I can't understand the usage of glOrtho. Can someone explain what it is used for?
Is it used to set the range of x y and z coordinates limit?
glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
It means that the x, y and z range is from -1 to 1?
Answer
Have a look at this picture: Graphical Projections
The glOrtho command produces an "Oblique" projection that you see in the bottom row. No matter how far away vertexes are in the z direction, they will not recede into the distance.
I use glOrtho every time I need to do 2D graphics in OpenGL (such as health bars, menus etc) using the following code every time the window is resized:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0f, windowWidth, windowHeight, 0.0f, 0.0f, 1.0f);
This will remap the OpenGL coordinates into the equivalent pixel values (X going from 0 to windowWidth and Y going from 0 to windowHeight). Note that I've flipped the Y values because OpenGL coordinates start from the bottom left corner of the window. So by flipping, I get a more conventional (0,0) starting at the top left corner of the window rather.
Note that the Z values are clipped from 0 to 1. So be careful when you specify a Z value for your vertex's position, it will be clipped if it falls outside that range. Otherwise if it's inside that range, it will appear to have no effect on the position except for Z tests.