Rotating a line in C#

Posted by Andrey in Algorithmics, Programming | 4 comments

Recently I had a problem that required me to do a rotation of a line keeping the original point fixed. A rotation is a movement of an object in a circular motion, mathematically a rotation is a rigid body movement which, unlike a translation, keeps a point fixed.

public partial class Rotate : Form
    {
        public Bitmap image;
        public Graphics gra;

        public int angle = 10;
        public int step = 0;
        public int radius = 150;

        public System.Timers.Timer timer;

        public Rotate()
        {
            InitializeComponent();

            image = new Bitmap(400, 300);
            gra = Graphics.FromImage(image);

            gra.Clear(Color.DarkBlue);
            pictureBox1.Image = image;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (timer == null)
            {
                timer = new System.Timers.Timer(50);
                timer.Elapsed += new ElapsedEventHandler(timer_Elapsed);

                timer.Start();
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (timer != null)
            {
                timer.Stop();
                timer = null;
            }
        }

        void timer_Elapsed(object sender, ElapsedEventArgs e)
        {
            gra.Clear(Color.DarkBlue);

            PointF point1 = new PointF(200, 150);
            PointF point2 = new PointF();

            point2.X = radius * (float)Math.Cos(DegreesToRadians(angle * step)) + 200;
            point2.Y = radius * (float)Math.Sin(DegreesToRadians(angle * step)) + 150;
            gra.DrawLine(new Pen(Color.White), point1, point2);
            step++;

            pictureBox1.Image = image;
        }

        private double DegreesToRadians(double angle)
        {
            return angle * Math.PI / 180.0;
        }
    }

Code Explanation:

        public Bitmap image;
        public Graphics gra;

        public int angle = 10;
        public int step = 0;
        public int radius = 150;

        public System.Timers.Timer timer;

       public Rotate()
        {
           InitializeComponent();
           image = new Bitmap(400, 300)
           gra = Graphics.FromImage(image);
           gra.Clear(Color.DarkBlue);
           pictureBox1.Image = image;
       }

To start with the step alongside the angle variable is used to calculate the angle witch the line must be rotated, a step of 36 will result in a full rotation of the line (10 * 36 = 360 degrees), in the class constructor we simply initialize the variables and set the default background color of the image to DarkBlue

           gra.Clear(Color.DarkBlue);

            PointF point1 = new PointF(200, 150);
            PointF point2 = new PointF();

            point2.X = radius * (float)Math.Cos(DegreesToRadians(angle * step)) + 200;
            point2.Y = radius * (float)Math.Sin(DegreesToRadians(angle * step)) + 150;
            gra.DrawLine(new Pen(Color.White), point1, point2);
            step++;

This is where the magic happens, we calculate the X and Y position and we draw the line the + 200/150 at the end represents the offset that we started with in point1.

Download the project here.

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