Newbie question: clipping volume/viewport

[Brush]

[note: english is not my first language, so please excuse me if I'm not using the correct terms for certain concepts]

I'm trying to learn openGL using LWJGL and an old book I managed to pick up cheap. Now if I understood correctly the clipping volume determins your bounderies of your coordinate system( min-X, max-X, etc.) and the viewport maps those coordinates to screencoordinates.

In the book they touch on the subject of simple animation using openGL.
Now when I use the following code, it won't draw anything on the screen, yet if i remove the call to glOrtho, I can see the animation just fine.
These are the relevant methods:

private int xpos=0;
	private int ypos=0;
	private int xStep=1;
	private int yStep=1;
	private int size=100;
	public void mainLoop()
	{
		if(Keyboard.isKeyDown(Keyboard.KEY_ESCAPE))
			done=true;
		if(Display.isCloseRequested())
			done=true;
			
		if(xpos>640-size||xpos<0)
			xStep=-xStep;
		if(ypos>480-size||ypos<0)
			yStep=-yStep;
		xpos+=xStep;
		ypos+=yStep;
	}

	private int viewPortX=640;
	private int viewPortY=480;
	public void initGL()
	{
	    GL11.glClearColor(0.0f, 0.0f, 1.0f, 1.0f); 
	    
	    GL11.glViewport(0,0,640,480);
	     
	    //reset coordinatesystem
	    GL11.glLoadIdentity();
	    //			min-X, max-X,  min-Y,max-Y, Z-min,Z-max 
	    //set the clippnig volume
	    
	    GL11.glOrtho(0,640,480,0,-1.0,1.0);
	    
	}
	
	public boolean render()
	{
	    GL11.glClear(GL11.GL_COLOR_BUFFER_BIT);
	    //				R   G   B
	    GL11.glColor3f(1.0f,0.0f,0.0f);
	    //			upperleft			lowerright
	    GL11.glRectf(xpos,ypos, xpos-size,ypos-size);
	    GL11.glFlush();
	    return true;
	}

I tried with different viewport sizes, but it doesn't work even if I pick the same size for the clipping volume and the viewport.
I realise LWJGL is set up for 2D drawing by default, but I just want to know why this doesn't work, when it does (or is supposed to) work in the book. It's probably some fundamental viewport/clipping concept that I'm misunderstanding.

[Orangy Tang]

A random guesstimate - I don't see you chosing your active matrix anywhere before you set up your ortho projection. If I remember right, it defaults to the modelview matrix, so LWJGL defaults your projection matrix for 2d, *then* you set the modelview up for 2d. I have no idea what the combined result is likely to do but it sure ain't gonna be what you want.

Try adding a GL11.glMatrixMode(GL11.gl_PROJECTION) in front of your ortho call.

[Brush]

Thank you for your speedy reply, but that didn't solve it.
Translating this program from the redbook to LWJGL, has the same problem: it simply doesn't draw anything to the screen.

#include <whateverYouNeed.h>

main() {

   OpenAWindowPlease();

   glClearColor(0.0, 0.0, 0.0, 0.0);
   glClear(GL_COLOR_BUFFER_BIT);
   glColor3f(1.0, 1.0, 1.0);
   glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); 
   glBegin(GL_POLYGON);
      glVertex2f(-0.5, -0.5);
      glVertex2f(-0.5, 0.5);
      glVertex2f(0.5, 0.5);
      glVertex2f(0.5, -0.5);
   glEnd();
   glFlush();

   KeepTheWindowOnTheScreenForAWhile();
}

Is LWJGL doing something funky behind my (our) back(s)?

[edit]
It seems that putting the GL11.glMatrixMode(GL11.GL_PROJECTION); before the loadIdentity() call, does make it work

[cfmdobbie]

Yeah, the problem as you've doubtless now discovered is that you're trying to manipulate two different matrix stacks here.

The glOrtho call should be called on the GL_PROJECTION matrix (usually on an identity matrix), while your drawing commands should be called on the GL_MODELVIEW matrix.

// Setup code - only call this once!
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho( ... );
glMatrixMode(GL_MODELVIEW); // Remember to change matrix back!

// Drawing code - call once per frame
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();

glTranslatef( ... );
glBegin(GL_QUADS);
// etc

Note that the glLoadIdentity() call in the drawing code will only affect the current matrix - GL_MODELVIEW in this instance.  The GL_PROJECTION matrix can be initialised once and then left, and most applications will do exactly this.


A simple rule - when it comes to system initialisation, LWJGL does a few funky things to put the system in as usuable a state as possible.  It sets up a simple 1:1 world-to-pixel coordinates projection, sets a few state flags etc.  However, when the program is in full swing, LWJGL stays right out of the way and leaves you to it.