How to Draw an Arc using just 2 known points without DrawPie or DrawArc

Question

I've question to ask you guys...

Let's say I have 2 lines as shown below:

What I want is to draw an arc that complete the shape as shown below:

Notes:

• I know the start and end points of the lines and the angle between them.
• The length of the lines are equall.

I don't use DrawPie method. Please don't suggest. I must use the end points of the line.

How can I achieve this? My best regards...

Answer 1

//I use here an arbitrarily angle of 60 degrees, 30 above x-axis and 30 under it.
    //If you have the end-points of each line, all you have to do is to implement it
    //instead and do some adaptation to extend the arc properly:
    private void btnArc_Click(object sender, EventArgs e)
    {
        int startX1, startX2, endX1, endX2, startY1, startY2, endY1, endY2, LineLength;
        double angle, startAngle, sweepAngle;
        const double RADIANS_TO_DEGREES = 180.0 / Math.PI;
        const double DEGREES_TO_RADIANS = Math.PI / 180.0;
        startX1 = startX2 = 350; //Set arbitrarily as the middle of a pictureBox, can be changed to pictureBox1.Width/2
        startY1 = startY2 = 350; //Set arbitrarily as the middle of a pictureBox, can be changed to pictureBox1.Height/2
        LineLength = 100; // length of 100 pixels set arbitrarily
        angle = 30; // measured in degrees, starts counting from x-axis, hour 3 position clockwise
        endX1 = startX1 + (int)(LineLength * Math.Cos(angle * DEGREES_TO_RADIANS)); //60 is an arbitary angle of mine
        endY1 = startY1 + (int)(LineLength * Math.Sin(angle * DEGREES_TO_RADIANS));
        endX2 = startX2 + 87; // Point equal to 30 degrees from x-axis set arbitrarily for demo purposes
        endY2 = startY2 - 50;
        //following multiplied by 2 since arbitrarily angle of 30 degrees from above axis to 30 deg. under axis equals 60 degrees
        sweepAngle = 2 * RADIANS_TO_DEGREES*(Math.Asin(((double)endY1 - (double)startY1) / (double)LineLength));
        startAngle = RADIANS_TO_DEGREES*(Math.Asin(((double)endY2 - (double)startY2) / (double)LineLength));

        System.Drawing.Graphics surface3 = pictureBox1.CreateGraphics();

        Pen pen1 = new Pen(Color.Blue, 2.0f);
        Pen pen2 = new Pen(Color.Red, 1.0f);
        //Assuming the arc should be symmetrical, as you didn't mention otherwise
        //A surrounding rectangle would have length and height equal to twice the line-length
        //The surrounding rectabgle middle point would be the common starting point of your lines
        int RecX = startX1 - LineLength;
        int RecY = startY1 - LineLength;
        int RecWidth = LineLength * 2;
        int RecHeight = LineLength * 2;
        Rectangle rect3 = new Rectangle(RecX, RecY, RecWidth, RecHeight);

        surface3.SmoothingMode = SmoothingMode.AntiAlias;

        surface3.DrawLine(pen1, startX1, startY1, endX1, endY1);
        surface3.DrawLine(pen1, startX2, startY2, endX2, endY2);
        surface3.DrawRectangle(pen1, rect3);
        surface3.DrawArc(pen2, rect3, (Single)startAngle, (Single)sweepAngle);
    }