opengl - camera cannot see object when glOrtho is used - c++

Question

I'm new to OpenGL and I'm trying to understand how the projection matrix works in it.
To create a simple case, I define a triangle in the world space and its coordinates are:
(0,1,0), (1,0,0), (-1,0,0)
I set the modelview matrix and projection matrix as below:
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(
0, 0, 2,
0, 0, 0,
0, 1, 0);
glMatrixMode(GL_PROJECTION);
glOrtho(-2, 2, -2, 2, -0.1, -2.0); // does not work
// glOrtho(-2, 2, -2, 2, 0.1, 2.0); // works
From my understanding, gluLookAt() is used to set the viewing matrix. Since OpenGL does not have a concept of "camera", and thus it transforms the entire world to reach the effect of a camera. In the above code, I assume the "camera" is at (0,0,2), looking at (0,0,0). So OpenGL internally moves the triangle backwards along z axis to z=-2.
To define a view frustum, glOrtho() get 6 parameters. To make the triangle visible in the frustum, I set the near and far value to -0.1 and -2.0 respectively and this should indicate that the frustum include [-0.1, -2.0] on z axis.
I searched for similar questions and found out someone states that the last two parameters of glOrtho() is in fact -near and -far. But if this is correct, the following code should work(but it doesn't):
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(
0, 0, -2, // changed 2 to -2, thus the triangle should be transformed to z=2?
0, 0, 0,
0, 1, 0);
glMatrixMode(GL_PROJECTION);
glOrtho(-2, 2, -2, 2, -0.1, -2.0); // -near=-0.1, -far=-2.0, thus the frustum should include [0.1, 2.0], thus include the triangle
If I'm correct, the triangle should be drawn on the screen, so there must be something wrong with my code. Can anyone help?

Answer 1

First of all note, that the fixed function pipeline matrix stack and drawing by glBegin/glEnd sequences is deprecated since more than 10 years.
Read about Fixed Function Pipeline and see Vertex Specification for a state of the art way of rendering.
If you use a view matrix like this:
gluLookAt(0, 0, 2, 0, 0, 0, 0, 1, 0);
Then the values for the near and the far plane have to be positive when you set up the the projection matrix,
glOrtho(-2, 2, -2, 2, 0.1, 2.0);
because, gluLookAt transforms the vertices to view space (in view space the z axis points out of the viewport), but the projection matrix inverts the z-axis.
But be careful, since the triangle is at z=0
(0,1,0), (1,0,0), (-1,0,0)
and the distance from the camera to the triangle is 2, because of the view matrix, the triangle is placed exactly on the far plane (which is 2.0 too). I recommend to increase the distance to the far plane from 2.0 to (e.g.) 3.0:
glOrtho(-2, 2, -2, 2, 0.1, 3.0);
If you change the view matrix,
gluLookAt(0, 0, -2, 0, 0, 0, 0, 1, 0);
then still the (view space) z-axis points out of the viewport, but you look at the "back" side of the triangle. The triangle is still in the center of the view (0, 0, 0), but the camera position has changed. The triangle is still in front of the camera.
If you would do
gluLookAt(0, 0, 2, 0, 0, 4, 0, 1, 0);
then you would look away from the triangle. You would have to project the backside of the view to the viewport to "see" the triangle (glOrtho(-2, 2, -2, 2, -0.1, -3.0);).
Further note, that glOrtho multiplies the current matrix by the orthographic projection matrix. This means you should set the identity matrix, before you use glOrtho, as you do it with the model view matrix:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-2, 2, -2, 2, 0.1, 2.0);
Explanation
The projection, view and model matrix interact together to present the objects (meshes) of a scene on the viewport.
The model matrix defines the position orientation and scale of a single object (mesh) in the worldspace of the scene.
The view matrix defines the position and viewing direction of the observer (viewer) within the scene.
The projection matrix defines the area (volume) with respect to the observer (viewer) projected onto the viewport.
At orthographic projection, this area (volume) is defined by 6 distances (left, right, bottom, top, near and far) to the viewer's position.
View matrix
The view coordinates system describes the direction and position from which the scene is looked at. The view matrix transforms from the wolrd space to the view (eye) space.
If the coordiante system of the view space is a Right-handed system, then the X-axis points to the left, the Y-axis up and the Z-axis out of the view (Note in a right hand system the Z-Axis is the cross product of the X-Axis and the Y-Axis).
Projection matrix
The projection matrix describes the mapping from 3D points of the view on a scene, to 2D points on the viewport. It transforms from eye space to the clip space, and the coordinates in the clip space are transformed to the normalized device coordinates (NDC) by dividing with the w component of the clip coordinates. The NDC are in range (-1,-1,-1) to (1,1,1). Every geometry which is out of the clippspace is clipped.
At Orthographic Projection the coordinates in the view space are linearly mapped to clip space coordinates and the clip space coordinates are equal to the normalized device coordinates, because the w component is 1 (for a cartesian input coordinate).
The values for left, right, bottom, top, near and far define a box. All the geometry which is inside the volume of the box is "visible" on the viewport.
The Orthographic Projection Matrix looks like this:
r = right, l = left, b = bottom, t = top, n = near, f = far
2/(r-l) 0 0 0
0 2/(t-b) 0 0
0 0 -2/(f-n) 0
-(r+l)/(r-l) -(t+b)/(t-b) -(f+n)/(f-n) 1
The z-axis is inverted by the projection matrix.