB color system with 24 bits of storage per pixel is generally referred to as full-color system or a true- color system. It allows 256 voltage settings for each electron gun and nearly 17 million colors.
- 28. 28 Display Processor • Also called either a Graphics Controller or Display CoProcessor • Specialized hardware to assist in scan converting output primitives into the frame buffer. • Fundamental difference among display systems is how much the display processor does versus how much must be done by the graphics subroutine package executing on the general-purpose CPU.
- 29. 29 Video ControllerCycles through the frame buffer, one scan line at a time. Contents of the memory are used the control the CRT's beam intensity or color. X address Y address Pixel value(s) Raster scan generator Data Horizontal and vertical deflection signals Intensity or color Linear address Set or increment Set or decrement M e m o r y
- 30. 30 Hard-copy devices ο Ink-jet printer ο Laser printer ο Film recorder ο Electrostatic printer ο Pen plotter
- 31. 31 Input Devices • Locator Devices: – to indicate a position and/or orientation – to select a displayed entity – Tablet, Mouse, Trackball, Joystick, Touch Panel, Light Pen • Keyboard devices: – to input a character string – Alphanumeric keyboard (coded - get single ASCII character, unencoded - get state of all keys - more flexible) • Valuator Devices: – to input a single value in the space of real numbers – Rotary dials (Bounded or Unbounded), Linear sliders • Choice Devices: – to select from a set of possible actions or choices – Function keys
Editor's Notes
- #10: Addressability is a measure of the spacing between the centers of vertical and horizontal lines. In graphics mode there are 800x600