How to move vector graphic in the direction it is pointing in Java

Question

After a long battle with Graphics2D, I have finally been able to draw, rotate and move a small triangle in an applet. Now what I want to do is find the angle the triangle is pointing in, then move it in that direction. Is there a method or algorithm to make it do this? I have tried this:

public double calcAngleMoveX(double angle)
    {
        return (double)(Math.cos(angle*Math.PI/180));
    }
public double calcAngleMoveY(double angle)
    {
        return (double)(Math.sin(angle*Math.PI/180));
    }

ship.incY(ship.calcAngleMoveY(ship.getFaceAngle()-90));
ship.incX(ship.calcAngleMoveX(ship.getFaceAngle()-90));

But it does really strange things. Can anyone solve this for me?

EDIT: This is my code in paint():

        g2d = (Graphics2D)g;
        AffineTransform identity = new AffineTransform();
        g2d.drawString(Double.toString(ship.getX()),100,100);
        g2d.drawString(Double.toString(ship.getY()),100,120);
        int width = getSize().width;
        int height = getSize().height;
        g2d.setColor(Color.BLACK);
        g2d.fillRect(0,0,width,height);
        g2d.translate(ship.getX(),ship.getY());
        g2d.rotate(Math.toRadians(ship.getFaceAngle()));
        g2d.setColor(Color.RED);
        g2d.fill(shape);

And this is my keyPressed method:

int ke = e.getKeyCode();
        switch(ke)
        {
        case KeyEvent.VK_LEFT:
            ship.setFaceAngle(ship.getFaceAngle()-5);
            break;
        case KeyEvent.VK_RIGHT:
            ship.setFaceAngle(ship.getFaceAngle()+5);
            break;
        case KeyEvent.VK_UP:
            ship.incX(ship.calcAngleMoveX(ship.getFaceAngle())*ship.velocity);
            break;
        case KeyEvent.VK_DOWN:
            ship.incY(ship.calcAngleMoveY(ship.getFaceAngle())*ship.velocity);
            break;
        }
        repaint();

These are the calculation methods:

public double calcAngleMoveX(double angle)
    {
        return (double)(Math.sin(angle * Math.PI / 180.0));
    }
    public double calcAngleMoveY(double angle)
    {
        return (double)(-Math.cos(angle * Math.PI / 180.0));
    }

Answer 1

The ship movement depends on the coordinate system used in your graphics.

Since you've said 90 degrees is "right", I'm going to assume that you're using clockwise bearings, with 0 degrees as "up".

Rather than manipulating the angle to convert from the cartesian convention of 0 degrees being right and then rotating clockwise, it's easier to simply flip around which trigonometric function you call for each axis.

Graphics2D, like most other graphics systems, uses increasing X coordinates as you move right, so:

delta_x = Math.sin(angle * Math.PI / 180.0)

You should be able to trivially see that this gives delta_x = 1.0 for 90 degrees, and delta_x = -1.0 for 270 degrees.

In Graphics2D Y coordinates increase as you move downwards (i.e. [0, 0] is at top-left), so use:

delta_y = -Math.cos(angle * Math.PI / 180.0)

Giving delta_y = -1.0 for 0 degrees, and delta_y = 1.0 for 180 degrees.

If you were using a system with [0, 0] at bottom-left you would just remove the unary minus (-) from the delta_y formula.