Tracing a node's path in javafx - javafx

Question

I am making a double pendulum simulation in javafx. I want to trace the lower bob's path by a clear line, not a trace of dots(which I have achieved). I am new to javafx. I have not used any transitions to animate it. I have used Timeline and KeyFrame to make it work.
private static void doCalc(){
a1_v+=a1_a;
a2_v+=a2_a;
a1+=a1_v;
a2+=a2_v;
a1_a = calcA1();
a2_a = calcA2();
System.out.println(a1_a+"\n"+a2_a+"\n");
x1 = calcX(x0,a1,r1);
y1 = calcY(y0,a1,r1);
x2 = calcX(x1,a2,r2);
y2 = calcY(y1,a2,r2);
Circle tracer = new Circle(x2,y2,1);
tracer.setFill(Color.BLUE);
root.getChildren().add(0,tracer);
}
This is the method that does the calculation and adds the tracer every time a new frame is created. But I want a single line to represent my node's path, not a trail of dots.
This might be a stupid question and I know my coding practices in javafx are a bit weak. I was just looking for a small solution to implement so that I can quickly add it to my simulator. Thanks in advance :)

Answer 1

Just add a line from the old coordinates to the new ones:
private static void doCalc(){
// I think these must be the old coordinates you need:
double oldX2 = x2 ;
double oldY2 = y2 ;
a1_v+=a1_a;
a2_v+=a2_a;
a1+=a1_v;
a2+=a2_v;
a1_a = calcA1();
a2_a = calcA2();
System.out.println(a1_a+"\n"+a2_a+"\n");
x1 = calcX(x0,a1,r1);
y1 = calcY(y0,a1,r1);
x2 = calcX(x1,a2,r2);
y2 = calcY(y1,a2,r2);
Line traceLine = new Line(oldX2, oldY2, x2, y2);
traceLine.setStroke(Color.BLUE);
root.getChildren().add(0,traceLine);
}
You could also use a Path and update it:
// do this once:
Path trace = new Path();
trace.getElements().add(new MoveTo(initialX2, initialY2));
trace.setStroke(Color.BLUE);
root.getChildren().add(trace);
// ...
private static void doCalc(){
a1_v+=a1_a;
a2_v+=a2_a;
a1+=a1_v;
a2+=a2_v;
a1_a = calcA1();
a2_a = calcA2();
System.out.println(a1_a+"\n"+a2_a+"\n");
x1 = calcX(x0,a1,r1);
y1 = calcY(y0,a1,r1);
x2 = calcX(x1,a2,r2);
y2 = calcY(y1,a2,r2);
trace.getElements().add(new LineTo(x2, y2));
}
Note that either of these approaches (and indeed your original approach with the circles) create indefinitely many UI nodes and add them to the scene. Consequently, if the simulation runs for long enough, you will eventually run out of memory. If the simulation is expected to run for a while, you probably want to limit the number of these you keep in memory (e.g. stop adding new lines after some fixed number, or keep the number at a maximum by removing the earlier ones once you reach a limit, etc).