Title: Draw lines with arrowheads in C#
If <vx, vy> is a vector between two points, then <vy, vx> and <vy, vx> are perpendicular vectors of the same length. If you add and subtract these vectors from the original one, you get new vectors that make a 45 degree angle with the original one. You can use those vectors to make arrowheads and fletchings (tails) for arrows.
The DrawArrowhead method shown in the following code draws a pair of vectors at a 45 degree angle from a vector.
// Draw an arrowhead at the given point
// in the normalized direction <nx, ny>.
private void DrawArrowhead(Graphics gr, Pen pen,
PointF p, float nx, float ny, float length)
{
float ax = length * (ny  nx);
float ay = length * (nx  ny);
PointF[] points =
{
new PointF(p.X + ax, p.Y + ay),
p,
new PointF(p.X  ay, p.Y + ax)
};
gr.DrawLines(pen, points);
}
The following DrawArrow method uses the DrawArrowhead method to draw arrow heads and tails.
// Draw arrow heads or tails for the
// segment from p1 to p2.
private void DrawArrow(Graphics gr, Pen pen, PointF p1, PointF p2,
float length, EndpointStyle style1, EndpointStyle style2)
{
// Draw the shaft.
gr.DrawLine(pen, p1, p2);
// Find the arrow shaft unit vector.
float vx = p2.X  p1.X;
float vy = p2.Y  p1.Y;
float dist = (float)Math.Sqrt(vx * vx + vy * vy);
vx /= dist;
vy /= dist;
// Draw the start.
if (style1 == EndpointStyle.ArrowHead)
{
DrawArrowhead(gr, pen, p1, vx, vy, length);
}
else if (style1 == EndpointStyle.Fletching)
{
DrawArrowhead(gr, pen, p1, vx, vy, length);
}
// Draw the end.
if (style2 == EndpointStyle.ArrowHead)
{
DrawArrowhead(gr, pen, p2, vx, vy, length);
}
else if (style2 == EndpointStyle.Fletching)
{
DrawArrowhead(gr, pen, p2, vx, vy, length);
}
}
The DrawArrow method draws a line with arrow heads and/or fletchings on the ends. It draws the line and then finds a vector of length 1 in the direction of the line. It call the DrawArrowhead method to draw arrowheads and fletchings appropriately.
Download the example to experiment with it and to see additional details.
