How can you draw primitives in OpenGL interactively?

Answer 1

I would probably do it by simply having some kind of data structure (array, linked list, whatever) holding the current "scene". Initially this is empty. Then when the event occurs, you create some kind of representation of the new desired geometry, and add that to the list.

On each frame, you clear the screen, and go through the data structure, mapping each representation into a suitble set of OpenGL commands. This is really standard.

The data structure is often referred to as a scene graph, it is often in the form of a tree or graph, where geometry can have child-geometries and so on.

Answer 2

If you're using the GLuT library (which is pretty standard), you can take advantage of its automatic primitive generation functions, like glutSolidSphere. You can find the API docs here. Take a look at section 11, 'Geometric Object Rendering'.

As unwind suggested, your program could keep some sort of list, but of the parameters for each primitive, rather than the actual geometry. In the case of the sphere, this would be position/radius/slices. You can then use the GLuT functions to easily draw the objects. Obviously this limits you to what GLuT can draw, but that's usually fine for simple cases.

Answer 3

Without some more details of what environment you are using it's difficult to be specific, but a few of pointers to things that can easily go wrong when setting up OpenGL

1. Make sure you have the camera set up to look at point you are drawing the sphere. This can be surprisingly hard, and the simplest approach is to implement glutLookAt from the OpenGL Utility Toolkit. Make sure you front and back planes are set to sensible values.

2. Turn off backface culling, at least to start with. Sure with production code backface culling gives you a quick performance gain, but it's remarkably easy to set up normals incorrectly on an object and not see it because you're looking at the invisible face

3. Remember to call glFlush to make sure that all commands are executed. Drawing to the back buffer then failing to call glSwapBuffers is also a common mistake.

4. Occasionally you can run into issues with buffer formats - although if you copy from sample code that works on your system this is less likely to be a problem.

Graphics coding tends to be quite straightforward to debug once you have the basic environment correct because the output is visual, but setting up the rendering environment on a new system can always be a bit tricky until you have that first cube or sphere rendered. I would recommend obtaining a sample or template and modifying that to start with rather than trying to set up the rendering window from scratch. Using GLUT to check out first drafts of OpenGL calls is good technique too.