Douglas-Peucker算法(道格拉斯-普克算法)是将曲線近似表示為一系列點,并減少點的數量的一種算法。它的優點是具有平移和旋轉不變性,給定曲線與門檻值後,抽樣結果一定。Douglas—Peucker算法通常用于線狀矢量資料壓縮、軌迹資料壓縮等。
算法步驟
連接配接曲線首尾兩點A、B形成一條直線AB;
計算曲線上離該直線段距離最大的點C,計算其與AB的距離d;
比較該距離與預先給定的門檻值threshold的大小,如果小于threshold,則以該直線作為曲線的近似,該段曲線處理完畢。
如果距離大于門檻值,則用點C将曲線分為兩段AC和BC,并分别對兩段曲線進行步驟[1~3]的處理。
當所有曲線都處理完畢後,依次連接配接各個分割點形成折線,作為原曲線的近似。
實作代碼
Java實作代碼如下,代碼引用自JTS庫。
class DouglasPeuckerLineSimplifier {
private Coordinate[] pts;
private boolean[] usePt;
private double distanceTolerance;
private LineSegment seg = new LineSegment();
public static Coordinate[] simplify(Coordinate[] pts, double distanceTolerance) {
DouglasPeuckerLineSimplifier simp = new DouglasPeuckerLineSimplifier(pts);
simp.setDistanceTolerance(distanceTolerance);
return simp.simplify();
}
public DouglasPeuckerLineSimplifier(Coordinate[] pts) {
this.pts = pts;
}
public void setDistanceTolerance(double distanceTolerance) {
this.distanceTolerance = distanceTolerance;
}
public Coordinate[] simplify() {
this.usePt = new boolean[this.pts.length];
for(int i = 0; i < this.pts.length; ++i) {
this.usePt[i] = true;
}
this.simplifySection(0, this.pts.length - 1);
CoordinateList coordList = new CoordinateList();
for(int i = 0; i < this.pts.length; ++i) {
if(this.usePt[i]) {
coordList.add(new Coordinate(this.pts[i]));
}
}
return coordList.toCoordinateArray();
}
private void simplifySection(int i, int j) {
if(i + 1 != j) {
this.seg.p0 = this.pts[i];
this.seg.p1 = this.pts[j];
double maxDistance = -1.0D;
int maxIndex = i;
int k;
for(k = i + 1; k < j; ++k) {
double distance = this.seg.distance(this.pts[k]);
if(distance > maxDistance) {
maxDistance = distance;
maxIndex = k;
}
}
if(maxDistance <= this.distanceTolerance) {
for(k = i + 1; k < j; ++k) {
this.usePt[k] = false;
}
} else {
this.simplifySection(i, maxIndex);
this.simplifySection(maxIndex, j);
}
}
}
}