Little Intro to OpenGL

1. Little Intro to OpenGL COMP471 - Computer Graphics Edition 1.2, October 2, 2002 Serguei Mokhov, mokhov@cs.concordia.ca

2. Contents • Demos • Setting up Visual C++ to work with OpenGL • Working under Linux with OpenGL • Basics • Sample Programs • skeleton.cpp Explained • sampleglprogram.cpp • Other Examples Serguei Mokhov, mokhov@cs.concordia.ca

3. Setting up Visual C++ to work with OpenGL • Click main menu "File", then click menu item "New...". • Use Tab or mouse to select option "Projects“ • Use mouse to highlight the project type -- Win32 Console Application, then input the project name • Click “OK” followed by clicking the “Finish” button, you will create an empty project. • Add all source code (*.h, *.cpp, *.c) into the current project. Serguei Mokhov, mokhov@cs.concordia.ca

4. Setting up Visual C++ to work with OpenGL (2) • Set the OpenGL library linkage. • Click main menu “Project”, then click menu item “Settings…” followed by using mouse or Tab key to select option "Link". • In the edit box "Object/library modules", three OpenGL libraries are appended: glu32.lib glut32.lib opengl32.lib (note that there is one space between library names). • If you plan to use glaux, add glaux.lib as well. • Compile and link your project by using "F7" or the toolbar. Serguei Mokhov, mokhov@cs.concordia.ca

5. Working under Linux with OpenGL • You’ll need to link your compiled object files with the following libraries:-lglut -lGL -lGLU -L/usr/X11R6/lib -lXmu -lXext -lXt -lXi -lX11 Grab example makefile from the web. Serguei Mokhov, mokhov@cs.concordia.ca

6. Basics • State Machine • Transformations • Light • Color • Textures • Viewing • OpenGL Coordinate System • OpenGL: Event Driven Serguei Mokhov, mokhov@cs.concordia.ca

7. Basics (2) • Matrices and Matrix Operations • Model-View Matrix • Projection Matrix • Matrix Stack • Polygons in Open GL • Viewing Serguei Mokhov, mokhov@cs.concordia.ca

8. Polygons (GL_POLYGONS) • By default are “two-sided” • Counter clockwise (cube example) • Concave • Not necessarily “flat” Serguei Mokhov, mokhov@cs.concordia.ca

9. Viewing • Changing Model (skeleton) • Changing Projection Matrix (sample) • Changing Camera position w/ gluLookAt()gluLookAt( 0.0, 0.0, g_fDistance, // position 0.0, 0.0, 0.0, // direction 0.0, 1.0, 0.0 // up-vector); Serguei Mokhov, mokhov@cs.concordia.ca

10. skeleton.cpp Explained: Headers glut.h includes glu.h and gl.h, it also defines all needed macros • #include stuff In labs, remove GL, because the sysadmins installed glut.h above it #include <GL/glut.h> #include <math.h> #include <iostream.h> #include <strstream.h> #include <iomanip.h> Note: if you decide not to use glut.h, but still want/need to do some OpenGL, you’ll have to include gl.h and/or glu.h; Pay attention in Windows However, make sure you include windows.h before any of the, since It defines some macros needed gl/glu.h Serguei Mokhov, mokhov@cs.concordia.ca

11. skeleton.cpp Explained: Globals • Global Constants and Variables // Initial size of graphics window. const int WIDTH = 600; const int HEIGHT = 400; // Current size of window. int width = WIDTH; int height = HEIGHT; That’s what it says. All sizes are in pixels. Serguei Mokhov, mokhov@cs.concordia.ca

12. skeleton.cpp Explained: Globals • Global Constants and Variables // Mouse positions, normalized to [0,1]. double xMouse = 0.5; double yMouse = 0.5; Serguei Mokhov, mokhov@cs.concordia.ca

13. skeleton.cpp Explained: Globals • Global Constants and Variables // Bounds of viewing frustum. double nearPlane = 5; double farPlane = 15; Object in bold is called frustum. Only those graphical objects, entirely or partially, happened to be inside this frustum are visible on screen. Camera farPlane 15 5 nearPlane Serguei Mokhov, mokhov@cs.concordia.ca

14. skeleton.cpp Explained: Globals • Global Constants and Variables // Viewing angle. double fovy = 40.0; Serguei Mokhov, mokhov@cs.concordia.ca

15. skeleton.cpp Explained: Globals • Global Constants and Variables // Variables. double alpha = 0; // Set by idle function. double beta = 0; // Set by mouse X. // Set by mouse Y. double distance = -(farPlane - nearPlane) / 2; y x • z Serguei Mokhov, mokhov@cs.concordia.ca

16. skeleton.cpp Explained: main() • Typical steps in main()… void main (int argc, char **argv) { // GLUT initialization. ... // Register call backs. ... // Display ... // GLUT loop } Serguei Mokhov, mokhov@cs.concordia.ca

17. skeleton.cpp Explained: main() • GLUT Initialization, p. 646 Command line options. Must call glutInit() the very first thing. // GLUT initialization. glutInit(&argc, argv); glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB); glutInitWindowSize(width, height); glutCreateWindow("GLUT Skeleton Program"); Use double buffering (to avoid flickering) and RGB palette. Window title bar. Renders useless during execution. Initial graphical window size. Creates graphical window with all settings before. Must be called the last. NOTE: Window is created, but NOT shown yet. Serguei Mokhov, mokhov@cs.concordia.ca

18. Callbacks • Callbacks are user-defined functions designed to react on specific events: • Whenever OpenGL decided it needs to redraw window contents • What to do when a user resizes a window. • Handle mouse motions… • React on keyboard, • What to do during idle period (no input from user), • and so on. • For OpenGL to become aware of your callbacks, you need to register them within it before you start drawing things. • Some of the callbacks are mandatory, such as display, so that OpenGL know how to render your graphics. Serguei Mokhov, mokhov@cs.concordia.ca

19. skeleton.cpp Explained: main() • Registering callbacks… User-defined functions // Register call backs. glutDisplayFunc(display); glutReshapeFunc(reshapeMainWindow); glutKeyboardFunc(graphicKeys); glutSpecialFunc(functionKeys); glutMotionFunc(mouseMovement); glutIdleFunc(idle); What to do when there’s nothing to do… Catching a mouse … How to react on special keys (F1..F12, etc) How to react on “ordinary” keys… How to resize on resize event… Knowing how to display the model Serguei Mokhov, mokhov@cs.concordia.ca

20. skeleton.cpp Explained: main() • The rest... // Display help. help(); // Enter GLUT loop. glutMainLoop(); All non-graphical output goes to STDOUT This is the very last thing you call in the main(). Upon this call finally all the created windows are shown, and the OpenGL state machine starts running. Serguei Mokhov, mokhov@cs.concordia.ca

21. Callbacks: display() • display() // This function is called to display the scene. void display () { // {Clearing viewport and setting up initial matrices} … // Translate using Y mouse. … // Rotation from idle function. … // Rotation using X mouse. … // Draw model axes. … // Draw an object. … // {Double Buffering} } Serguei Mokhov, mokhov@cs.concordia.ca

22. Callbacks: display() • display() Clears the color buffer of the viewport with current color (image if it were would stay) glClear(GL_COLOR_BUFFER_BIT); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); We’re going to operate on the Model/View Matrix now. Initial “value” of the model/view matrix is identity Serguei Mokhov, mokhov@cs.concordia.ca

23. Callbacks: display() • display() // Translate using Y mouse. distance = - (yMouse * (farPlane - nearPlane) + nearPlane); glTranslatef(0, 0, distance); float x y -z Make sure we’re inside the viewing volume. NOTE: z-axis goes to us from screen, i.e. it’s a normal to screen’s surface Before we translate the object, it’s centered at the origin, for us to see it, we need to move it far enough by z Serguei Mokhov, mokhov@cs.concordia.ca

24. Callbacks: display() • Rotation Apply rotation transformation by y axis. // Rotation from idle function. glRotatef(alpha, 0, 1, 0); // Rotation using X mouse. beta = 180.0 * xMouse; glRotatef(beta, 1, 0, 0); Apply rotation transformation by x axis when using mouse. Serguei Mokhov, mokhov@cs.concordia.ca

25. Callbacks: display() • Drawing graphical primitives. • Always encapsulated inside glBegin/glEnd block // Draw model axes. glBegin(GL_LINES); // X axis glColor3f(1, 0, 0); glVertex3f(0, 0, 0); glVertex3f(2, 0, 0); // Y axis glColor3f(0, 1, 0); glVertex3f(0, 0, 0); glVertex3f(0, 2, 0); // Z axis glColor3f(0, 0, 1); glVertex3f(0, 0, 0); glVertex3f(0, 0, 2); glEnd(); The color persists until next getColor*() is called. RGB We connect vertices with lines. (Other things are polygons, etc.) Serguei Mokhov, mokhov@cs.concordia.ca

26. Callbacks: display() • Wire sphere… Wire sphere, cube and other objects are also good for “visual debugging”. P. 659 // Draw an object. glColor3f(0, 1, 1); glutWireSphere(1, 20, 20); Serguei Mokhov, mokhov@cs.concordia.ca

27. Callbacks: display() • Double Buffering • To avoid flickering • To increase performance • While one buffer is being displayed, the other one is being drawn “off-line” • Then you swap the buffers, and do the same thing. • This way you don’t see actual drawing when the program is running, so you can avoid flickering on a lower-resolution monitors and display adapters. • In OpenGL we use: glutSwapBuffers(); Serguei Mokhov, mokhov@cs.concordia.ca

28. Callbacks: reshapeMainWindow() • reshapeMainWindow // Respond to window resizing, preserving proportions. // Parameters give new window size in pixels. void reshapeMainWindow (int newWidth, int newHeight) { width = newWidth; height = newHeight; glViewport(0, 0, width, height); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(fovy, GLfloat(width) / GLfloat(height), nearPlane, farPlane); } Drawing region in the window where all graphics is mapped. Projection matrix is initialized as identity matrix All subsequent transformations are now upon projection matrix. Define viewing volume (frustum), applying matrix transformations. Serguei Mokhov, mokhov@cs.concordia.ca

29. sampleglprogram.cpp • Has a very similar structure as skeleton.cpp, but there’s one significant difference: • Viewing Transformation is performed upon the Projection Matrix every time display() is invoked. Serguei Mokhov, mokhov@cs.concordia.ca

30. // This function is called to display the scene. void display () { setView(); glClear(GL_COLOR_BUFFER_BIT); // set modelling mode glMatrixMode(GL_MODELVIEW); glLoadIdentity(); .... } Performs viewing transformation. Perform model transformation after. Serguei Mokhov, mokhov@cs.concordia.ca

31. void setView() { // Must set it up in Projection Matrix glMatrixMode(GL_PROJECTION); glLoadIdentity(); // Rotation about X from UP-DOWN Arrow key glRotatef(beta, 1, 0, 0); // Rotation about Y from idle function. glRotatef(alpha, 0, 1, 0); if(projType) gluPerspective(fovy, (GLfloat) 1.0, nearPlane, farPlane); else glOrtho ( viewWindowLeft, viewWindowRight, viewWindowBottom, viewWindowTop, nearPlane, farPlane ); } Perform model transformation after. Serguei Mokhov, mokhov@cs.concordia.ca

32. gluLookAt() • Manipulates the Camera • The best way to observe the model gluLookAt( 0.0, 0.0, g_fDistance, // position 0.0, 0.0, 0.0, // direction 0.0, 1.0, 0.0 // up-vector); Serguei Mokhov, mokhov@cs.concordia.ca

33. Matrix Stack void drawSphere(GLdouble Radius, GLdouble x, GLdouble y, GLdouble z) { glPushMatrix(); // translate the table to its position glTranslated(x, y, z); //draw glut sphere glutWireSphere(Radius, 20, 20); glPopMatrix(); } Serguei Mokhov, mokhov@cs.concordia.ca