CD2012 Principles of Interactive Graphics Lecture 05

1. CD2012Principles of Interactive GraphicsLecture 05 Interactions Abir Hussain www.cms.livjm.ac.uk/cmsahus1

2. Previous Lecture • Graphics hardware: display devices, hard copy devices, and interactive input devices. • Display devices: CRT, plasma screen, LED and LCD. • Hard copy devices: printers and plotters • Interactive input devices: mouse and joystick. CD2012-05

3. Today’s lecture and lab CD2012-05

4. Introduction • Interactive graphics application allows the user to control the flow of a program • by natural human motions such as pointing and clicking the mouse or pressing various keys on the keyboard. • Recall, every interactive graphics program has the same model: • Initialise the application data and graphics environment • Create the contents of the display • Paint the contents of the display on a window • Set-up functions to handle input event • Start an infinite loop to handle input events CD2012-05

5. General event handling model • When a user presses or releases a mouse button, moves the mouse or presses a key in the keyboard an event occurs. • A graphics program registers certain functions as event handling functions. • The Program then goes into an infinite loop waiting for events • when an event arrives a notification system decides how it should be handled. CD2012-05

6. General event handling model • The event structure is then passed to the function that was registered for that event. • Then, the function exits and returns control to the main loop. CD2012-05

7. What is a callback function?

8. Recall • The basic mechanism for handling input events is to register callback functions to handle specific events. • These events could be • Window events: moving, hiding or resizing a window • Input device events: mouse, keyboard, tracker etc • Timer events that are generated by a clock in the application CD2012-05

9. Windows event handling • The function glutDisplayFunc(display) reacts to events from the Window system to draw (or re-draw) the contents of the window. • As an example, in MS Windows if the window has been • Minimised/Maximised • Hidden/Shown relative to other windows  CD2012-05

10. Windows event handling • The function glutReshapeFunc(reshape) reacts to events when the Window is resized. • The callback function (e.g. reshape) takes as arguments, the new width and height of the window. • Every window system has similar functions to handle these events. CD2012-05

11. Mouse interaction • The function glutMouseFunc(myMouse) registers the function myMouse () as the function to be executed when a mouse event occurs. • The callback function myMouse() should be designed to take four parameters: void myMouse (int button, int state, int x, int y); CD2012-05

12. Mouse interaction • In this case, the button value could be • GLUT_LEFT_BUTTON, GLUT_MIDDLE_BUTTON, GLUT_RIGHT_BUTTON. • The value of the state could be • GLUT_DOWN and GLUT_UP. • While x and y represent the position of the mouse at the time of the event. CD2012-05

13. Mouse interaction • Windows systems also support event handlers for mouse motion, e.g. • glutMotionFunc(motion) • glutPassiveMotionFunc(passive) • The first registers motion() to handle moving the mouse with a button pressed • The second registers passive() to handle moving the mouse with a button released. • By managing the input state, we can support the behaviour of most user interface objects. CD2012-05

14. Keyboard interaction • When pressing a key on the keyboard, this would queue a keyboard events. • The callback function myKeyboard() is register with such events through glutkeyboardFunc(myKeyboard). • This function has the following prototype:   • void myKeyboard(unsigned int key, int x, int y) CD2012-05

15. Text and fonts • There are two forms of text which are stroke and raster. • In stroke text, we define line segments or curves that outline each character. • In raster text, characters are defined as rectangles of bits called bit blocks. • Each block defines a single character by the pattern of 0 and 1 bits in the block. CD2012-05

16. Text and fonts • The GLUT provides a few bitmap and stroke character sets. As an example:   • GlutBitmapCharacter(GLUT_BITMAP_8_BY_13, c) • Where c is the number of ASCII characters that we wish to be placed on the display. CD2012-05

17. Timer general events • While the main event loop is running the program can only respond to external events. • If we want something to happen between events we have to register handlers to run either • When the program is idle • Or in response to timer generated events CD2012-05

18. Timer general events • glutIdleFunc() will register a function to run whenever the program is not handling other events. • However, this function should return quickly or the program may be unable to respond to other events • We can register a function to be called every few milliseconds by glutTimerFunc. CD2012-05

19. Summary • Windows event handling • Mouse interactions • glutMouseFunc(myMouse) • Keyboard interactions • glutkeyboardFunc(myKeyboard). • Text and fonts • GlutBitmapCharacter(GLUT_BITMAP_8_BY_13, c) CD2012-05