8 replies to this topic

[bleedy3]

I am currently working on a simple 2d game that requires the character to jump up(vertically), and I cant seem to implement the jumping correctly. I know there is a proper way to realistically simulate jumping in a video game, through the use of a certain physics formula, and I was wondering if any of you can share your knowledge about this subject area, any knowledge about this topic would be greatly appreciated. Please respond to this thread and thanks for reading.

[wim DC]

Since the gravity reduces vertical acceleration by 9.81m/s². you can keep the time of the start of your jump. And then every repaint( ) or every update( ) you have to check the time, calculate delta, and use that time to change the speed, set the newly measured time as the old and repeat.

You just have to figure out a normal starting speed of the jump..
So imagine if you start with a speed of 9.81m/s (how convenient^^). And you computer is suuuuppper slow and loops every 0.25sec (how convenient again). Result:

[U]       time   height   speed[/U]
       0      0        9.81
       0.25   2.4525   7.3575
       0.5    4.291875 4.905
       0.75   5.518125 2.4525
       1      6.13125   0
       1.25   6.13125   -2.4525 
       1.5    5.518125  -4.905
       1.75   4.291875  -7.3575
       2      2.4525   -9.81
       2.25   0        -12.2625   

The delta here was 0.25 with you the delta will be timeNow - timePreviousLoop. Which will be much more accurate as i reach a height of 6.13m here, while it will actually reaches 5m only (as you see in the attachment). 9.81m/s as start speed is a bit fast too :D
Or am i totally going in the wrong direction here? :)

I attached an xls with 0.01s intervals. I guess you get what i mean ^^. I had to zip it cause it didn't accept .xls

Attached Files

•  spreadsheet.zip   11.12K   27 downloads

[bleedy3]

So basically what you are saying is that I should add 9.81/s (s, being the time which is 0.25, then 0.5, then 0.75.... and so on) to the characters y direction? Thanks for responding, but could you please incorporate some Java code with your explanation? And also, do you know the formula that they used to make to character jump and fall respectively in the iphone game "doodle jump"?

[wim DC]

Here, i made a little sample:

import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JFrame;

public class Jump extends JFrame{
  private double height=0, speed=10.0;
  public static final double gravity = 9.81;
 
  public Jump(){
      super("jump");
      setSize(100, 600);
      setDefaultCloseOperation(EXIT_ON_CLOSE);
      setVisible(true);  
  } 

  @Override
  public void paint(Graphics g) {    
    super.paint(g);
    g.setColor(Color.GREEN);
    g.fillRect(0, 550, 100, 2);
    g.setColor(Color.BLUE);
    g.fillOval(25, (int) (500-(height*100)), 50, 50);
  }   

  
  public void jump(){ 
    long previous= 0, start=0;
  
    while(true){
      start= System.nanoTime();  
      if(previous != 0){
        double delta = start-previous;
        height += (delta/1000000000) *speed;        
        speed -= (delta/1000000000)  * gravity;  

        paint(this.getGraphics());
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
      }  
      if(height < 0){
          height=0;
          speed=10.0; 
      }      
      previous= start;
    }
    
  }
  public static void main(String[] args){
    Jump jump = new Jump();
    jump.jump();
  }
 
}

I don't even know what doodle is, looks cute juding from google images :)
For only 1 jump, add a break at the end of the if(height < 0){
Might flicker a bit. Didn't do double buffering. I hope you do for your game ^^

You can play with the speed and gravity variable to get the jump you want :D.

• Speed gets you higher (... hm this sentence can be pulled out of context so easily :p) and faster
  
• Gravity makes it go slower and higher.

EDIT: Okay i couldn't hold myself and just HAD to add a keylistener and double buffering so i can move around :D
I've put it all in 1 class. What a nice clean code:glare:

import java.awt.*;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.image.BufferedImage;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Jump extends JPanel implements KeyListener{
    private double height=0, speed=4;
    public static final double gravity = 9.81;
    private double x=25;
    private boolean left=false, right=false, up=false;
    private BufferedImage buf;
    private JFrame frame;

    public Jump(){
        super();
        frame = new JFrame("jump");
        frame.setSize(600, 600);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(this);
        frame.setVisible(true);
        frame.addKeyListener(this);
        buf = new BufferedImage(600,600, BufferedImage.TYPE_INT_RGB);
        setIgnoreRepaint(true);
    } 

    @Override
    public void paint(Graphics g) {
        
        
        Graphics2D gBuf = buf.createGraphics();
        gBuf.setColor(Color.WHITE);
        gBuf.fillRect(0, 0, 600, 600);
        gBuf.setColor(Color.GREEN);
        gBuf.fillRect(0, 550, 600, 2);
        gBuf.setColor(Color.BLUE);
        gBuf.fillOval((int) x, (int) (500-(height*100)), 50, 50);            
        gBuf.dispose();
        
        g.drawImage(buf, 0, 0, this);
    }   


    public void jump(){ 
        long previous= 0, start=0;

        while(true){
            start= System.nanoTime();  
            if(previous != 0 && up){
                double delta = start-previous;

                height += (delta/1000000000) *speed;        
                speed -= (delta/1000000000)  * gravity; 
            }  
            if(left)
                x-=3;
            if(right)
                x+=3; 
            
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            if(height < 0){
                height=0;
                speed=4; 
                up=false;
            }      
            previous= start;
            paint(this.getGraphics());
        }

    }
    public static void main(String[] args){
        Jump jump = new Jump();
        jump.jump();
    }

    public void keyTyped(KeyEvent e) {    
    }

    public void keyPressed(KeyEvent e) {    
        if(e.getKeyCode() == KeyEvent.VK_UP)
            up=true;
        if(e.getKeyCode() == KeyEvent.VK_LEFT)
            left=true;
        if(e.getKeyCode() == KeyEvent.VK_RIGHT)
            right=true;      
    }

    public void keyReleased(KeyEvent e) {    
        if(e.getKeyCode() == KeyEvent.VK_LEFT)
            left=false;
        if(e.getKeyCode() == KeyEvent.VK_RIGHT)
            right=false;    
    }

}

Edited by wim DC, 28 July 2010 - 05:14 AM.

[WingedPanther]

A realistic jump follows a parabola.

[bleedy3]

Thanks for putting this code example up, I really appreciate your help, it helped me out a lot, I compiled the code and it worked! However, I have some questions. The first one is: what is the purpose of using the "start", "previous", and "delta" variables? And for the variables "height", and "speed", why did you assign part of them to (delta/1000000000), Why divide by such a long number? And if I wanted the thread to sleep for 100 milliseconds instead of 10 milliseconds how could I change the code so the ball moves faster?

[wim DC]

Delta contains a value measured in nanoseconds. (1s = 10^9nanoseconds).
The speed and gravity's measurement are in m/second and m/second².
Imagine if delta was 10 milliseconds (0.01s) and the target had a speed of 10m/s.
In those 10 milliseconds it would've moved 10m/s * (10ms / 1000) = 0.1m. And for nanoseconds it's just a few extra zeros.

In the method i do thread.sleep(10) so it loops every 10ms. Unfortunately that's not 100% correct.
Not only does eclipse tell me this:

Quote

The precision is not guaranteed - the Thread may sleep more or less than requested.

But even if it is exactly 10ms. It's still up to the software that controls the processor (OS i think?) That decides if this thread may run now or not. Processors may look like they are running multiple threads at the same time. But in fact they choose 1 thread. Run it a very short time, run the next a short time, run the ne.....
So if the software decides our thread isn't so very important the loop won't loop exactly every 10ms.

Because of this, i can't hardcode
height += 0.01 * speed;
Instead i manually calculate how much time there has been since the previous loop. This is where start and previous comes in. And delta(being start-previous) just resembles the 10ms from the previous example.

Remember, because the times are measured in nanoseconds I have to do /1000000000 Or *10E-9 if you like.

If you print out delta you'll see that 1 loop repeats itself roughly every 15ms. ~10ms from the sleep. Probably 5ms for the calculations. And then the variable part decided by the processor.
Note that System.out.println(..) may have a bigger impact than you think. Don't every leave them in there for the final result.

1.5640903E7
1.5543059E7
3.1123445E7
1.6647953E7
1.4716222E7
1.5600562E7
1.5963846E7

I even get 31ms once.. for some weird reason. Even weirder it gets because it's always in the 3th loop with me.. ALWAYS:confused:

Quote

And if I wanted the thread to sleep for 100 milliseconds instead of 10 milliseconds how could I change the code so the ball moves faster?

First of all 100ms sleep becomes noticeable by the human eye. So the object is going to stutter.
Pretty normal as the 100ms would mean 10fps in my program.

Because i use the start and previous. The amount of sleep doesn't have an impact on the time the ball goes into the air. If the jump takes 2.18 seconds with a sleep of 10. The jump will take 2.18 seconds with a sleep of 100. If you want the jump to look slower or faster you can change the gravity, and then adjust the speed so the jump gets the correct height again.

Speed 3.5 and gravity 2 will get you about the same height / jump as speed 8 and gravity 9.81. But the jump will be much slower.

[bleedy3]

Thanks a lot for helping me out, I finally understand it with all of your explanations!
I really appreciate your help.

[iamobama]

Do you resolved this problem now?