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Darragh

OpenGL Display Lists refusing to work...

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Grrrr... All I can say is that display lists are driving me mad! I'm just after making some big changes to my 3D engine (see this thread for screenshots) at the moment that will hopefully enable me to load and switch between different levels while the program is running. Before, basically whatever level was loaded when the game started couldn't be changed. Display lists were created for each of the rooms/areas in the map and were kept until the game exited. Now I want a system whereby display lists for each level can be created and deleted as necessary- thus allowing level changes. Trouble is, this just isn't working.. Ok, so to give you an idea of the problem here is what happens when I run the game: 1 - The game loads up and a menu is displayed showing 'new game', 'exit' , 'options' etc... 2 - When the actual OpenGL initialisation method is run, it creates two display lists. One display list is for setting up the 3D view, the other is for setting up the 2D view. These display lists are hard coded into the game and are not subject to change. I have had no problems creating or using these two particular lists- so everythings fine there.. 3 - When the 'new game' option is activated the level is loaded from a file. The level geometry is constructed. 4- After constructing the level, the geometry for each room is placed in its own separte display list. However, the glGenLists() command does not seem to be working correctly. No matter how many times I query it, it always returns '0' as an index for the next free display list. '0' of course is an invalid index for a display list. Here is the actual code for compiling display lists for each room in the map:
[source lang = "java"]
import net.java.games.jogl.GL;

class DisplayList_map
{

private static int 	current_quad;	//Current quad number
private static int 	current_tex;	//Current texture number
private static byte current_shade;	//Current quad shade
private static Quad quad;		//Current quad data	

static void compile(GL myGL)
{
///////////////////////////////////////////
// PRECOMPILED DRAWING LIST FOR THE MAP  //
///////////////////////////////////////////

GL_Drawing.LISTS_MAP_AREAS = new int[MapAreas.num_areas];

int current_tex = 0;

for ( int x = 1 ; x <= MapAreas.num_areas ; x ++ )
	{	
	
	GL_Drawing.LISTS_MAP_AREAS[x-1] = myGL.glGenLists(1);
		
	// THE DISPLAY LIST INDEX RETURNED IS ALWAYS 0! - MEANING ITS INVALID
	System.out.println("Map Area" + x + " list=" + GL_Drawing.LISTS_MAP_AREAS[x-1] );

	myGL.glNewList(GL_Drawing.LISTS_MAP_AREAS[x-1],GL.GL_COMPILE);
	
	myGL.glEnable(GL.GL_TEXTURE_2D);
	
	myGL.glBegin(GL_Drawing.display_mode);
		
	///////////////////////////////
	// MAIN POLYGON DRAWING LOOP //
	///////////////////////////////
	
	// ( CODE DELETED ) //

	myGL.glEnd();
	myGL.glEndList();

	}

	
	
}



}





Now this code never actually generates any display lists since the index is always '0' and invalid. One solution you might say would be to manage the index of display lists manually, rather than using glGenLists()... Well I have tried this, and even though the correct indexes are generated when using the manual method, I still see no geometry upon calling these display lists... So it seems that any display list that isn't created during the OpenGL initialisation will not work for me. I have absolutely no idea why this is happening. Is it even possible to create or delete display lists while the program is running? I thought it was, but yet this continously refuses to work for me.. I am using the JOGL bindings for java, so maybe this is part of the problem. Has anyone got any ideas about how I might solve this problem ? The game isn't going to be much good if I have to reload the entire program every time I need to change a level! Thanks in advance for your help, Darragh

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Hi, need to know a few things to help out

1. What is the code returned by glGetError() immediatly after the glGenLists call?
2. Is the GL context valid when you call compile?
3. Where in your code do you call compile?
4. Whats your hardware specs?
5. What is the index of your last valid DL returned by glGenLists?

cheers
-Danu

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Hi Silvermace, thanks for responding.

Luckily, I managed to stumble across the solution last night and the display lists are now compiling fine. I created a function call within the OpenGL display method to compile the display lists when needed, and bingo- it suddenly worked! Now my game is able to delete and load new levels without any trouble.

Now once I solved this, I also had a similar problem creating textures for the new level. Once again I done the same thing and executed the the texture creation code from the display method.. and you guessed it.. solved also!

Well my conclusions from this: You can only apply state changes to OpenGL either from the initilisation or the display methods, directly or through further sub method calls. The application cannot apply changes to the GL state whenever it pleases- only when these two methods are actually active.

I doubt this will be startling news to most here [smile]- but its certainly news to me. I didn't see any mention of this whilst looking through the red book..

Phantom, I think this would be a helpful piece of information to put into the FAQ. It may well save others hours of needless frustration pondering over their code..

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that is definatly wierd, and I wouldnt expect that behaviour in my C++ engine.
It actually sounds like you are making GL calls outside a valid context..

it should something like this:

CreateGLContext();
LoadLevel();
while( !done )
{
Render();
if( loadNewLevel ) {
ChangeLevel();
}
}
DestroyGLContext();
ExitApp();
in JOGL, is there a special function you have to call just
before rendering or something that looks like it makes a context current?

Cheers
-Danu

EDIT: from the JOGL users guide
Quote:
Writing a GLEventListener:
Applications implement the GLEventListener interface to perform OpenGL drawing. When the methods of the GLEventListener are called, the underlying OpenGL context associated with the drawable is already current. The listener fetches the GL object out of the GLDrawable and begins to perform rendering.
It looks like JOGL doesn't guarantee a valid context outside of a GLEventListener trigger... thats a bit un-expected :)

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Quote:
Original post by silvermace

EDIT: from the JOGL users guide
Quote:
Writing a GLEventListener:
Applications implement the GLEventListener interface to perform OpenGL drawing. When the methods of the GLEventListener are called, the underlying OpenGL context associated with the drawable is already current. The listener fetches the GL object out of the GLDrawable and begins to perform rendering.
It looks like JOGL doesn't guarantee a valid context outside of a GLEventListener trigger... thats a bit un-expected :)


Certainly is. That would explain also why the red book makes no mention of this- seems to be a JOGL specific problem.

Oh well, you learn the hard way! [smile]

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