VERTICES

When you say "vertex," most people think that you're speaking of a location in 3-space—that is, an x, y, z triplet. In this book, that's called a point. A vertex in 3D graphics typically means all the properties that are used to describe that particular vertex—that is, all the information needed to describe a vertex so that it can be rendered. Of course, the most obvious one is its location—its "point." However, if I give you a vertex description consisting of just a position and tell you to draw it, what do you draw? No, it's obvious that you need more information in order to draw a vertex, be it the vertex color, material properties, texture coordinates, whatever. So in 3D graphics, a vertex is the vertex position and whatever other information is required to render that vertex. I should note that vertices themselves aren't rendered; they are typically grouped in threes and rendered as triangles—a triangle having enough information to describe a surface area.

This "other information" could be anything about that point that can be used to describe or calculate its final color value. In the simplest case, it's just the color information for that point (e.g., the point is red). Or the point may be part of a textured surface, in which case it might have texture coordinates instead of a color, in which case the color of the point is looked up from the texture. A vertex that's part of an illuminated surface (a "lit" surface in the vernacular) could have a set of material properties that are unique color values for diffuse, ambient, and specular properties plus the point's normal value. Thus when we talk about a vertex, we're not simply talking about the vertex's location, but also that we've specified enough information to draw it according to the situation we want to render it in, be that a textured surface, a lit surface, etc. In some parts of this text, I'll talk about a vertex and mean just the position part of the vertex; other times, I might be focused on the resultant color information, but the important thing is that a vertex is specified by more than just a point.