Tutorial: Introduction to 3D worlds
From DeleDWiki
This tutorial is intended for absolute newbies to 3D editing. It's meant to introduce beginners to 3D worlds and to give them a basic understanding of how these things work.
Contents |
Basics: how 3D worlds are built
The coordiante system
3D scenes are built in a three dimensional coordinate system. To help you understand that, take a look around you. The room in which you are right now probably has four sides, a wall and a ceiling. Now chose a corner of that room. The corner will represent the origin of the coordinate system and will be marked as (0,0,0). Now, imagine three arrows coming out of that corner, which are aligned with the three edges. These arrows are called (X/Y/Z) vectors. With these three vectors, called the basis, you can describe every objects position in your room, relative to the origin of the coordinate system.
For example, if your vectors all are 20cm long and there would be an object lying on a floor for which it would take you 3 vector length steps along the red arrow and 5 along the blue arrow, the coordinates of the object would be (3*20, 0*20, 5*20) = (60, 0, 100). 3D points are written like that, with its coordinates relative to the origin in the brackets.
If you expand the X basis vector infinitely in both directions (forwards and backwards), you get the X axis. Every point of the X axis can be reached by stretching the X vector. It's analog to the Y and Z axis.
In the very most situations, it's assumed you have basis vectors of the length 1. Note that it's not 1cm but 1 length unit. It's a generic unit, which means that you can chose the size of your objects within your 3D scenes just as you like. You can make 1 length unit be 1cm or 1m. The choice is up to you.
Vertices
In almost every 3D editor, you will come across the term vertex (plural: vertices). Vertices are simple points in the 3D space. They are infinitely small, so they can be fully described by their coordinates relative to the origin. In 3D editors, vertices can have properties like color or UV coordinates.
Polygons
Multiple vertices can create a polygon, which is a infinitely thin plane. First, we'll have a look at polygons consisting of three vertices, called triangles.
Any three vertices that do not lie on one line can create a triangle. Almost everything you see in 3D scenes consists of triangles, even polygons with more than 3 vertices. If polygons are not explicitely triangles, they are split into several triangles. This process is called triangulation.
Now, the space you see between the vertices can be filled differently. It can have colors in between or if can be filled with textures.
Creating 3D models, it is important for the polygons to be convex, because they may not be rendered correctly otherwise. Most 3D applications will export their models triangulated so convexity is not important to those because triangles are always convex. DeleD however, supports polygons with an arbitrary amount of vertices. So when you model, you have to take care of that or you will get into trouble.
So what is convexity? Think about having your polygon as a physical objects with a thickness of some centimeters. Now imagine stretching a thread or a tissue around the polygon. If the tissue touches all edges of your polygon, then it's convex. Otherwise it's not. Here's an example.
Objects
Objects are nothing more than a pile of polygons. Please note that objects don't have interior. There is no "inside", for what you see is just a hull of infinitely thin polygons.
