OpenGL Introduction Computer Graphics and Visualization

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1 Fall OpenGL OpenGL System Interaction Portable Consistent visual display regardless of hardware, OS and windowing system Callable from Ada, C, C++, Fortran, Python, Perl and Java Runs on all major OS and windowing system. Well documented OpenGL is completely window system independent. But we need the window system for things like: Opening and closing windows Interaction Handling events We can work directly with the window system (X-windows, Win 32,...) or use a higher level toolkit (GLUT, Motif,...). 3 4 OpenGL Utility Toolkit OpenGL Libraries GLUT has been ported to all platforms that support OpenGL. Provides basic window system interaction Open/Close Window Mouse/Keyboard callbacks Idle callback for animation Pop-Up Menus Application GTK+ GLUI GLUT GLU WGL GLX AGL GL 5 6

2 OpenGL Functions OpenGL State Primitives Points Line Segments Triangles Quadrilions Polygons Attributes Transformations Viewing Modeling Control (GLUT) OpenGL functions are of two types Primitive generating Can cause output if primitive is visible How vertices are processed and appearance of primitive are controlled by the state State changing Transformation functions Attribute functions Input (GLUT) Query 7 8 No Overloading or Generics OpenGL function name dimensions There are multiple functions for a given logical function glvertex3f(x,y,z) glvertex3f glvertex2i glvertex3dv belongs to GL library glvertex3fv(p) x,y,z are floats p is a pointer to an array 9 10 A Simple Program Generate a square on a solid background OpenGL Examples 11 12

3 simple.c Event Loop 13 #include <GL/glut.h> void display(){ glclear(gl_color_buffer_bit); glbegin(gl_polygon); glvertex2f(-0.5, -0.5); glvertex2f(-0.5, 0.5); glvertex2f(0.5, 0.5); glvertex2f(0.5, -0.5); glend(); glflush(); } int main(int argc, char** argv){ glutcreatewindow("simple"); glutdisplayfunc(display); glutmainloop(); } 14 Note that the program defines a display callback function named display Every glut program must have a display callback The display callback is executed whenever OpenGL decides the display must be refreshed, for example when the window is opened The main function ends with the program entering an event loop Primitives Primitives in OpenGL Complex objects are defined using simpler primitvies Points Line segments Polygons (filled) A non filled polygon is a set of lines. An object is defined by vertices. Vertices are grouped into primitives using glbegin(<type>)...glend(), where <type> defines the primitive type. OpenGl differs between filled and non-filled types Line segment types Polygon types 17 GL_LINE Successive pairs of verteces are interpreted as endpoints in individual lines GL_LINE_STRIP and GL_LINE_LOOP Verteces are connected 18 To draw a filled polygon with displayed edges, the polygon needs to be rendered twice. glpolygonmode can be used to control if only the edges should be drawn or the polygon to be filled. Notice that a polygon with > 3 vertices may not be flat.

4 2D World and scrreen coordinates View port An object (or primitive) has its coordinate in world coordinates. A viewport and camera defines a viewing volume that defines which primitives should be draw or not, and which lines to clip. In 2D this volume is a rectangle in 2D coordinates Orthographics projection Screen Coordinates A 2D viewing volum can be seen as an orthographic projection in 3D onto a 2D viewplane. In an orthographic projection, all projectors are parallel (and often orthogonal to the viewplane). A defined viewing rectangle in 2D must then be converted into screen coordinates. Either each pixel is one unit in world coorinates or we scale the viewing rectangle Aspect ratio OpenGL and defining a 2D view port 23 If not the ration between the height and width of the viewing rectangle match the windows size we get mismatch in aspect ratio. 24 glitinitwindowsize(800, 600); glmatrixmode(gl_projection); glloadidentity(); gluortho2d(-400,400,-300,300); glmatrixmode(gl_modelview); glloadidentity(); glviewport(400, 300, 400, 300); glbegin(gl_line) glvertex2i(-100, -100);glVertex2i;(100,100) glend();

5 Classical Imperative Programming Event Driven Programming Previous programming paradigms have emphasized the process for solving a problem. Once the program was complete we would: provide input run the program display the answer(s) Things always happened in the same sequence. The program always terminated when the process was complete Event Driven programming Event Driven programming Event-driven programs do not have a set sequence of instructions to execute. They also do not have a predetermined finish. The most common example of event-driven programming is found in graphical user interfaces (s): Windows, OSX, ios, X11 Other applications include embedded systems, control systems, sensor systems like home security, etc. In event driven architecture a programm idles awaiting events. Different events are stored in a queue. For each loop a new event is processed. If no new events in the queue, the program idles. Event driven programmes often threaded Imperative vs. Event Driven Event Driven APIs More or less integrated in languages but with a similar concept. High integration Java (AWT, Swing), Javascript Visual Basic Integrated with extensions C/C++ (GTK+, QT, etc.) Matlab An event producer produces an event, that when processed calls a callback function at an event handler

6 GTK+ and Glade Glade is a RAD (Rapid Application Development) tool to develope user interfaces for the GTK+ toolkit. The user interfaces are saved as XML Using the libglade library these can be loaded dynamically. Can be used in many languages including C, C++, Java, Perl, Python, C#, Pike, Ruby, Haskell, Objective Caml and Scheme. GladeXML *xml = glade_xml_new( "project1gui.glade", NULL, NULL); GLUT and the Given Given HOWTO is initialized by calling guiinit and initguiwindow Callback functions from the exists in project1gui.c GTK also uses a main loop, but GLUT main loop can call an idle callback function. GTK main loop content can be called separately. Call guimainiteration from the GLUT idle callback. Compile and link project1gui.c Use the callback functions to implement the features of your program. can also display simple text in its status bar (see project1gui.h) Qt #include <QApplication> #include <QFont> #include <QPushButton> int main(int argc, char *argv[]) { QApplication app(argc, argv); QPushButton quit("quit"); quit.resize(75, 30); QObject::connect(&quit, SIGNAL(clicked()), &app, SLOT(quit())); 35 } quit.show(); return app.exec();