2D LWJGL with OpenGL - Enabling depth testing for layering 2D textures?

Question

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.

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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();

}

Answer 1

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.

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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...

Answer 2

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.