Java FX minigolf game - how to slow down the moving ball - animation

Question

I am beginner and trying to create a simple minigolf game with Java FX.
I built small golfcourse using lines. I get user input about hit strength from the slider and can make a hit using keyboard into 8 different directions.
If I hit for example button H, ball starts to move to right. If slider value was minimum, it moves slowly and if its maximum, it moves very fast.
But I cannot make the movement slow down until it stops.
Any ideas how to do that? My code so far is below.
Thank You in advance!
package sample;
import javafx.animation.AnimationTimer;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Slider;
import javafx.scene.input.KeyCode;
import javafx.scene.layout.Pane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.scene.shape.Line;
import javafx.stage.Stage;
public class Main extends Application {
double directionx = 0;
double directiony = 0;
Line line1,line2,line3,line4,line5,line6,line7,line8,line9;
#Override
public void start(Stage primaryStage) throws Exception{
primaryStage.setTitle("Minigolf");
Pane pane = new Pane();
Scene scene = new Scene(pane, 900, 600);
pane.setStyle("-fx-background-color: green");
primaryStage.setScene(scene);
Slider slider = new Slider();
slider.setMin(1);
slider.setMax(10);
slider.setValue(1);
slider.setShowTickLabels(true);
slider.setShowTickMarks(true);
slider.setLayoutX(10);
slider.setLayoutY(10);
Circle ball = new Circle(10);
Circle hole = new Circle(15);
hole.setCenterX(75);
hole.setCenterY(430);
hole.setFill(Color.YELLOW);
ball.setCenterX(100);
ball.setCenterY(300);
// creating golfcourse with lines
line1 = new Line(50,200,500,200);
line2 = new Line(50,350,650,350);
line3 = new Line(50,200,50,350);
line4 = new Line(500,200,500,50);
line5 = new Line(650,350,650,200);
line6 = new Line(500,50,800,50);
line7 = new Line(800,50,800,500);
line8 = new Line(800,500,50,500);
line9 = new Line(50,500,50,350);
pane.getChildren().addAll(slider,ball,hole,line1,line2,line3,line4,line5,line6,line7,line8,line9);
new AnimationTimer() {
#Override
public void handle(long now) {
double ballY = ball.getCenterY();
double newballY = ballY + directiony;
// if ball touches vertical walls
if (ball.getBoundsInParent().intersects(line5.getBoundsInParent())
|| ball.getBoundsInParent().intersects(line3.getBoundsInParent())
|| ball.getBoundsInParent().intersects(line4.getBoundsInParent())
|| ball.getBoundsInParent().intersects(line7.getBoundsInParent())
|| ball.getBoundsInParent().intersects(line9.getBoundsInParent())) {
// changes direction
directionx = directionx * -1;
}
ball.setCenterX(ball.getCenterX() + directionx);
ball.setCenterY(newballY);
// if ball touches horizontal walls
if (ball.getBoundsInParent().intersects(line1.getBoundsInParent())
|| ball.getBoundsInParent().intersects(line2.getBoundsInParent())
|| ball.getBoundsInParent().intersects(line6.getBoundsInParent())
|| ball.getBoundsInParent().intersects(line8.getBoundsInParent())) {
directiony = directiony * -1;
}
// if ball touches hole, then game is over
if (ball.getBoundsInParent().intersects(hole.getBoundsInParent())) {
System.out.println("Game over");
pane.getChildren().removeAll(ball);
}
}
}.start();
// I get the slider value and making array where are 10 different strengts
scene.setOnKeyPressed(event -> {
double b = Math.round(slider.getValue());
b--;
int c = (int) b;
double [] strengts = new double[10];
double k = 0.1;
for (int i = 0; i < strengts.length; i++) {
strengts[i] = k;
k = k + 0.5;
}
// 8 different directions to move the ball
if (event.getCode().equals(KeyCode.H)) {
directionx = directionx + strengts[c];
}
else if(event.getCode().equals(KeyCode.F)) {
directionx = directionx - strengts[c];
}
else if(event.getCode().equals(KeyCode.V)) {
directiony=directiony + strengts[c];
}
else if(event.getCode().equals(KeyCode.T)) {
directiony= directiony - strengts[c];
}
else if(event.getCode().equals(KeyCode.B))
{
directionx = directionx + strengts[c];
directiony = directiony + strengts[c];
}
else if(event.getCode().equals(KeyCode.Y))
{
directionx = directionx + strengts[c];
directiony = directiony - strengts[c];
}
else if(event.getCode().equals(KeyCode.R))
{
directionx = directionx - strengts[c];
directiony = directiony - strengts[c];
}
else if(event.getCode().equals(KeyCode.C))
{
directionx = directionx - strengts[c];
directiony = directiony + strengts[c];
}
});
primaryStage.show();
}
public static void main(String[] args) {
launch(args);
}
}

Answer 1

Just use physics for friction. Assume for every frame the speed (v) remains constant.
Energy of a ball (mass m):
E = 0.5 * m * v²
Work for friction based on distance (s) traveled and a friction constant μ
ΔE = s * m * μ = Δt * v * m * μ
The new energy is E - ΔE which you can use with the first formula to calculate the new speed v':
0.5 * m * v'² = 0.5 * m * v² - s * m * μ
v'² = v² - s * μ * 2
v' = sqrt(v² - s * μ * 2)
The above assumes the ground is flat.
This allows you to decrease the speed like this:
#Override
public void start(Stage primaryStage) {
Point2D direction = new Point2D(1, 1).normalize();
Circle c = new Circle(10, 10, 10);
Timeline tl = new Timeline();
tl.getKeyFrames().setAll(new KeyFrame(Duration.seconds(1 / 60d), new EventHandler() {
double speed = 3;
double friction = 0.02;
#Override
public void handle(Event event) {
double distance = speed;
Point2D movement = direction.multiply(distance);
c.setCenterX(c.getCenterX() + movement.getX());
c.setCenterY(c.getCenterY() + movement.getY());
double energyBefore = speed * speed;
double energyLost = distance * friction;
if (energyLost > energyBefore) {
tl.stop();
speed = 0;
System.out.println("stopped");
} else {
speed = Math.sqrt(energyBefore - energyLost);
}
}
}));
tl.setCycleCount(Animation.INDEFINITE);
Pane root = new Pane();
root.getChildren().add(c);
Scene scene = new Scene(root, 400, 400);
scene.setOnMouseClicked(evt -> tl.play());
primaryStage.setScene(scene);
primaryStage.show();
}

Answer 2

Thank You for the reply, but as a beginner I found now by myself simpler solution.
I just add 4 if-statements as follows:
if (directionx>0) {
directionx = directionx - 0.01;
}
if(directionx<0){
directionx = directionx + 0.01;
}
if(directiony>0) {
directiony = directiony - 0.01;
}
if(directiony<0) {
directiony = directiony + 0.01;
}
Perhaps the in real physically its somehow wrong, but it looks quite real for me.