A Very Simple OpenGL Program

Models, or objects, are constructed from geometric primitives
- points, lines, and polygons - that are specified by their vertices.

The OpenGL program renders a white rectangle on a black background:

#include <whateverYouNeed.h>

main() {

   OpenAWindowPlease();

   glClearColor(0.0, 0.0, 0.0, 0.0);

   glClear(GL_COLOR_BUFFER_BIT);
   glColor3f(1.0, 1.0, 1.0);
   glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); 
   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();

   KeepTheWindowOnTheScreenForAWhile();
}

Clear the window to black: glClearColor() establishes what color the window will be cleared to, and glClear() actually clears the window. Once the color to clear to is set, the window is cleared to that color whenever glClear() is called. The clearing color can be changed with another call to glClearColor().

Similarly, the glColor3f() command establishes what color to use for drawing objects - in this case, the color is white. All objects drawn after this point use this color, until it's changed with another call to set the color.

The next OpenGL command used in the program, glOrtho(), specifies the coordinate system OpenGL assumes as it draws the final image and how the image gets mapped to the screen. The next calls, which are bracketed by glBegin() and glEnd(), define the object to be drawn - in this example, a polygon with four vertices. The polygon's "corners" are defined by the glVertex2f() commands. As you might be able to guess from the arguments, which are (x, y) coordinate pairs, the polygon is a rectangle.

Finally, glFlush() ensures that the drawing commands are actually executed, rather than stored in a buffer awaiting additional OpenGL commands.