Curved surfaces in Orthographic Projection

Before we start drawing curves in Orthographic Projection we need to look back on lines again.

We already know that a line can represent a face when two faces meet at 90 degrees. This is when a line becomes an edge.

We also know that a line appears when a sloped surface meets a Horizontal, Vertical or another Sloped surface. This is when a line signifies a change in direction.

However when we look at curved objects such as a cylinder, a line can represent a limiting element. This is the very limit of the curve, a line appears here because we can't see past this point of the curve. Look at the picture below to get a better idea.

Secondly a curved surface can appear as a circle or portion of a circle. Look at the purple surface in the animation below.

A curved surface may also appear as a regular plane figure when viewed Orthographically (i.e. it's curvature is not apparent). Look at what happens to the purple surface in the animation below.

Finally, when a curved surface meets a surface that is Tangential to it, no line appears because there is no change in direction (the curve flows smoothly into the other surface).

However if the surface is not Tangential to the curve then a line appears (there is an abrupt change in direction of the curve).