Newbie questions!

EeeK · July 12, 2000, 5:20am

These are the two questions I have encountered so far:

when I wanted to draw a square using:

glBegin(GL_POLYGON); or glBegin(GL_QUADS);
glVertex3f(-5.0, -5.0, 0.0);
glVertex3f(5.0, -5.0, 0.0);
glVertex3f(-5.0, 5.0, 0.0);
glVertex3f(5.0, 5.0, 0.0);
glEnd();

the output is not a square but a polygon. Why?

I got myself the Redbook and most of the examples does not require function protoype. I cut and pasted and compiled it without error but when I tried to write out 1 without function prototype, my compiler give me errors. Why is that so?

Thanks for the reply!

[This message has been edited by EeeK (edited 07-12-2000).]

Relic · July 12, 2000, 6:17am

Watch out for the vertex winding order.
Your vertices are not building a convex polygon.
Try swapping the last two vertices to get counterclockwise orientation of the four vertices like this:

glBegin(GL_POLYGON); or glBegin(GL_QUADS);
glVertex3f(-5.0, -5.0, 0.0);
glVertex3f(5.0, -5.0, 0.0);
glVertex3f(5.0, 5.0, 0.0);
glVertex3f(-5.0, 5.0, 0.0);
glEnd();

Though with glBegin(GL_QUAD_STRIP) your order would have worked. Puzzled?

If you have
#include <GL/gl.h>
#include <GL/glu.h>
in you source files using OpenGL everything should compile fine.

"- I got myself the Redbook and most of the examples does not require function protoype. I cut and pasted and compiled it without error but when I tried to write out 1 without function prototype, my compiler give me errors. Why is that so?
"

Depending on the function you either have not included the file with the implementation into your project, not included the header and the lib containing the prototype and the implementation.
But that’s a wild guess wiothout knowing what the function and the progrma looks like.

Hope that helps.

[This message has been edited by Relic (edited 07-12-2000).]

EeeK · July 14, 2000, 1:48am

So you mean that in building a convex polygon, the vertices must be in the order of counter-clockwise? I tried and it work but how about with the z-coordinates?

glBegin(GL_POLYGON); or glBegin(GL_QUADS);
glVertex3f(-5.0, -5.0, 2.0);
glVertex3f(5.0, -5.0, 2.0);
glVertex3f(5.0, -5.0, -2.0);
glVertex3f(-5.0, -5.0, -2.0);
glEnd();

The above vertices are in counter-clockwise but the output is not a square. Why?

system · July 14, 2000, 8:39am

Probably… your verts are getting clipped out because they are to close…

Question are you using perspective or ortho viewing?

If you want a square… try this

void init()
{
glClearColor(0.0,0.0,0.0,0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0,1.0,0.0,1.0,-1.0,1.0);
}

and

void display()
{

	glClear (GL_COLOR_BUFFER_BIT); 		//clear the screen with black

	glColor3f(.5,.75,0.3);			//change to a green color
	glBegin(GL_TRIANGLES);			//begins the declaration of a triangle
		//vertices
		glVertex3f(.3,.5,0.0);
		glVertex3f(.7, .5, 0.0);
		glVertex3f(.5, .25, 0.0);
	glEnd();				//end declaration

	glColor3f(1.0, 1.0, 0);			//change to yellow
	glBegin(GL_POLYGON);			//declare a polygon
		//define verts
		glVertex3f(0.15, .4,0.0);
		glVertex3f(0.1, .3,0.0);
		glVertex3f(0.2, .2,0.0);
		glVertex3f(0.25, .2, 0.0);
		glVertex3f(0.3,.1,0.0);

	glEnd();				//end declaration

	glColor3f(1.0, 0, 0);			//change to red
	glBegin(GL_POLYGON);			//declare new poly
		//define verts
		glVertex3f(0.45, 0.9, 0.0);
		glVertex3f(0.35, 0.8,0.0);
		glVertex3f(0.35, 0.7, 0.0);
		glVertex3f(0.45,0.6,0.0);
		glVertex3f(0.55,0.6,0.0);
		glVertex3f(0.65,0.7,0.0);
		glVertex3f(0.65, 0.8,0.0);
		glVertex3f(0.55,0.9, 0.0);

	glEnd();				//end declaration
	glFlush();				//perform all buffered gl instruction
						//paint all polys

}

system · July 14, 2000, 8:45am

poop im sorry just use this

void init()
{
glClearColor(0.0,0.0,0.0,0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0,1.0,0.0,1.0,-1.0,1.0);
glMatrixMode(GL_MODELVIEW);
}

//this function paints the window
void display()
{

	glClear (GL_COLOR_BUFFER_BIT); 		//clear the screen with black

	glColor3f(.5,.75,0.3);			//change to a green color
	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();
glFlush();

}

imported_Shadow · July 14, 2000, 8:30pm

Eeek, your second example is a square but
the output is what you see in real life.

its smaller when its far away

Relic · July 15, 2000, 12:27pm

“So you mean that in building a convex polygon, the vertices must be in the order of counter-clockwise?”

To be precise, it actually does not matter if the orientatoin is clockwise or counter-clockwise as long as the polygon is convex. You can select the winding order in OpenGL with glFrontFace() and default is GL_CCW (counter-clockwise).
You’ll find OpenGL easier to learn if you keep some defaults.

EeeK · July 19, 2000, 2:55am

Ok, here is part of my source for creating my first 3D application:

void init(void)
{
glClearColor(0.0, 0.0, 0.0, 0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-10.0, 10.0, -10.0, 10.0, -10.0, 10.0);
}
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBegin(GL_TRIANGLES);
glColor3f(1.0, 0.0, 0.0); //red
glVertex3f(-6.0, -6.0, 6.0); //left
glColor3f(0.0, 1.0, 0.0); //green
glVertex3f(6.0, -6.0, 6.0); //right
glColor3f(0.0, 0.0, 1.0); //blue
glVertex3f(0.0, 6.0, 0.0); //top

	glColor3f(0.0, 1.0, 0.0); //green
	glVertex3f(-6.0, -6.0, -6.0);
	glColor3f(1.0, 0.0, 0.0); //red
	glVertex3f(6.0, -6.0, -6.0);
	glColor3f(0.0, 0.0, 1.0); //blue
	glVertex3f(0.0, 6.0, 0.0);
			
	glColor3f(1.0, 0.0, 0.0); //red
	glVertex3f(-6.0, -6.0, 6.0);
	glColor3f(0.0, 1.0, 0.0); //green
	glVertex3f(-6.0, -6.0, -6.0);
	glColor3f(0.0, 0.0, 1.0); //blue
	glVertex3f(0.0, 6.0, 0.0);

	glColor3f(0.0, 1.0, 0.0); //green
	glVertex3f(6.0, -6.0, 6.0); //left
	glColor3f(1.0, 0.0, 0.0); //red
	glVertex3f(6.0, -6.0, -6.0); //right
	glColor3f(0.0, 0.0, 1.0); //blue
	glVertex3f(0.0, 6.0, 0.0); //top
glEnd();
glBegin(GL_QUADS);
	glVertex3f(-6.0, -6.0, -6.0);
	glVertex3f(-6.0, -6.0, 6.0);
	glVertex3f(6.0, -6.0, 6.0);
	glVertex3f(6.0, -6.0, -6.0);
glEnd();
glutSwapBuffers();

}

Why does color of the last polygon always override the color even it is at the back.
Please point out what mistakes I have made, thanks!

[This message has been edited by EeeK (edited 07-19-2000).]

Relic · July 19, 2000, 7:40am

You mean why is the color of the quad always the same color as the last triangle vertex (here blue)?

This is by design. OpenGL is a state machine.
All commands changing one of the current states stay valid until the next change occurs. In your case the last time you set the color to blue, this color will be taken for all vertices thereafter until you issue the next color command.
Put another glColor call before the glBegin(GL_QUADS) and you’ll see.

EeeK · July 19, 2000, 8:06pm

I found out what is wrong, I did not put the glEnable(GL_DEPTH_TEST); in the init().

Thanks for you reply thought.