OpenGL question

This is a discussion on OpenGL question within the C++ Programming forums, part of the General Programming Boards category; First, lets get the problem out of the way. Code: Draw an octahedron [eight triangles] in space with corners at ...

  1. #1
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    OpenGL question

    First, lets get the problem out of the way.

    Code:
    Draw an octahedron [eight triangles] in space with corners at (+/-1.0, 0.0, +/-1.0) and (0.0, +/-1.0, 0.0). 
    Use a 3D perspective frame (viewing volume) with boundaries (+/-5.0, +/-5.0, +/-5.0). 
    Temporarily assign each face different color, display and rotate around the y-axis.
    Now, I am new to OpenGL, and really, new to C++ all together, so I am learning at a slow pace. Disclaimer aside, I am still tweaking the octahedron because it isn't coming out the way I am expecting and I still have to figure out what I am missing, but that is not the issue right now. You will see that I am using an old program I did which turned out to be a simple box with all its sides, and a cut open area revealing the inner sides as well. I have removed the code I didn't feel needed to be used in this assignment and have tried moving on, while hitting some roadblocks.

    The main issue that I am having, is that I have no idea how I am supposed to be able to display the octahedron, and have it rotate around the y-axis. I can't really conceive anyway of how I am going to do this nor do I truly have the knowledge of going about doing it. Any helpful tips or hints would be greatly appreciated. I will show you what I have so far (the busted octahedron as of now).

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <GL/glut.h>
    
    typedef float Point2f [2];
    
    GLfloat v [6][3] = {{-1.0, 0.0, -1.0}, {1.0, 0.0, -1.0},
    				{-1.0, 0.0, 1.0}, {1.0, 0.0, 1.0},
    				{0.0, 1.0, 0.0}, {0.0, -1.0, 0.0}};
    
    GLfloat colors [6][3] = {{0.0, 0.0, 0.0}, {1.0, 0.0, 0.0}, 
    					{0.0, 1.0, 0.0}, {0.0, 0.0, 1.0}, 
    					{1.0, 0.0, 1.0}, {0.0, 1.0, 1.0}};
    
    void side (GLfloat * a, GLfloat * b, GLfloat * c, GLfloat * d) {
    	glVertex3fv (a);
    	glVertex3fv (b);
    	glVertex3fv (c);
    //	glVertex3fv (d);
    }
    
    void octo (GLfloat * a, GLfloat * b, GLfloat * c, GLfloat * d,
    		   GLfloat * e, GLfloat * f, GLfloat * g, GLfloat * h) {
    	glColor3fv (colors [0]);
    	side (a, b, d, c);
    	glColor3fv (colors [1]);
    	side (c, d, f, e);	
    	glColor3fv (colors [2]);
    	side (e, f, h, g);
    	glColor3fv (colors [3]);
    	side (g, h, b, a);
    	glColor3fv (colors [4]);
    	side (a, c, e, g);
    	glColor3fv (colors [5]);
    	side (b, d, f, h);
    }
    
    void display ( ) {		// need one such function per callback method
    	char * name = "octahedron";
    
    	glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    	
    	glPushMatrix ( );
    	glBegin (GL_TRIANGLE_STRIP);
    		octo (v [0], v [1], v [2], v [3], v [4], v[5], v[6], v[7]);
    	glEnd ( );
    	glPopMatrix ( );
    
    	glFlush ( );
    }
    
    void myReshape (int w, int h) {
    	GLfloat aspect;
    
    	glViewport (0, 0, w, h);
    	glMatrixMode (GL_PROJECTION);
    	glLoadIdentity ( );
    	aspect = (GLfloat) h / (GLfloat) w;
    	glOrtho (-5.0, 5.0, -5.0, 5.0, -5.0, 5.0);
    
    	gluPerspective (10.0, aspect, 10.0, -10.0);							
    
    	gluLookAt (5.0, 5.0, 10.0, 0.5, 1.0, 1.0, 0.0, 1.0, 0.0);			
    	glMatrixMode (GL_MODELVIEW);
    	glutPostRedisplay ( );
    }
    
    int main (int argc, char ** argv) {
    
    	/* 3D initialization stuff */
    	glutInit (&argc, argv);
    	glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);	
    	glutInitWindowSize (500, 500);
    	glutInitWindowPosition (100,100);
    	glutCreateWindow ("Octahedron");
    	glutReshapeFunc (myReshape);
    	glutDisplayFunc (display);		/* display callback invoked when window opened */
    	glEnable (GL_DEPTH_TEST);
    	glClearColor (1.0, 1.0, 1.0, 1.0);	/* white background */
    
    	glutMainLoop ( );
    	return 0;
    }
    Thanks again.

  2. #2
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  3. #3
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    thanks for this site, some very informative stuff

  4. #4
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    If you are still stuck after going through this, you may want to go ask your question at an OpenGL forum (OpenGL.org Discussion and Help Forums - Powered by UBB.threads™ is an example).

  5. #5
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    Have a look at glRotatef() for rotation. You'll need to keep track of a rotation amount and increment it before each rendering.

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