OpenGL general question

scaraba · 11-23-2009, 01:36 AM

Hi, I am learning OpenGL and I am trying to make an object I created cross a 2D grid. Using glTranslate I have figured out how to move the entire image, but how would I say move my object ( Which is a GL quad)?

Code:

for(int i=-0; i<10;i+=10)
	{
		glBegin(GL_QUADS);
		glVertex2f(-5.0f,i);
		glVertex2f(i,5.0f);
		glVertex2f(5.0f,i);
		glVertex2f(i,-5.0f);
		glEnd();
	}


	for(int i=-500; i<500; i+=30)
	{
	glBegin(GL_LINES);
	glVertex2f(600.0f,i); //much simpler than all of that .........
	glVertex2f(-600.0f,i);
	glVertex2f(i,600.0f);
	glVertex2f(i,-600.0f);
	glEnd();
	}
	drawFont("Test Blue");

Mod Edit: Added code tags.

EVOEx · 11-23-2009, 05:08 AM

Try:

Code:

glPushMatrix();
glTranslate(...);
// Render your triangle here
glPopMatrix();

That ought to work...

MK27 · 11-23-2009, 09:11 AM

Fundamental to the possibility of motion is an "event loop" -- that is, a loop that continuously redraws the screen/window. OpenGL does not do that by itself.

However, if you are using glut or SDL you probably are using an event loop if you have been following some kind of tutorial. If you are not sure, ask.

With glut, you have your scene drawing function registered like this, then you initialize the main event loop:

Code:

glutDisplayFunc(drawScene);
[...]
glutMainLoop();

You must also include this:

Code:

glutPostRedisplay();

at the end of drawScene() (or whatever you called it).

Then you just use the technique EVOEx mentioned. As is, your quad is centered on the origin. So, working with that example:

Code:

static float square[3] = {0.0f};
glTranslatef(square[0],square[1],square[2]);
glBegin(GL_QUADS);
    for(int i=-0; i<10;i+=10) {
        glVertex2f(-5.0f,i);
        glVertex2f(i,5.0f);
        glVertex2f(5.0f,i);
        glVertex2f(i,-5.0f);
    }
glEnd();
square[0]++;
square[1]++;

This will cause the square to move diagonally right. Using the z-axis in 2D effectively scales the shape (as if it were receeding), I think...

Note the scene drawing routine registered by glut does not accept parameters, so you must use either "static" variables or globals. Also noticed I put the for loop inside glEnd/Begin, which is a little more efficient.

11-23-2009, 01:36 AM	#1
scaraba Registered User Join Date: Nov 2009 Posts: 1	OpenGL general question Hi, I am learning OpenGL and I am trying to make an object I created cross a 2D grid. Using glTranslate I have figured out how to move the entire image, but how would I say move my object ( Which is a GL quad)? Code: for(int i=-0; i<10;i+=10) { glBegin(GL_QUADS); glVertex2f(-5.0f,i); glVertex2f(i,5.0f); glVertex2f(5.0f,i); glVertex2f(i,-5.0f); glEnd(); } for(int i=-500; i<500; i+=30) { glBegin(GL_LINES); glVertex2f(600.0f,i); //much simpler than all of that ......... glVertex2f(-600.0f,i); glVertex2f(i,600.0f); glVertex2f(i,-600.0f); glEnd(); } drawFont("Test Blue"); Mod Edit: Added code tags. Last edited by Bubba; 11-24-2009 at 12:13 AM. Reason: Edit: Here's my code. I also have keyboard commands rigged to switch statements under their keyboard function method.
scaraba is offline

11-23-2009, 05:08 AM	#2
EVOEx Registered User Join Date: Oct 2008 Posts: 983	Try: Code: glPushMatrix(); glTranslate(...); // Render your triangle here glPopMatrix(); That ought to work...
EVOEx is offline

11-23-2009, 09:11 AM	#3
MK27 dat is, vast staat Join Date: Jul 2008 Location: SE Queens Posts: 6,612	Fundamental to the possibility of motion is an "event loop" -- that is, a loop that continuously redraws the screen/window. OpenGL does not do that by itself. However, if you are using glut or SDL you probably are using an event loop if you have been following some kind of tutorial. If you are not sure, ask. With glut, you have your scene drawing function registered like this, then you initialize the main event loop: Code: glutDisplayFunc(drawScene); [...] glutMainLoop(); You must also include this: Code: glutPostRedisplay(); at the end of drawScene() (or whatever you called it). Then you just use the technique EVOEx mentioned. As is, your quad is centered on the origin. So, working with that example: Code: static float square[3] = {0.0f}; glTranslatef(square[0],square[1],square[2]); glBegin(GL_QUADS); for(int i=-0; i<10;i+=10) { glVertex2f(-5.0f,i); glVertex2f(i,5.0f); glVertex2f(5.0f,i); glVertex2f(i,-5.0f); } glEnd(); square[0]++; square[1]++; This will cause the square to move diagonally right. Using the z-axis in 2D effectively scales the shape (as if it were receeding), I think... Note the scene drawing routine registered by glut does not accept parameters, so you must use either "static" variables or globals. Also noticed I put the for loop inside glEnd/Begin, which is a little more efficient. __________________ C programming resources: GNU C Function and Macro Index -- glibc reference manual The C Book -- nice online learner guide Current ISO draft standard CCAN -- new CPAN like open source library repository GDB tutorial #1 -- gnu debugger tutorials -- GDB tutorial #2 cpwiki -- our wiki on sourceforge Last edited by MK27; 11-23-2009 at 09:13 AM.
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