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What would I need to add to my OpenGL init method to enable depth testing, and how would I actually use it for texture layering?

I would have to extend the last parameter of glOrtho to something more extreme than -1, and of course glEnable depth testing. Then to use it, I can only assume that I change the third parameter of glVertex to something that isn't 0 to send it in front / behind of other textures.

I try this, and the damn textures don't even show. xD I must be missing something.

EDIT: RE: Tim's response

whenever i made the image's z more extreme than -1 it didnt show the screen was just black. 

void initGL(){

GL11.glEnable(GL11.GL_TEXTURE_2D);
GL11.glEnable(GL11.GL_DEPTH_TEST); //depth test enabled

    GL11.glMatrixMode(GL11.GL_PROJECTION);
GL11.glOrtho(-width/2, width/2, -height/2, height/2, 1, -10);//far changed to -10
    GL11.glMatrixMode(GL11.GL_MODELVIEW);

}

and

void loadBG(int theLoadedOne){

GL11.glBindTexture(GL11.GL_TEXTURE_2D, theLoadedOne);
GL11.glBegin(GL11.GL_QUADS);
    GL11.glTexCoord2f(0,0);
GL11.glVertex3f(-width/2,height/2, -2);//new z value

    GL11.glTexCoord2f(1,0);
GL11.glVertex3f(width/2,height/2,-2);//new z value

    GL11.glTexCoord2f(1,1);
GL11.glVertex3f(width/2,-height/2,-2);//new z value

    GL11.glTexCoord2f(0,1);
GL11.glVertex3f(-width/2,-height/2,-2);//new z value

GL11.glEnd();
GL11.glFlush();

}

and

while(!Display.isCloseRequested()){
        GL11.glClear(GL11.GL_COLOR_BUFFER_BIT | GL11.GL_DEPTH_BUFFER_BIT);

...

            for(int i=0;i<1;i++){  //dont mind this for loop
                bg.loadThisBG(0);  //its here for reasons
            }
            updateFPS();

        Display.update();

    } Display.destroy();

}
GlassZee
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  • I don't understand. How does texture layering affect depth testing? Why do you have to extend the projected volume? Ah, you use multiple quads shifted in `z` direction, combined with blending? So you're not using shaders? Please elaborate/rephrase your question. – Stefan Hanke Apr 02 '12 at 18:09
  • Yes what you said. Using z to layer them in the view so that some will be more behind others. – GlassZee Apr 03 '12 at 00:25
  • Can you show the code to do this? With this kind of multitexturing, you need to order the quads from back to front to make the blending work. You use screen-aligned quads? – Stefan Hanke Apr 03 '12 at 05:10
  • i dont know about why I need any blending. i'm not sure what screen-aligned means; each quad has its own coordinates – GlassZee Apr 04 '12 at 20:36

2 Answers2

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Seems like you switched near and far plane. Have a look at gluOrtho2D. It just calls glOrtho with near=-1 and far=+1, resulting in the z coordinates switching sign (m33=-2/(far-near)). However, with the values given above, m33=-2/(-10-1) is positive, and the z axis reversed to standard workflow.

This consequences in the quad being viewed from the back.

OpenGL matrix manipulation methods do no care what you feed them; except when values would led to a division by zero.

Assuming there is no modelview transform, and only the one matrix contributing to the projection one, here is what I think is happening:

The z value transform from world to NDC space is z_ndc = -9/11 * z_w + 2/11 (set near and far into the orthographic matrix and take the third row). Now, z_w=-2, and so z_ndc = 20/11. This is out of the NDC space boundaries and thrown away.

Well, I assume that this test is implicitly enabled/disabled with the Z test itself. Next suspect would be backface culling...

Stefan Hanke
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  • are you insisting i should use gluOrtho2D instead and then try it with adding depth tests? – GlassZee Apr 04 '12 at 20:37
  • No, I just pointed you to `gluOrtho2D` to show how it calls `glOrtho`. Change the last two parameters to something like `near=-10` and `far=+10`. After reenabling the depth test, the quad should now be visible. – Stefan Hanke Apr 04 '12 at 21:15
  • thanks man, it worked. the problem i'm having now is, when i put a texture w/ transparency over another texture, the second texture isn't showing under where the transparency should be. http://i.imgur.com/sTNU8.png?1 Here is a quick demonstration I did with transparent text over a blue textured square. The text's z is 0 and the square's z is -9. How do I make it so that textures show behind other textures' transparency? – GlassZee Apr 06 '12 at 07:36
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    You need to setup _blending_. Look for `glBlendFunc`; [here's](http://www.andersriggelsen.dk/glblendfunc.php) a nice playground. – Stefan Hanke Apr 06 '12 at 08:56
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Provided your context includes a depth buffer (not sure about lwjgl buffer creation...)

All you need should be:

  • Call glEnable(GL_DEPTH_TEST) during initialization
  • Add depth buffer bit to glClear glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  • Define z coordinate to be between near and far values of orthographic matrix.
Tim
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  • i tried what you said and the background was just black. more details in the original post. – GlassZee Apr 03 '12 at 22:13
  • @GlassZee: does this code actually work if you don't use depth testing? For one thing you need to call glLoadIdentity every time before you call glOrtho (it multiplies orthographic matrix with what's currently on the stack). Also I'm not 100% positive but I believe that the value for near must be less than the value of far (currently you have near > far), which might be screwing things up. – Tim Apr 03 '12 at 22:25
  • @Tim oh well, just writing up a nice answer for this, _then_ reading your comment... – Stefan Hanke Apr 04 '12 at 04:20
  • yes it does work without the depth testing, but apparently the textures are being viewed from the back. I'll try to fix everything up and then try what you said again. I'm actually getting on a plane today so I won't be able to do it until later. – GlassZee Apr 04 '12 at 20:28