NagashTDN NagashTDN - 1 month ago 14
C# Question

Drawing a triangle with rounded corners

So here's my code to draw a triangle:

Graphics y = CreateGraphics();
Pen yy = new Pen(Color.Red);

Point x = new Point(Convert.ToInt32(textBox1.Text)*10, Convert.ToInt32(textBox2.Text)*10);
Point xx = new Point(Convert.ToInt32(textBox3.Text)*10, Convert.ToInt32(textBox4.Text)*10);
Point xxx = new Point(Convert.ToInt32(textBox5.Text)*10, Convert.ToInt32(textBox6.Text)*10);

Point[] u = { x, xx, xxx };
y = pictureBox1.CreateGraphics();
y.DrawPolygon(yy, u);

Is there any way to round the corners on this? I read about it on google and it seems there's only a way to round rectangles, but not triangles.

Is there any command to do that for me or I have to do it manually?

Thanks for replies :)


It is not possible out of the box.

You can create rounded polygons like this:

  • Change each line by making it shorter (on both ends) by your chosen number of pixels, now line AB is line A1B1..
  • Create a GraphicsPath which consists of these lines and in between each pair of lines a curve that connects those new endpoints
  • To better control the curves it would help to add a point in the middle between the original corners and the connection of the new endpoints

For a Triangle ABC this would mean to

  • Create A1, A2, B1, B2, C1, C2
  • Then calculate the middle points A3, B3, C3
  • and finally adding to a GraphicsPath:
    • Line A1B1, Curve B1B3B2, Line B2C1, Curve C1C3C2, Line C2A2 and Curve A2A3A1.

enter image description here

Here is a piece of code that uses this method:

A GraphicsPaths to be used in the Paint event:

GraphicsPath GP = null;

A test method with a list of points, making up a triangle - (*) we overdraw by two points to make things easier; one could add the final lines and curves by ading a few lines of code instead..

private void TestButton_Click(object sender, EventArgs e)
    Point A = new Point(5, 50);
    Point B = new Point(250, 100);
    Point C = new Point(50, 250);

    List<Point> points = new List<Point>();
    points.Add(A);  // *
    points.Add(B);  // *

    GP = roundedPolygon(points.ToArray(), 20);


private void panel1_Paint(object sender, PaintEventArgs e)
   if (GP == null) return;
   using (Pen pen = new Pen(Brushes.BlueViolet, 3f))
          e.Graphics.DrawPath(pen, GP);

GraphicsPath roundedPolygon(Point[] points, int rounding)
    GraphicsPath GP = new GraphicsPath();
    List<Line> lines = new List<Line>();
    for (int p = 0; p < points.Length - 1; p++)
        lines.Add( shortenLine(new Line(points[p], points[p+1]), rounding) );
    for (int l = 0; l < lines.Count - 1; l++)
        GP.AddLine(lines[l].P1, lines[l].P2);
        GP.AddCurve(new Point[] { lines[l].P2, 
                                  supPoint(lines[l].P2,points[l+1], lines[l+1].P1), 
                                  lines[l+1].P1 });

    return GP;

// a simple structure
struct Line
    public Point P1; public Point P2;
    public Line(Point p1, Point p2){P1 = p1; P2 = p2;}

// routine to make a line shorter on both ends
Line shortenLine(Line line, int byPixels)
    float len = (float)Math.Sqrt(Math.Pow(line.P1.X - line.P2.X, 2)
                               + Math.Pow(line.P1.Y - line.P2.Y, 2));
    float prop = (len - byPixels) / len;
    Point p2 = pointBetween(line.P1, line.P2, prop);
    Point p1 = pointBetween(line.P1, line.P2, 1 - prop);
    return new Line(p1,p2);

// with a proportion of 0.5 the point sits in the middle
Point pointBetween(Point p1, Point p2, float prop)
    return new Point((int)(p1.X + (p2.X - p1.X) * prop), 
                     (int)(p1.Y + (p2.Y - p1.Y) * prop));

// a supporting point, change the second prop (**) to change the rounding shape! 
Point supPoint(Point p1, Point p2, Point p0)
    Point p12 = pointBetween(p1, p2, 0.5f);
    return pointBetween(p12, p0, 0.5f);       // **


enter image description here