I assume this would be a easy question but I just could not figure it out.
There is a hello.c in the red “programming guide” book. I’ve copied the code as below. My question is:
The program is not using gluLookAt(), therefore the camera should be at the default position, i.e. (0, 0, 0). But the rectangular is also at z=0 plane. How would this work? If the camera is placed right on the object plane, how could camera “see” the object? But I ran the program and it did show the rectangular. If I use prospective view, i.e. change
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
to
glFrustum(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
It still shows the rectangular but the position and size is different.
I am kinda confused. Did I miss anything?
Thanks!
Frank
#include <GL/glut.h>
void display(void)
{
/* clear all pixels */
glClear (GL_COLOR_BUFFER_BIT);
/* draw white polygon (rectangle) with corners at
* (0.25, 0.25, 0.0) and (0.75, 0.75, 0.0)
*/
glColor3f (1.0, 1.0, 1.0);
glBegin(GL_POLYGON);
glVertex3f (0.25, 0.25, 0.0);
glVertex3f (0.75, 0.25, 0.0);
glVertex3f (0.75, 0.75, 0.0);
glVertex3f (0.25, 0.75, 0.0);
glEnd();
/* don't wait!
* start processing buffered OpenGL routines
*/
glFlush ();
}
void init (void)
{
/* select clearing color */
glClearColor (0.0, 0.0, 0.0, 0.0);
/* initialize viewing values */
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
}
/*
* Declare initial window size, position, and display mode
* (single buffer and RGBA). Open window with "hello"
* in its title bar. Call initialization routines.
* Register callback function to display graphics.
* Enter main loop and process events.
*/
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize (250, 250);
glutInitWindowPosition (100, 100);
glutCreateWindow ("hello");
init ();
glutDisplayFunc(display);
glutMainLoop();
return 0; /* ANSI C requires main to return int. */
}