# Simple problem with gluOrtho2D

Hi,
I have some 3D data that is a set of triangles on the XZ plane all at a fixed height and I want to draw them using gluOrtho2D.

The following test code which has a triangle on the XZ plane at a fixed depth (-500) works. It draws a triangle touching the bottom corners and the middle of the top edge

``````
glMatrixMode (GL_PROJECTION);
gluOrtho2D(0, 400, 0, 400);

glMatrixMode (GL_MODELVIEW);

gluLookAt (0, -500, 0,
0.0, 0.0, 0.0,
0.0, 0.0, 1.0);

GLfloat white[] = { 1.0,1.0,1.0 };
glColor3fv(white);

glBegin(GL_TRIANGLES);
glVertex3d(0,  -500,  0);
glVertex3d(400,-500,  0);
glVertex3d(200,-500,400);
glEnd();

``````

However, if I make the triangle Y = 500 instead of -500 I can’t get the triangle to appear. I assumed the -500 in gluLookAt would be 500 (and maybe change the sign of the UP vector), but I’ve tried lots of parameter combinations (FYI GL_CULL_FACE is disabled).

I’m missing something obvious, but I can’t see it.

Thanks,

Stuart.

Everything is working correctly. The problem is that, when changing the sign of all -500 values, your triangle gets transformed outside of your viewing frustum.

Change the gluOrtho call to
gluOrtho2D(-400, 400, -400, 400);
to see what is happening.

N.

In your example code your up-vector is (0,0,0). This is undefined. Most people in computer graphics use (0,1,0) as the up-vector. Hence, z = (0,0,1) becomes depth, and the term z-buffering starts making a lot of sense.

I also note that you are not positioning the camera/eye at the origin. I find it very useful to do so before I have wrapped my head around the world coordinate system.

Be careful of where you have the near and far clipping planes. I have pointed out in my suggested code where they are for gluOrtho2D.

May I suggest you try the following:

``````
glMatrixMode (GL_PROJECTION);
// Note that by default gluOrtho2D sets near = -1 and far = 1,
// so your geometry (the triangle) has to be within these values.
//
gluOrtho2D(0, 400, 0, 400);

glMatrixMode (GL_MODELVIEW);

// These parameters will actually
gluLookAt (0.0, 0.0, 0.0,         // Camera at origin.
0.0, 1.0, 0.0,         // Y+ is up
0.0, 0.0, -1.0);       // Looking down negative z-axis.

GLfloat white[] = { 1.0,1.0,1.0 };
glColor3fv(white);

glBegin(GL_TRIANGLES);
// Move triangle out by 0.5 units, which is in the range [-1..1]
//
glVertex3d(0,     0, -0.5);
glVertex3d(400,   0, -0.5);
glVertex3d(200, 400, -0.5);
glEnd();

``````

@nico: D’oh! Thanks.

@thinks: Params 4,5,6 are the reference point and 7,8,9 is the up vector. So the vector is not undefined.