Lecture 6 introduction to open gl and glut

Open Gl

1. Computer Graphics Shahid Mahmood

2. Introduction to OpenGL Supporting Libraries Lecture 6 - Content

3. Initially, Silicon Graphics Inc. Developed a system called “Graphics Library, GL” In 1992, a variation of GL, called OpenGL was announced OpenGL was designed to be platform-independent – to run across a wide range of computer hardware, not just on Silicon Graphics machines OpenGL - Origins

4. Soon, OpenGL was accepted as a standard computer graphics programming , due to its power and portability As we know, OpenGL is not a programming language, it’s the specification of an Application Programming Interface (API) It defines a set functions for creating computer graphics OpenGL – Success

5. OpenGL provides 3D geometric objects, such as lines, polygons, triangle meshes, spheres, cubes, quadric surfaces, and curves It provides 3D modeling transformations and viewing functions to create views of 3D scenes, using the idea of virtual camera OpenGL – Capabilities

6. OpenGL supports high-quality rendering of scenes, including hidden-surface removal, multiple light sources, transparency, textures, blending, fog It provides display lists for creating graphics caches and hierarchical models. It also supports the interactive picking of objects It supports the manipulation of images as pixels, enabling frame-buffer effects such as anti-aliasing, motion blur, etc. OpenGL – Capabilities, cont.

7. OpenGL is itself is only concerned with graphics rendering – OpenGL functions always start with “ gl ” To extend OpenGL’s functionality, two libraries have been developed, OpenGL Utility Library (GLU), and the OpenGL Utility Toolkit (GLUT) OpenGL – The support libraries

8. GLU provides functions to draw more complex primitives, such as curves and surfaces, it also help specify 3D views of scenes All GLU functions names start with “ glu ” GLU

9. GLUT provides the facilities for interaction that OpenGL lacks It provides windows management functions, handling input events from the mouse and keyboard It also provides basic functions for creating Graphical User Interfaces (GUIs). All GLUT functions names start with “glut” GLUT

10. Since OpenGL has been designed to be device-independent, it is not concerned with the exact makes and model of the graphics display and interaction hardware it uses, instead its functions refer to windows and events An OpenGL window is a rectangular area on a physical display screen into which OpenGL draws graphics An OpenGL event occurs when the operates an input device To respond to the input event, the application must provide a function known as a callback function to handle the event OpenGL automatically calls the application’s function, passing it with event data OpenGL – Platform & device independence

11. OpenGL does not draw its graphics directly to the window Instead draws into a data structure (an array of pixels) inside OpenGL called the frame-buffer, or simply buffer Periodically, OpenGL copies the pixels in the frame buffer into the window. OpenGL – frame-buffer

12. OpenGL – Program structure GLUT, GLEW Includes Create GLUT Window Register callback functions User Initializations Initialize GLUT GLUT main loop

13. Open window Configure display mode, Window position/size Register input callback Function (GLUT) Render, resize, input: keyboard, mouse, etc User Initialization Set background color, clear Color, etc Generate points to be drawn Initialize shader stuff Intitalize GLEW , Register GLUT callbacks, glutMainLoop () OpenGL/GLUT – Program structure GLUT, GLEW Includes Create GLUT Window Register callback functions User Initializations Initialize GLUT GLUT main loop

14. GLUT is used to create and open window glutInit (& argc , argv ); // Initializes GLUT glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); Sets display mode (e.ge single frame-buffer with RGB colors) glutInitWindowsSize (640, 480); Sets window size (Width x Height) in pixels glutInitPosition (100, 150); Sets location of upper left corner of window glutCreateWindow (“Window title”); open window with title “Window title” GLUT: Window management

15. Void main( int argc , char** argv ) { // initialize toolkit, set display mode and create window glutInit (& argc , argv ); // initialize toolkit glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); glutInitWindowSize (640, 480); glutInitWindowPosition (100, 150); glutCreateWindow (“Window title”); … } OpenGL Skeleton

16. Register callback functions , Do user initialization Wait in glutMainLoop for events … } OpenGL Skeleton

17. OpenGL programs are event-driven Sequential program Start program at main() Perform actions 1, 2, 3 … N End Event-driven program Start at main() Initialize Wait in infinite loop Wait till defined event occurs Event occurs => take defined actions } Sequential VS Event-driven

18. Program only responds to events Do nothing until event occurs Mouse clicks Keyboard stroke Window resize Programmer defines Events that program should respond to Actions to be taken when event occurs Wait in infinite loop System (window) Receives event, maintains event queue } OpenGL: Event-driven

19. Programmer registers callback functions (event handler) Callback function called when event occurs Example: Programmer Declare function onMouseClick , to be called on mouse click Register the function glutMouseFunc ( onMouseClick ); When OS receives mouse click, calls callback function onMouseClick } OpenGL: Event-driven

20. Register callbacks for all events your program will react to No registered callback = no action If no registered keyboard callback function, hitting keyboard keys generates no response. } GLUT Callback Functions

21. GLUT callback functions in skeleton glutDisplayFunc ( myDisplay ) // image to be drawn initially glutReshapeFunc ( myReshape ) // called when window is reshaped glutMouseFunc ( myReshape ) // called when mouse button is pressed } GLUT Callback Functions