Lines

In a 2D picture it is often hard to judge depth. When a 2D picture is produced from a 3D scene (either a real scene or a scene in a computer), a great deal of information is lost.

The real reason I asked the question was to trick you into thinking about the line segment between the two points. A line segment is the straight path between two points. It has no thickness. There is only one line segment between the two points.

Again, if you did not already have the concept of "line" the above definition would not be enough to learn it. The phrase "straight path" designates the concept that is being defined. In 3D graphics you mostly deal with line segments. In geometry, lines continue without end. A line segment is the part of a line between two points.

In geometry texts the world line is usually not defined. It (like point) is one of the undefined concepts that used to define other objects. Books give pictures and examples of lines and hope that somehow you will come to understand what a line is by looking at them.

Often people say "line" when properly they should say "line segment." These notes will do that also, since line segments are so common.