title: Show trail of moving pixel in C# WinForm project
body: <p>I'm working on a personal project linked to the ant colony problem.
  I successfully developed everything for a console project and now I want to transpose the code in a more graphic layout with windows forms.</p>
  
  <p>Using the code below the form shows correctly one moving pixel, but I'm not able to find a way to show the left trail of the movement. I tried to remove the  pictureBox1.Invalidate(), but this brings me to a static pixel in the starting point.</p>
  
  <p>I would like not to reparse the map matrix and draw the pheromone trail, but only leave the drawn positions as if I'm drawing something with a pen on a piece of paper.</p>
  
  <pre><code>    public void RenderAnts(object sender, PaintEventArgs e)
      {
          pictureBox1.Invalidate();
          foreach (Ant a in ants)
          {
              Brush c;
              c = Brushes.DarkBlue;
              if (a.role == AntRole.Scout)
              {
                  a.Move(i);
                  c = Brushes.Red;
              }
              e.Graphics.FillRectangle(c, a.position.x, a.position.y, 1, 1);
          }
          pictureBox1.Show();
      }
  </code></pre>