Introduction

ActiveWorlds 3.3 introduced terrain as an alternative to using a ground object. Terrain is made up of a grid of heights, each height being at the south-eastern corner of a cell. The world's terrain is organised in pages, each having 128 x 128 cells (total 16384 cells). The central page is centered on the 0n 0w coordinate, so a world up to 64n will only need one terrain page

A terrain square, which covers exactly one 10m x 10m cell, has four corners. The height of it's south-eastern corner is its 'cell height', that is, it is caracterised by its cell's coordinates. These terrain squares have a fold along one of its diagonals, forming two triangles. To know which of the two possible diagonals is used to triangulate the square, the cell coordinates are used to determine whether a cell is even or odd. A cell is even if both its coordinates are even or if both its coordinates are odd; if one coordinate is even and one is odd, the cell is odd. Even cells triangulate their square using the NW to SE diagonal, odd cells use the NE to SW diagonal.

So, as you can see now, terrain is made out of triangles obtained by splitting the cell surface square in two. A terrain page has therefore 32768 triangles.

To minimise the amount of memory used, the ActiveWorlds browser will have from one to four terrain pages loaded in memory, depending on whether the user is situated near the centre of a page or near one of its borders or corners. The xelagot does something similar.

To speed up rendering terrain, the AW browser uses an algorithm called LOD (level of detail). Terrain is rendered accurately near the user, and approximately at farther distances. As you approach a terrain feature, you may notice that it changes to a more accurate shape. Another fenomenon is that distant terrain features in front of you may disappear as you turn around to face them, and reappear at the right and left edges of the screen when you are not facing them directly: this is due to the far clipping plane chopping them off, because the shortest distance to that plane is right in front of you, and this distance gets greater towards the edge of the screen, allowing more distant features to be seen.

Xelagots can sense terrain. There are a few 3.4 Verbal Commands that use this feature: land, touch and walk. There are also quite a few Action Script commands using this feature.

The X1 xelagot has a Terrain Manager in its Uni menu. The next sections describe its features and how to use it.

Index