Help With Bearing Movement

[BloodLust_89]

Greetings, all! Just need a little help with movement of objects in a more complex fashion - using bearing.

I want to move an object at specific degrees but the following isnt working.

//Main Class
public void createBullet()
{
    Bullet b = new Bullet(getWidth()/2, getHeight()/2, 33, 3);
}

public void run()
{
    while (bRunning)
    {
        try
        {
            b.move();
            b.paint(getGraphics());
            Thread.sleep(tickTime);
        }
        catch (Exception e)
            bRunning = false;
    }
}



//Bullet Class
//declaration stuff
int x,         //Current x co-ord
    y,         //Current Y co-ord
    pX,       //Previous x co-ord
    pY,       //Previous x co-ord
    bearing, //Direction in gegrees of movement
    speed;  //velocity

//some other stuff

//Constructor
public Bullet(int sX, int sY, int sBearing, int sSpeed)
{
    //'s' means Start
    x = sX;
    y = sY;
    pX = sX;
    pY = sY;
    bearing = sBearing;
    speed = sSpeed; 
}

//move method
public void move()
{
    //convert bearing to radians 
    int angle = bearing * 2 * Math.PI / 360;   
    //Set previous x & y co-ord
    pX = x;
    pY = y;

    //Use trig functions to get new X an Y co-ords
    x += Math.cos(angle) * speed;
    y += Math.sin(angle) * speed;

    //Mirror the bearing if the x reach the side of the screen
    //and all that.
}

//Paint methos
public paint(Graphics g)
{
    g.setColor(new Color(0, 255, 0));
    g.drawLine(pX, pY, x, y);
}

Ignoring the errors if n e, the code works, however the movement is definately not rite. I hope u get wat im saying, and if n e 1 has n code for the angle movement post it. l8er

[codex]

Haven't looked at the code thoroughly but your angle variable should definitely be a double and not an int!

[crosswire]

I would second codex except that I am away from my dev machine to know how java handles int.

I remember having a problem with sin and cosine w.r.t. the sign in a problem the other day and it was solve using an alternate function tangent, but that will not help you. Only test with different angle ranges, eg angle between 0,90,180,270, and -0,-90,-180,-270

Remember also that the y display is negative to traditional y in coordinate geometry.

[crosswire]

Try adding a displacement vector to the bullet class
Then set the values once you get the initial bearings.

//Constructor
public Bullet(int sX, int sY, int sBearing, int sSpeed)
{
    //'s' means Start
    x = sX;
    y = sY;
    pX = sX;
    pY = sY;
    bearing = sBearing;
    speed = sSpeed; 

    dX; //displacement vector
    dY; //displacement vector

    //Note sine use for X

    //The convention i used is to have Y up, X right and angle clockwise from up
    //calculate angle in rads from bearing here
    dX = Math.sin(angle) * speed;
    dY = Math.cos(angle) * speed;

}

//Once displacement vector is set then add displacement on every frame in Move()

//Reflect displacement vector on boundary condition


//move method
public void move()
{
    pX = x;
    pY = y;

    x += dX;
    y += dY;
}

I still have not fixed the issue with display y is negative of the convention of y coordinate as up

The code is the same but I was just addressing one of the minor errors.

To fix the y sign issue , use

    //calculate angle in rads from bearing here
    dX = Math.sin(angle) * speed;
    dY = - Math.cos(angle) * speed; //negative here
}

//move method
public void move()
{
    pX = x;
    pY = y;

    x += dX;
    y += dY;
}

Hope that helps, but test for different ranges of angle because I think that cos function works funny for some angles.

BTW, how does the movement look for what you have

[codex]

Let me be more verbose about the "angle is an int" problem.

The line

int angle = bearing * 2 * Math.PI / 360;

is supposed to transform the bearing from degrees (0 - 360) into radians (0 - 2*pi).

If bearing is 90 degrees, the result should be 90*2*pi/360, which is pi/2, about 1.57.

But angle is an int, and it will end up as 1 (no rounding)!!!

The difference is 0.57 in radians, which is almost 33 degrees! That is a big difference.

Furthermore, how do you compile your code? My compiler doesn't allow that line at all, because you cannot assign a double (the expression on the right side of the assignment is double because of Math.PI) to an int variable without explicit casting.

[BloodLust_89]

Well, the 'int angle' was a typo, and all the code was typed quickly from out of my head, i had to hurry cuz i was at taits internet cafe. I know radians are in doubles. Thanks for the info on the y co-ord, ill try it out. I guess that the y co-ord thing was the problem, ill let ya know.

[Magma]

the first thing that popped into my head has already been said...darnnit


java x,y coordinates are flipped...that's all i had to say *sighs and leaves*

[BloodLust_89]

well, i figured it out. It seems the following worked,

    double angle = bearing * 2 * Math.PI / 360;

    x += Math.cos(angle) * speed;
    y += Math.sin(angle) * speed;

when the following didnt.

    double angle = bearing * 2 * Math.PI / 360;

    x += (int) Math.cos(angle) * speed;
    y += (int) Math.sin(angle) * speed;

hmmmmmm, the '(int)' casting part seems to prevent rounding. im gonna investigate.