Events and Coordinates

1. Events and Coordinates Lecture 5 Fri, Sep 5, 2003

2. Mouse and Keyboard Interaction • Through the callback functions, we may process mouse clicks and keystrokes. • This will be our only form of input to our programs.

3. Processing Mouse Clicks • The mouse function has prototype void mouse(int button, int state, int x, int y); • Values of button • GLUT_BUTTON_LEFT • GLUT_BUTTON_RIGHT • Values of state • GLUT_UP • GLUT_DOWN

4. Processing Mouse Clicks • x and y are the x and y screen coordinates of the mouse when the key was pressed, measured in pixels. • y is measured from the top of the window down. • x is measured from the left of the window across.

5. Processing Mouse Clicks • Skeleton code for mouse(). void mouse(int button, int state, int x, int y) { // Filter out other mouse events if (button == GLUT_BUTTON_LEFT && state == GLUT_DOWN) { // Do something } glutPostRedisplay(); }

6. Processing Keystrokes • The keyboard function has prototype void keyboard(unsigned char key, int x, int y); • key is any ASCII character on the keyboard. • x and y are the x and y screen coordinates of the mouse when the key was pressed.

7. Processing Keystrokes • Skeleton code for keyboard(). void keyboard(unsigned char key, int x, int y) { // Switch on designated keys only switch (key) { case ‘a’: // Do something default: break; } glutPostRedisplay(); }

8. Processing Special Keystrokes • The special function has prototype void special(int key, int x, int y); • key is nearly any non-ASCII character on the keyboard. • Values of key • GLUT_KEY_LEFT – left arrow key , etc. • GLUT_KEY_F1 – F1 function key, etc. • GLUT_KEY_HOME – home key, etc.

9. Processing Special Keystrokes • x and y are the x and y screen coordinates of the mouse when the key was pressed.

10. Processing Special Keystrokes • Skeleton code for special(). void special(int key, int x, int y) { // Switch on designated keys only switch (key) { case GLUT_KEY_LEFT: // Do something default: break; } glutPostRedisplay(); }

11. Example: Drawing an Octagon • DrawOctagon.cpp

12. Chapter 3 More Drawing Tools

13. World Coordinates • The world coordinate system is the coordinate system of the model itself, expressed in world units. • It is established by calling gluOrtho2D(). • gluOrtho2D(xmin, xmax, ymin, ymax).

14. Screen Coordinates • The screen (or window) coordinate system is the coordinate system of the screen (or window), expressed in pixels. • It is established by calling glViewport(). • glViewport(left, bottom, width, height).

15. Changing Coordinate Systems • We might need to convert from one coordinate system to another • When we go from screen coordinates (e.g., a mouse click) to world coordinates. • When we resize the window.

16. d v (X, Y) (x, y) u c r s a b Changing Coordinate Systems World coordinates Screen coordinates

17. Change of Coordinates • The points (X, Y) and (x, y) occupy the same relative positions in their respective rectangles. • Therefore, (x – r)/(s – r) = (X – a)/(b – a) and so x = X(s – r)/(b – a) + (br – as)/(b – a). • Similarly for y.

18. Example: Change of Coordinates • Given the statements gluOrtho2D(-8, 8, -6, 6); glViewport(0, 0, 640, 480); express x and y in terms of X and Y. x = X/40 – 8, y = Y/40 – 6.

19. Change of Coordinates • Furthermore, since the mouse() function measures the y-coordinate from the top down, we must make an additional adjustment. • Normally we will replace y by screenHeight – y. • In the last example, we now have x = X/40 – 8, y = (screenHeight – Y)/40 – 6.

20. Change of Coordinates • If we want to convert world coordinates into screen coordinates, this will require the inverse transformation. • In the last example, we find X = 40x + 320, Y = screenHeight – (40y + 240).