glOrtho − multiply the current matrix with an orthographic matrix
void glOrtho( GLdouble left,
GLdouble right, 

GLdouble bottom, 

GLdouble top, 

GLdouble near_val, 

GLdouble far_val ) 
left, right 
Specify the coordinates for the left and right vertical clipping planes. 

bottom, top 
Specify the coordinates for the bottom and top horizontal clipping planes. 
near_val, far_val
Specify the distances to the nearer and farther depth clipping planes. These values are negative if the plane is to be behind the viewer.
glOrtho describes a transformation that produces a parallel projection. The current matrix (see glMatrixMode) is multiplied by this matrix and the result replaces the current matrix, as if glMultMatrix were called with the following matrix as its argument:
where
Typically, the matrix mode is GL_PROJECTION, and (left, bottom, −near_val) and (right, top, −near_val) specify the points on the near clipping plane that are mapped to the lower left and upper right corners of the window, respectively, assuming that the eye is located at (0, 0, 0). −far_val specifies the location of the far clipping plane. Both near_val and far_val can be either positive or negative.
Use glPushMatrix and glPopMatrix to save and restore the current matrix stack.
GL_INVALID_OPERATION is generated if glOrtho is executed between the execution of glBegin and the corresponding execution of glEnd.
glGet with argument GL_MATRIX_MODE
glGet with argument GL_COLOR_MATRIX
glGet with argument GL_MODELVIEW_MATRIX
glGet with argument GL_PROJECTION_MATRIX
glGet with argument GL_TEXTURE_MATRIX
glFrustum(3G), glMatrixMode(3G), glMultMatrix(3G), glPushMatrix(3G), glViewport(3G)