introduction to computer graphics with latest trends
- 2. 2 The creation and manipulation of pictures with aid of a computer. Passive Computer Graphics The observer has no control over the image. One way communication. Examples : Titles shown on T.V, Presentation graphics. COMPUTER GRAPHICS
- 3. 3 Interactive Computer Graphics The observer is given some control over the image by providing an input device, so that the observer can signal his request to the computer. Two-way communication To the user it appears that the picture is changing instantaneously in response to his commands. Example: Flight simulator. COMPUTER GRAPHICS
- 4. 4 The flight simulator is a mockup of aircraft flight desk containing all the usual controls and surrounded by screens on which are projected computer generated views of terrain visible on takeoff and landing. As the trainee pilot maneuvers his aircraft these views change so as to maintain an accurate impression of the plane’s motion. Merits: Training, Fuel savings, safety, ability to familiarize the trainee with the world’s airports. COMPUTER GRAPHICS
- 5. 5 PIXEL The pixel is the smallest addressable screen element. It is the smallest piece of the display screen which the user can control. Each pixel has a name or address. The names which identify pixels correspond to the co-ordinates which identify points
- 6. 6 It collects and stores pixel values as an array in computer's memory for use by the display device. This array which contains an internal representation of the image, is called as the frame buffer. Graphics display devices can access this array to determine the intensity at which each pixel should be displayed. Frame Buffer
- 7. 7 Display Controller is an interface that passes the contents of the frame buffer to the monitor. The display controller simply reads each successive byte of data from the frame buffer and convert its 0’s and 1’s into the corresponding video signal. This signal is then fed to the monitor producing black and white pattern on the screen Display Controller
- 8. 8 Set Pixel Functions Users Program Frame Buffer 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 1 0 0 0 0 0 1 0 0 0 1 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Display Processor Display
- 9. 9 DISPLAY FILE INTERPRETER Display file The display file will contain the information necessary to construct the picture. The information will be in the form of instructions such as "draw a line" or "move the pen" It provides an interface between the image specification process and the image display process. Such files are also called as "pseudo display files" or "meta files". Display File interpreter Display file interpreter will convert these instructions into actual images.
- 10. 10 DISPLAY FILE STRUCTURE Contains a sequence of display commands. Each display-file command contains two parts, an operation code(op-code), and operands. The operation code indicates what kind of a command it is. The operands are the co-ordinates of a point. Representation in memory Three separate arrays are used. One for the operation code(DF-OP), one for the x coordinate(DF-X) and one for the y coordinate(DF-Y).
- 11. 11 DISPLAY FILE STRUCTURE DF-OP DF-X DF-Y Display file processor Display File processor is a special purpose processor located in a graphic terminal which will convert the drawing instructions into actual images.
- 12. 12 Call to Line Functions Users Program Display File Display File Interpreter Display DF-OP DF-X DF-Y
- 13. 13 An application program is input and stored in the system memory along with the graphics package. Graphics commands in the application program are translated by the graphics package into a display file stored in the system memory. This display file is then accessed by the display processor to refresh the screen. The display processor cycles through each command in the display file program once during every refresh cycle. Sometimes the display processor in the random-scan system is referred to as a display processing unit or a graphics controller. DISPLAY FILE STRUCTURE
- 14. 14 Lines should appear straight. Lines should terminate accurately. Line should have constant density Line density should be independent of line length and angle. Lines should be drawn rapidly. Characteristics of a Good Line Drawing algorithms
- 15. 15 ANITIALIASING OF LINES Many displays allow only two pixel states, on or off. For these displays, lines may have a jagged or stair-case effect when a line step from one row or column to the next. This is one aspect of a phenomenon called aliasing. Displays which allow setting pixels to gray levels between black and white provide a means to reduce this effect. This technique is called as anitialiasing .
- 16. 16 CHARACTER GENERATION There are two primary methods for character generation 1. Stroke method 2. Dot-matrix or bit-map method Stroke method This method creates characters out of a series of line segments, like strokes of a pen. The Character Generator can be built as series of calls to any line drawing algorithm. This method lends itself to changes of scale, that is, the characters may be made twice as large by simply doubling the length of the line segments.
- 17. 17 Dot-matrix method or Bitmap method In this method the characters are represented by an array of dots. 7 X 9 9 X 13 are the common arrays used. This array is like a frame buffer, just enough to hold the character. Placing the character on the screen becomes a process of copying pixel values from the small character array into some portion of the screen’s frame buffer.
- 18. 18 Dot-matrix method or Bitmap method Character generator chip is the hardware device that is the memory containing the character dot matrix array. Random access memory may also be used when many fonts are desired. The size of the pixel is fixed. So the dot matrix method dose not lend itself to variable sized characters.
- 20. 20 PRIMITIVE OPERATIONS Absolute line command Used to draw a line from the current position to the point specified. LINE-ABS(X,Y) Relative line command Used to draw a line from the current position to a new position. The new position is indicated by specifying the distances in both directions. LINE-REL (DX,DY) LINE-REL(DX,DY) is same as LINE-ABS(X+DX,Y+DY)
- 21. 21 PRIMITIVE OPERATIONS Absolute move command Used to move from the current position to the point specified. MOVE-ABS(X,Y) Relative move command Used to move from the current position to a new position. The new position is indicated by specifying the distances in both directions. MOVE-REL(DX,DY)
- 22. 22 NORMALIZED DEVICE CO-ORDINATES Different display devices may have different screen sizes as measured in pixels. The user's program should be device independent. User specifies the co-ordinates in some units and the interpreter will convert these co-ordinates to the appropriate pixel values for a particular display device. Normalized device co-ordinates is the system followed by the users in general. Xs = WIDTH * Xn + WIDTH-START Ys = HEIGHT * Yn + HEIGHT-START