The Chapter 3 of The Red Book clarifies the relationship among modeling, viewing, projection, viewport transformations that often confuse the OpenGL beginners. Please be patient in going through following steps one by one:
INC=-I/usr/local/include -I/usr/X11R6/include -I/usr/include/Xft2 -O2 LIB=-L/usr/local/lib -L/usr/X11R6/lib -L$(QTDIR)/lib LIBS=-lGL -lX11 -lGLU -lXmu -lm -lXext -lglut -lXi .c: g++ $@.c $(LIB) $(INC) -v -o $@ $(LIBS)
#include <GL/glut.h> #include <stdlib.h> void init(void) { glClearColor (0.0, 0.0, 0.0, 0.0); glShadeModel (GL_FLAT); } void display(void) { glClear (GL_COLOR_BUFFER_BIT); glColor3f (1.0, 1.0, 1.0); glLoadIdentity (); /* clear the matrix */ /* viewing transformation */ gluLookAt (0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0); glScalef (1.0, 2.0, 1.0); /* modeling transformation */ glutWireCube (1.0); glFlush (); } void reshape (int w, int h) { glViewport (0, 0, (GLsizei) w, (GLsizei) h); /* viewport transformation */ glMatrixMode (GL_PROJECTION); glLoadIdentity (); glFrustum (-1.0, 1.0, -1.0, 1.0, 1.5, 50.0); /* projection transformation */ glMatrixMode (GL_MODELVIEW); } void keyboard(unsigned char key, int x, int y) { switch (key) { case 27: exit(0); break; } } int main(int argc, char** argv) { glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); glutInitWindowSize (500, 500); glutInitWindowPosition (100, 100); glutCreateWindow (argv[0]); init (); glutDisplayFunc(display); glutReshapeFunc(reshape); glutKeyboardFunc(keyboard); glutMainLoop(); return 0; }
Following sequence maps a square image onto a square viewport: gluPerspective(myFovy, 1.0, myNear, myFar); glViewport(0, 0, 400, 400); However, the following sequence projects a nonequilateral rectangular image onto a square viewport. The image appears compressed along the x-axis. gluPerspective(myFovy, 2.0, myNear, myFar); glViewport (0, 0, 400, 400); To avoid the distortion, this line could be used: glViewport(0, 0, 400, 200);