Question

I’m trying to get a simple 1D fractal terrain generator to work for a 2D game. But for some reason it always either makes a hill, a valley, or a flat line, as seen in this output of the segments (x1, x2, y2):

  0.0   4.0  -0.02771158460407615
  4.0   8.0  -0.052252824583875854
  8.0  12.0  -0.07902285133059965
 12.0  16.0  -0.10555908121086155
 16.0  20.0  -0.14254585245303525
 20.0  24.0  -0.18045065070426206
 24.0  28.0  -0.22501139415695381
 28.0  32.0  -0.2710783915066824
 32.0  36.0  -0.329316259372778
 36.0  40.0  -0.3892923247211977
 40.0  44.0  -0.4553572198140616
 44.0  48.0  -0.5231035764770061
 48.0  52.0  -0.5959274130549632
 52.0  56.0  -0.6656219786904637
 56.0  60.0  -0.744915288029726
 60.0  64.0  -0.8218171898484998
 64.0  68.0  -0.7981756789319197
 68.0  72.0  -0.7738770779295611
 72.0  76.0  -0.7451672391318471
 76.0  80.0  -0.7157435744314788
 80.0  84.0  -0.6724228309158163
 84.0  88.0  -0.6295400852776468
 88.0  92.0  -0.5843134728466224
 92.0  96.0  -0.5360614060509056
 96.0 100.0  -0.4688005196071708
100.0 104.0  -0.4002685625504277
104.0 108.0  -0.3345027903832193
108.0 112.0  -0.26818546486875805
112.0 116.0  -0.20032715588508002
116.0 120.0  -0.1335249071978246
120.0 124.0  -0.06698448902698972
124.0 128.0   0.0

  0.0   4.0   0.018955623904205618
  4.0   8.0   0.04081565391393612
  8.0  12.0   0.05924247389764535
 12.0  16.0   0.07763715222906489
 16.0  20.0   0.09633847099282224
 20.0  24.0   0.11312197407020168
 24.0  28.0   0.12839143489843127
 28.0  32.0   0.14414813672698973
 32.0  36.0   0.1274505909321103
 36.0  40.0   0.10774014148758147
 40.0  44.0   0.08156728603267831
 44.0  48.0   0.05819330004014197
 48.0  52.0   0.02490146503362223
 52.0  56.0  -0.005671514173097783
 56.0  60.0  -0.030944276269930564
 60.0  64.0  -0.053593376577440344
 64.0  68.0  -0.06844103124397849
 68.0  72.0  -0.08240392823800813
 72.0  76.0  -0.09384053047264221
 76.0  80.0  -0.10452653202924991
 80.0  84.0  -0.10675174062525229
 84.0  88.0  -0.10997901222951861
 88.0  92.0  -0.1120510737650679
 92.0  96.0  -0.11166174606669571
 96.0 100.0  -0.10035737979486
100.0 104.0  -0.08817859848319709
104.0 108.0  -0.07771753902059923
108.0 112.0  -0.07037700764073251
112.0 116.0  -0.05267698984443657
116.0 120.0  -0.033583174260327434
120.0 124.0  -0.016020212145163026
124.0 128.0   0.0

The range is from -1 to 1.

Here is the code used to generate a level.

public void generate()
{
    long seed = System.currentTimeMillis();
    int numLineSegments = width/4;
    Vector<LineSegment> result = new Vector<LineSegment>(numLineSegments);
    result.add(new LineSegment(0, 0, width, 0, seed, 1, 1));
    int currentLevel = 1;
    while(result.size() < numLineSegments)
    {
        Iterator<LineSegment> itr = result.iterator();
        Vector<LineSegment> tmp = new Vector<LineSegment>(numLineSegments);
        while(itr.hasNext())
        {
            LineSegment l = itr.next();
            if(l.level == currentLevel)
            {
                LineSegment[] parts = l.subdivide();
                itr.remove();
                tmp.add(parts[0]);
                tmp.add(parts[1]);
            }
        }
        result.clear();
        Iterator<LineSegment> tmpItr = tmp.iterator();
        while(tmpItr.hasNext())
        {
            result.add(tmpItr.next());
        }
        currentLevel++;
    }
    for(Object o : result.toArray())
    {
        System.err.println(((LineSegment)o).x1+" "+((LineSegment)o).x2+" "+((LineSegment)o).y2);
    }
}

public class LineSegment {
public double x1;
public double y1;
public double x2;
public double y2;
private Random r;
public long seed;
public int level;
public double range;
public LineSegment(double x1, double y1, double x2, double y2, long seed, int level, double range)
{
    r = new Random();
    this.x1 = x1;
    this.y1 = y1;
    this.x2 = x2;
    this.y2 = y2;
    this.seed = seed;
    this.level = level;
    this.range = range;
}
public LineSegment[] subdivide()
{
    double midpoint = (x1 + x2)/2;
    LineSegment[] t = new LineSegment[2];
    double rand;

    if(level == 1)
        rand = r.nextDouble() * 2 - 1;
    else
    {
        rand = ((y1+y2)/2) + (r.nextDouble() * (range * 2) - range);
    }
    double l1x1 = x1;
    double l1y1 = y1;
    double l1x2 = midpoint;
    double l1y2 = rand;
    double l2x1 = midpoint;
    double l2y1 = rand;
    double l2x2 = x2;
    double l2y2 = y2;
    LineSegment l1 = new LineSegment(l1x1, l1y1, l1x2, l1y2, seed, level + 1, range * 0.2);
    LineSegment l2 = new LineSegment(l2x1, l2y1, l2x2, l2y2, seed, level + 1, range * 0.2);
    t[0] = l1;
    t[1] = l2;
    return t;
}

}

I’m not sure what’s wrong, thanks for any help.

Answer 1

The seed gets ignored, thus the generated random numbers aren’t all that random.

Use a single instance of Random and pass it on to subdivide instead of having one instance per segment.

Edit:

Another thing, try to decrease the range by range*0.5 instead of range*0.2, since the horizontal length is also halved. By using a smaller factor the first iteration has a overly large influence. If, for example the first split puts the center point way up, you will get a single hill, since the next iterations are too ‘weak’ to create secondary hills.