Drawing a Circle with a forward vector and Moving both along with Rotation

Question

Been rather busy posting today and creating an AI program to simulate Boids. Right now I am just working on getting a single "boids" drawing a moving around. The "boid" is just a circle(ellipsis) with a line draw to symbolize its "Forward Vector".

class SwarmCir
{
    public float _pointX;
    public float _pointY;
    public float _height;
    public float _width;
    Pen _pen = new Pen(Color.Black);
    PointF _ForwardPoint = new PointF(0, 0);
    float rotAng = 0.0f;

    public SwarmCir()
    {
        _pointX = 1.0f;
        _pointY = 1.0f;
        _height = 5.0f;
        _width = 5.0f;
        _ForwardPoint.X = 7.0f;
        _ForwardPoint.Y = 7.0f;
    }
    public SwarmCir(Point XY)
    {
        _pointX = XY.X;
        _pointY = XY.Y;
        _height = 5.0f;
        _width = 5.0f;
    }
    public SwarmCir( Point XY, float Height, float Width )
    {
        _pointX = XY.X;
        _pointY = XY.Y;
        _height = Height;
        _width = Width;
    }
    public void SetPen(Pen p)
    {
        _pen = p;
    }
    public void Draw(Graphics g)
    {
        g.DrawEllipse(_pen, _pointX, _pointY, _width, _height);
        g.DrawLine(_pen, new PointF(_pointX, _pointY), _ForwardPoint);
    }
    public void Rotate(PaintEventArgs e)
    {
        e.Graphics.TranslateTransform(_pointX, _pointY);
        e.Graphics.RotateTransform(rotAng);
        e.Graphics.TranslateTransform(-_pointX, -_pointY);
    }
    public PointF ForwardVec()
    {
        PointF temp = new PointF();
        temp.X = _pointX - _ForwardPoint.X;
        temp.Y = _pointY - _ForwardPoint.Y;
        return Normalize(temp);
    }
    public PointF Normalize(PointF p)
    {
        PointF temp = new PointF();
        if (p.X > p.Y)
        {
            temp.X = 1;
            temp.Y = p.Y / p.X;
        }
        else if (p.Y > p.X)
        {
            temp.Y = 1;
            temp.X = p.X / p.Y;
        }
        else
        {
            return new PointF(1, 1);
        }
        return temp;
    }
    public void MoveForward()
    {
        _pointX += ForwardVec().X;
        _pointY += ForwardVec().Y;
    }
    public void MoveBackwards()
    {
        _pointX -= ForwardVec().X;
        _pointY -= ForwardVec().Y;
    }
    public void TurnLeft()
    {
        rotAng += 10.0f;
    }
    public void TurnRight()
    {
        rotAng -= 10.0f;
    }
}

Currently when running a program implementing this class, instantiate a default SwarmCir() and just calling the moving functions at the bottom I get really weird results. Essentially I want 'W' to move the circle along the "forward vector" which ever way the line is pointing. Obviously for 'S' is just reverse. Then when turning I would like the shape and line to turn properly. If more information is needed Please ask. Working towards a complete AI workbench.

Anlo · Accepted Answer

Since you're only drawing a circle and a line, I would skip the transformations used in the Rotate method. I would also store the forward vector instead of the forward point and rotAngle in the class. And use a PointF for position instead of float _pointX/_pointY, and a radius instead of width/height, since they will be equal for circles. I would also make the fields private to avoid modification from outside the class.

class SwarmCir
{
  private PointF _point;
  private PointF _forwardVector = new PointF(1, 0);
  private float _radius = 2.5f;
  private float _vectorLength = 5.0f;
  private Pen _pen = new Pen(Color.Black);

Then, I would use _point as the center point for the circles (in your code, the _pointX/_pointY refers to a point to the top/left of the circle). The draw and move methods then becomes:

public void Draw(Graphics g)
{
  g.DrawEllipse(_pen, _point.X - _radius, _point.Y - _radius, 2 * _radius, 2 * _radius);
  g.DrawLine(_pen, _point,
      new PointF(_point.X + _forwardVector.X * _vectorLength,
                 _point.Y + _forwardVector.Y * _vectorLength));
}

public void MoveForward()
{
  _point.X += _forwardVector.X;
  _point.Y += _forwardVector.Y;
}

By the way, I often define some extension methods for PointF to ease adding and multiplying PointF's and scalars.

static class PointFExtensions
{
  public static PointF Add(this PointF point, PointF other)
  {
    return new PointF(point.X + other.X, point.Y + other.Y);
  }

  public static PointF Add(this PointF point, float value)
  {
    return new PointF(point.X + value, point.Y + value);
  }

  public static PointF Multiply(this PointF point, PointF other)
  {
    return new PointF(point.X * other.X, point.Y * other.Y);
  }

  public static PointF Multiply(this PointF point, float value)
  {
    return new PointF(point.X * value, point.Y * value);
  }
}

This makes the draw and move methods code a bit more elegant in my opinion:

public void Draw(Graphics g)
{
  g.DrawEllipse(_pen, _point.X - _radius, _point.Y - _radius, 2 * _radius, 2 * _radius);
  g.DrawLine(_pen, _point, _point.Add(_forwardVector.Multiply(_vectorLength)));
}

public void MoveForward()
{
  _point = _point.Add(_forwardVector);
}

Alright, now there's just the rotation left. Start by adding the Rotate method to PointFExtensions:

public static PointF Rotate(this PointF point, double angle)
{
  angle *= Math.PI / 180.0; // Convert from degrees to radians
  return new PointF(
      Convert.ToSingle(point.X * Math.Cos(angle) - point.Y * Math.Sin(angle)),
      Convert.ToSingle(point.X * Math.Sin(angle) + point.Y * Math.Cos(angle)));
}

The turn methods then becomes:

public void TurnLeft()
{
  _forwardVector = _forwardVector.Rotate(10.0f);
}

Lets hope that works out. I haven't compiled and tested this code so there might be some typos...

Drawing a Circle with a forward vector and Moving both along with Rotation

Answers (2)

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