glOrtho, zNear and zFar

Can anyone explain what the zNear and zFar parameters in glOrtho should be? I am developing a 3D modeling environment, so models may not always be centered about the origin. When I frame the view to show the entire model, I redefine the projection with glOrtho using the center of the data and the radius of the bounding sphere so that my glOrtho parameters become:

left = centerX - radius
right = centerX + radius
top = centerY + radius
bottom = centerY - radius
near = centerZ + radius
far = centerZ - radius

However, when I send these parameters into glOrtho portions of the model get clipped as I rotate about the center of the data. I’ve had to increase the radius by a factor of 5 to keep this from happening, but then the precission of the depth buffer goes down more than it should. Should I be defining the near and far parameters differently?

What is seen on the screen is also dependent on the position of your camera. What u have suggested seems to be correct. Also u could try placing your camera a bit far away so that u have the entire scene in your view frustum.

I guess you translate the camera in order to see the object in front of you…

then the znear/zfar must be translated as well, if you move 10 steps back you should move the planes 10 steps forward.

Yes, the camera has been translated and rotated with gluLookAt to see the scene. I define my clipping planes in terms of world coordinates. Should I be using eye coordinates instead?