This lesson covers drawing points, lines, and faces in 3D. Some basic user input functionality is also introduced. Use the same procedures for setting up Pelles C compiler for Win32 as outlined in the previous lesson, Getting Started with OpenGL GLUT.

The OpenGL cartesian coordinate system is right-handed, meaning +X is to the right, +Y is up, and +Z points out of the screen. The center (X,Y,Z) point is the origin point, or (0,0,0). When we draw basic geometric primitives we specify the coordinates of multiple vertices, a single vertex at a time.

As shown above, OpenGL has 10 functions for drawing points, lines, and faces. They all have a similar syntax and are very easy to use. All you need to do is use the glVertex3f() function flanked by calls to glBegin() and glEnd().


Example of drawing two points
glBegin(GL_POINTS);

glVertex3f(4.0, 0.0, 4.0);
glVertex3f(1.0, -4.0, -2.0);

glEnd();


Example of drawing three lines
glBegin(GL_LINES);

glVertex3f(6.0, 4.0, 2.0);
glVertex3f(2.0, -4.0, 3.3);

glVertex3f(5.0, 8.0, 8.0);
glVertex3f(-4.7, 5.0, -3.0);

glVertex3f(0.0, 0.0, 0.0);
glVertex3f(6.0, -1.0, -7.0);

glEnd();


Example of drawing a single triangle
glBegin(GL_TRIANGLES);

glVertex3f(-3.0, -3.0, 2.0);
glVertex3f(3.0, -3.0, -1.0);
glVertex3f(0.0, 3.0, 4.0);

glEnd();

The rest are quite simple to figure out, here is a table of all the available OpenGL point, line, and face drawing functions:

Now for a note about color. Notice in the code below the use of glColor() before some of the vertex definitions. When OpenGL sees different colors for different vertices of polygons it blends the colors across the face.

Now it's time to try things out! As you can see, the code below is based on the previous lesson with some additions. Some simple calls to drawing functions have been added along with a keyboard routine using the glutKeyboardFunc() function. The glRotatef() function is used to rotate the data on the screen in 3 dimensions.

//---------------------------------------------------------------------
// Drawing in 3D with openGL GLUT
// Points, Lines and Faces
//
// Includes basic user keyboard input
//---------------------------------------------------------------------

#include <windows.h>
#include <gl\gl.h>
#include <gl\glut.h>

void init(void);
void display(void);
void keyboard(unsigned char, int, int);

int main (int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(200, 200);
glutInitWindowPosition(50, 50);
glutCreateWindow("Drawing in 3D");
init();
glutDisplayFunc(display);
glutKeyboardFunc(keyboard); //set keyboard handler
glutMainLoop();
return 0;
}

void init(void)
{
glClearColor(0.0, 0.0, 0.0, 0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-15.0, 15.0, -15.0, 15.0, -15.0, 15.0);
}

void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);

glBegin(GL_QUADS);
glColor3f(1.0, 0.0, 0.0); //red
glVertex3f(-3.0, -3.0, 0.0);
glVertex3f(3.0, -3.0, 0.0);
glColor3f(1.0, 0.0, 1.0); //blue
glVertex3f(3.0, 3.0, 0.0);
glVertex3f(-3.0, 3.0, 0.0);
glEnd();

glBegin(GL_TRIANGLES);
glColor3f(1.0, 0.0, 0.0);
glVertex3f(0.0, 6.0, 0.0);
glColor3f(0.0, 0.0, 0.9f);
glVertex3f(-3.0, 3.0, 0.0);
glVertex3f(3.0, 3.0, 0.0);
glEnd();

glBegin(GL_LINES);
glColor3f(0.0, 0.0, 1.0); //blue

glVertex3f(0.0, 7.0, 0.0);
glVertex3f(0.0, -4.0, 0.0);
glEnd();

glutSwapBuffers();
}

void keyboard(unsigned char key, int x, int y)
{
/* this is the keyboard event handler
the x and y parameters are the mouse
coordintes when the key was struck */
switch (key)
{
case 'u':
case 'U':
glRotatef(3.0, 1.0, 0.0, 0.0); // rotate up
break;
case 'd':
case 'D':
glRotatef(-3.0, 1.0, 0.0, 0.0); // rotate down
break;
case 'l':
case 'L':
glRotatef(3.0, 0.0, 1.0, 0.0); // rotate left
break;
case 'r':
case 'R':
glRotatef(-3.0, 0.0, 1.0, 0.0); // rotate right
}
display(); // repaint the window
}
//---------------------------------------------------------------------

Drawing output

There is a reason drawing only occurs on a single plane in this example. Experiment and you will see faces do not intersect properly. This effect is due to a lack of depth buffering, also known as Z-buffering, which will be covered in the next lesson.

END OF LESSON

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