Viewing: ModelView/Projection Transformation/Matrices and Viewport Transformation

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:

1. Spend at least 5 minutes on Figure3-1 in Chapter 3 of The Red Book at: http://fly.cc.fer.hr/~unreal/theredbook/chapter03.html
2. Look at Figure3-2 while reading only the one paragraph above Figure3-2 and the four paragraphs thereafter.
   If not understanding some part, just skip it.
3. Copy the following Makefile:
   

   		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)
   

4. Copy the following code and name it as cube.c

   		#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;
   		}
   

5. Run the code after make cube.
   And change some values in those transformation functions to see the effect.
6. Now, once more, Look at Figure3-2 while reading only the one paragraph above Figure3-2 and the four paragraphs thereafter.
   Please remember by heart and mind:
   • ModelView matrix contains both viewing and modeling matrices, their OpenGL functions are: glLookAt(), glScale(), glTranslate()...
   • Projection matrix corresponds viewing volume which does NOT all relate to the viewing transformation!! The OpenGL functions: glFrustrum(), gluPerspective(), gluOrtho().
   • Viewport transformation corresponds to the size of the developed photogram. The OpenGL function: glViewport().
     Please notice:

     		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);
     

7. Try to do those parts marked Try This in Chapter 3