source: ps/trunk/binaries/data/mods/public/maps/random/rmgen/Terrain.js

/**
 * @file A Terrain is a class that modifies an arbitrary property of a given tile.
 */

/**
 * SimpleTerrain paints the given texture on the terrain.
 *
 * Optionally it places an entity on the affected tiles and
 * replaces prior entities added by SimpleTerrain on the same tile.
 */
function SimpleTerrain(texture, templateName = undefined)
{
    if (texture === undefined)
        throw new Error("SimpleTerrain: texture not defined");

    this.texture = texture;
    this.templateName = templateName;
}

SimpleTerrain.prototype.place = function(position)
{
    if (this.templateName && g_Map.validTilePassable(position))
        g_Map.setTerrainEntity(this.templateName, 0, Vector2D.add(position, new Vector2D(0.5, 0.5)), randomAngle());

    g_Map.setTexture(position, this.texture);
};

/**
 * RandomTerrain places one of the given Terrains on the tile.
 * It choses a random Terrain each tile.
 * This is commonly used to create heterogeneous forests.
 */
function RandomTerrain(terrains)
{
    if (!(terrains instanceof Array) || !terrains.length)
        throw new Error("RandomTerrain: Invalid terrains array");

    this.terrains = terrains;
}

RandomTerrain.prototype.place = function(position)
{
    pickRandom(this.terrains).place(position);
};