Why is there no circle or ellipse primitive in openGL?

Question

Hey guys.

Circles are one of the basics geometric entities. Yet there is no primitives defined in openGl for this like lines or polygons. Why so? Its a little annoying to include custom headers for this all the time!

Any specific reason to omit it?

Answer 1

Because historically, video cards have rendered points, lines, and triangles.

You calculate curves using short enough lines so the video card doesn't have to.

Answer 2

You could always use gluSphere (if a three-dimensional shape is what you're looking for).

If you want to draw a two-dimensional circle you're stuck with custom methods. I'd go with a triangle fan.

The primitives are called primitives for a reason :)

Answer 3

While circles may be basic shapes they aren't as basic as points, lines or triangles when it comes to rasterisation. The first graphic cards with 3D acceleration were designed to do one thing very well, rasterise triangles (and lines and points because they were trivial to add). Adding any more complex shapes would have made the card a lot more expensive while adding only little functionality.

But there's another reason for not including circles/ellipses. They don't connect. You can't build a 3D model out of them and you can't connect triangles to them without adding gaps or overlapping parts. So for circles to be useful you also need other shapes like curves and other more advanced surfaces (e.g. NURBS). Circles alone are only useful as "big points" which can also be done with a quad and a circle shaped texture, or triangles.

If you are using "custom headers" for circles you should be aware that those probably create a triangle model that form your "circles".

Answer 4

Saying this depends on resolution

Answer 5

Because graphic cards operate on 3-dimensional points, lines and triangles. Circle requires curves or splines. It cannot be perfectly represented by "normal" 3d primitive, only approximated as N-gon (so it will look like a circle at certain distance). If you want circle - write routine yourself (isn't hard to do) - either draw it as N-gon, or make a square (2 triangles) and cut a circle out of it it using fragment shader (you can get perfect circle this way).