Coordinates in openGL - opengl

Question

I am having trouble finding the right coordinates in openGL.
For e.g.: - If the h and w are height and width of the window, then I want to draw a line of length w/2 at a distance h/4 from bottom. How would I do this in openGL?
I don't find any references telling the maximum and minimum values of coordinates.
My computer screen is 1024*768 so technically the limit should be:-
x coordinate: -512 to 512
y coordinate: -384 to 384
z coordinate: -inf to 0
But this doesn't work. Why? I need to know how coordinate system is working for openGL.

Answer 1

In OpenGL you can redefine the coordinate system to whatever you need. The default coordinate system is defined by a identity transform from model space to clip space identity transformed to NDC space. What this means is, that the xy coordinate range [-1,1]² maps to the viewport you set with glViewport. However by applying the right transformations you can alter that mappings to whatever you want, or need.
So I strongly suggest you read some tutorial on the OpenGL transformation pipeline and how to use it.
Fixed Function pipeline approach http://www.opengl.org/wiki/Vertex_Transformation
And the modern approach http://arcsynthesis.org/gltut/Positioning/Tut03%20A%20Better%20Way.html

Answer 2

I was having a difficult time with the generic coordinate system at first too.
What I wound up doing which makes more sense to me is working with OpenGL coordinate systems as if I am working with real objects in a real world coordinate system.
What I did was: I took a blueprint of a TARDIS (From the TV Show Doctor Who), which had dimensions for building the blue box in inches and feet.
From there, I took the GL coordinate system, and for every "1" GL unit, I made that equivalent to "1" foot, or 12 inches.
And - based on 0,0,0 being the center point of my TARDIS, i just hand drew the TARDIS through code, as I saw on the blueprint, translating the precise dimensions from what i saw on the blueprint.
Here's a SMALL example of what i did:
glBegin( GL_QUADS );
glNormal3f( 1.0f, 0.0f, 0.0f);
glTexCoord2f(0.0f, 0.0f); glVertex3f(15.0f/12.0f, myTop, 2.0f-(ONEINCH*0.25f));
glTexCoord2f(0.0f, 1.0f); glVertex3f(3.25f/12.0f, myTop, 2.0f-(ONEINCH*0.25f));
glTexCoord2f(1.0f, 1.0f); glVertex3f(3.25f/12.0f, myTop-(14.5f/12.0f), 2.0f - (ONEINCH*0.25f));
glTexCoord2f(1.0f, 0.0f); glVertex3f(15.0f/12.0f, myTop-(14.5f/12.0f), 2.0f - (ONEINCH*0.25f));
glEnd();
The first thing I learned in this exercise is - GL units are generic units. So applying a system I was more familiar with - feet and inches - made it SO much easier to focus on GL rather than what the heck does this unit mean?
Once I started the drawing, I was able to work with gluPerspective much more effectively, which helped me understand the viewport resolution (not to be confused with screen resolution) should ONLY have to be dealt with once, and that's when leveraging the gluPerspective command (as follows)
gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,0.1f,250.0f);
So to answer your question: There's no maximums for the floating point values, only levels of accuracy of the float you're dealing with.
For instance, in my perspective example above, i put the viewport extremely close (0.1 GL Unit, which is 1.2 inches), and set the distance to 250 GL uints, or 250 feet in the distance.
I think that's in part what was intended with OpenGL to begin with. It's generic units because three dimensional design makes so much more sense if you have a real world unit system to measure it again.
My advice is: Do not think of the GL units as having bounds or limitations other than degrees of error associated with the floating point inaccuracies at small and large scales.
In fact, I advise comparing it to real world units. I work in Feet and Inches. Metric system and meters are quite common. If those don't work for you, make one up that does.
Here's the TARDIS I built:
https://universalbri.wordpress.com/2015/05/24/creators-journal-holodeck-management-system-progress-10/
I am working on a game with all of this, a Star Trek Online without combat and now doing database work (SQL Server), the OpenGL was part of the question: Can I roll my own engine rather than leverage someone else's.
The answer is Yes, I can, and in fact it's preferable.
Good luck