Highlighting a panel area

Question

I have a VB.NET WinForms application. I am drawing a bunch of shapes on a panel programmatically with GDI+. Now I want to hightlight a small rectangular portion of that panel in some way. The highlighted area will move around a lot. I don't want to redraw the whole panel, because there is too much processing involved and it causes flicker. What it the best way to do this?

Thanks.

Answer 1

Can you add a transparent panel as a child of your existing panel (and dock it), and then do your highlights on the transparent panel?

Here's a very rudimentary example:

public partial class Form1 : Form
{
    private Panel pan = new Panel();
    private Panel trans = new Panel();

    private bool clicked { get; set; }
    private Point mouse { get; set; }

    public Form1()
    {
        InitializeComponent();
        this.Load += new EventHandler(Form1_Load);
    }

    public void Form1_Load(object sender, EventArgs e)
    {
        pan.Dock = DockStyle.Fill;
        pan.BackColor = Color.White;

        trans.Dock = DockStyle.Fill;
        trans.BackColor = Color.FromArgb(0, 0, 0, 0);

        pan.Paint += new PaintEventHandler(DrawSomeShapes);
        trans.MouseClick += new MouseEventHandler(trans_MouseClick);
        trans.Paint += new PaintEventHandler(DrawHighlight);

        pan.Controls.Add(trans);
        this.Controls.Add(pan);

        trans.BringToFront();
    }

    public void DrawSomeShapes(object sender, PaintEventArgs e)
    {
        var g = e.Graphics;

        var p = new Pen(Color.Black, 1F);
        g.DrawRectangle(p, new Rectangle(0, 0, 100, 100));
        g.DrawRectangle(p, new Rectangle(20, 20, 50, 50));
        g.DrawRectangle(p, new Rectangle(10, 50, 100, 25));
        g.DrawRectangle(new Pen(Color.Red, 1F), new Rectangle(new Point(50, 50), new Size(50, 50)));
    }

    public void DrawHighlight(object sender, PaintEventArgs e)
    {
        if (clicked)
        {
            var g = e.Graphics;
            var b = new SolidBrush(Color.FromArgb(50, 50, 50, 50));
            g.FillRectangle(b, mouse.X, mouse.Y, 50, 50);

            clicked = false;
        }
    }

    public void trans_MouseClick(object sender, MouseEventArgs e)
    {
        mouse = e.Location;
        trans.Invalidate();
        clicked = true;
    }
}