Some advantages:

• Open, vendor-neutral, multiplatform graphics standard.
• Supported and used by the industry.
• Stable, reliable, and portable.
• Runs on the major operating systems: Unix / Linux, Windows, MacOS and on some other platforms like the iPod.

Components of OpenGL:

• Basic library: GL.
• GLU - graphics utility library - code for some higher level objects like spheres
• GLUT Graphics Utility Toolkit - user interface.

A relatively simple GUI library.
• The interface objects are hidden from the user. All we can get is some "id" associated with graphical objects.
• All communication with the interface is done through some simple function calls.
• It only allows for pop-up menus.
• One or two windows.

Some GLUT functions

glutInit(&argc, argv)	To be called in the beginning of the program.
glutCreateWindow(title)	Creates the application window. All GL command will be displayed on this window.
glutDisplayFunc(function)	To be called for every update type of event.
glutIdleFunc(function)	Called repeatedly if the user doesn't input anything (animation).
glutMainLoop()	To be called to launch the application (GUI).
glutMouseFunc(function)	Callback function for any mouse event.

Between glBegin and glEnd, a list of vertex primitives, for example
void glVertex3f (GLfloat x, GLfloat y, GLfloat z );

Consider the list p_i = glVertex(x_i, y_i, z_i); 1 < i < n .

Geometrical mode:

GL_POINTS	Each pi is drawn as a point.
GL_LINES	Each (p2i, p2i+1) makes a line segment.
GL_LINE_STRIP	Each (pi, pi+1) makes a line segment.
GL_LINE_LOOP	Similar to line strip, plus (pn, p1) also makes a line segment.
GL_TRIANGLES	Each (p3i, p3i+1, p3i+2) makes a triangle.
GL_TRIANGLE_STRIP	Each (pi, pi+1, pi+2) makes a triangle.
GL_TRIANGLE_FAN	Each (pi, pi+1), i>0, makes a triangle with p1.
GL_QUADS	Each (p4i, p4i+1, p4i+2, p4i+3) makes a quadrilateral.
GL_QUAD_STRIP	Each (p2i, p2i+1), (p2i+3, p2i+2) makes a line quadrilateral.
GL_POLYGON.	A single polygon with n sides.

Color in OpenGL

• Defined by R, G, B in the range 0.0 to 1.0.
• glColor3f(r, g, b);
• options: d, f, b, i, s, u-unsigned, v-vector.
  
  
• To clear the window with a given color:
• glClearColor(r, g, b, alpha);
• alpha = 1.0 for opaque, alpha = 0.0 for completely transparent.

Interpolating the Color

• A glColor command defines the color to be applied to all geometrical commands that follow it.
• A new glColor command has the effect of interpolating the color between the previously defined color and the new one for all the geometrical commands in between.

Display Lists

• Def. A display list is a compiled group of OpenGL commands that have been stored under a unique identity (integer) for later execution.
• When a display list is invoked, the commands in it are executed in the order in which they were issued.
• Display lists can be drawn fast and easily by one function call.
• Useful for objects that will be drawn more than once.
  
  
• Creating a display list (the second parameter can also be GL_COMPILE_AND_EXECUTE):
  
  glNewList(id, mode); // Starts a new display list
  any GL/GLU commands in between
  glEndList();
  
  where mode can be
   

  GL_COMPILE	The commands in the list will not be executed as they are stored in the display list.
  GL_COMPILE_AND_EXECUTE	The commands will be executed as they are stored in the display list.

  
  
• Drawing the display list:
  glCallList(id);

• for the objects in the scene (model view),
• for the camera (point of view or projection),
• for the texture.

All of the transformations are defined as matrices that are multiplied with the active matrix.
See page 68 from the textbook.

Matrix Operations

• Switching the matrix mode:

glMatrixMode(GL_PROJECTION);
glMatrixMode(GL_MODELVIEW); // default
glMatrixMode(GL_TEXTURE);
 
• Setting a matrix to the identity:

glLoadIdentity();
 
• Loading a particular matrix:

glFloat m[] = {1.0, 0.0, ... }; // 16 elem.
glLoadMatrixf(m); // f or d
 
• Multiply the active matrix by a given matrix

glMultMatrixf(m);  // f or d

• The 3 matrices are organized in a stack. One can push the active matrix in the stack, then pop it out of the stack again.

glPushMatrix(); // save
glPopMatrix(); // restore

Programs: cube.c, cubeview.c from the textbook CD.
Goal: define a cube of  edge=2, with the center in the origin.

• cube.c - parallel projection
• cubeview.c - perspective projection
• cube1.cc - cube.c, modified to spin slowly

Changing the Point of View

• In parallel projection: call the glOrtho function again (see function myReshape).
• In perspective projection: either by calling the gluLookAt plus the glFrustum or gluPerspective

parallel projection (cube.c)	perspective projection (cubeview.c)
perspective, the close plane cuts the cube	persective centered front view