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help me understand rendering a quad down the z-axis


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#1 bradbobak   Members   -  Reputation: 1154

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Posted 26 November 2012 - 12:10 AM

I've used the code from nehe's lesson2 to build a simple app that demonstrates the problem I'm having.
The code is original, except I've removed the gluPerspective line, made it non-fullscreen and changed the drawGLScene() routine.

Here is the new drawGLScene() :
int drawGLScene(GLvoid)
{
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glLoadIdentity();
	glTranslatef(0, 0.0f, -0.5f);
glColor3f(1, 1, 1);
	glBegin(GL_QUADS);
		glVertex3f(-0.5f, 0.5f, 0.2f);
		glVertex3f(0.5f, 0.5f, 0.2f);
		glVertex3f(0.5f, -0.5f, 0.2f);
		glVertex3f(-0.5f, -0.5f, 0.2f);
	glEnd();
glColor3f(1, 0, 0);
	glBegin(GL_QUADS);
		glVertex3f(-0.5f, 0.5f, -0.2f);
		glVertex3f(0.5f, 0.5f, -0.2f);
		glVertex3f(0.5f, -0.5f, -0.2f);
		glVertex3f(-0.5f, -0.5f, -0.2f);
	glEnd();
	if (GLWin.doubleBuffered)
	{
		glXSwapBuffers(GLWin.dpy, GLWin.win);
	}
	return True;  
}

As you can see, I'm drawing 2 rectangles at the same position, except along the z-axis.
The first is colored white, and has its vectices for z at 0.2f. The second is red, and z is at -0.2f.
For some reason, the red quad appears in front of the white one. Seeing how opengl treats the negative z axis as going into the screen, I'm trying to understand why the white quad (z at 0.2f) isn't drawn in front of the red one.

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#2 Aks9   Members   -  Reputation: 919

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Posted 26 November 2012 - 01:44 AM

Because you probably didn't enable depth test. In that case OpenGL draws objects in the order you issue commands (like in 2D drawing).

#3 bradbobak   Members   -  Reputation: 1154

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Posted 26 November 2012 - 02:00 AM

Depth testing is enabled. Even if I switch the drawing order, the red quad is still up front.

#4 Olof Hedman   Crossbones+   -  Reputation: 2956

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Posted 26 November 2012 - 04:02 AM

If you use lesson 2, you set glDepthFunc(GL_LEQUAL);, and therefore, lesser depth values will be drawn over higher depth values.

#5 Brother Bob   Moderators   -  Reputation: 8631

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Posted 26 November 2012 - 04:06 AM

Two points:
  • The near and far clip plane values, as specified by the glOrtho or glFrustum (or equivalent functions) are along the negative Z-axis. That means a clip plane value of 5 means that the clip plane is located at z=-5.
  • If you leave out the projection matrix, the default matrix is the identity matrix which is equivalent to glOrtho(-1, 1, -1, 1, 1, -1).

Pay attention to the default clip plane values; near=1 and far=-1. This means that the near clip plane is located at z=-1, and the far clip plane is located at z=1. This means that a quad at z=-0.2 is close to the near clip plane than a quad at z=0.2, and thus your red quad occludes your white quad.

Edited by Brother Bob, 26 November 2012 - 04:07 AM.


#6 bradbobak   Members   -  Reputation: 1154

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Posted 26 November 2012 - 12:59 PM

Ok. I get the idea. Thanks for the help.




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