A Simple Vector-Based LED User Control

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Introduction

The LEDBulb is a .NET user control for Windows Forms that emulates an LED light with two states On and Off. The purpose of the control is to provide a sleek looking representation of an LED light that is sizable, has a transparent background and can be set to different colors.

LedBulb bulb = new LedBulb();
bulb.Size = new Size(25, 25);
bulb.Color = Color.LawnGreen;
bulb.On = true;
this.Controls.Add(bulb); 

Sizeable

I recently needed to add a custom user control to a Window Forms project of mine that would represent an LED bulb commonly seen on household electronics. After a quick search, most of the existing controls I found were ugly and used static images, which means they wouldn’t scale well. This control uses the System.Drawing.Drawing2D namespace to draw a vector image so that it not only looks clean but will also scale to any size without affecting the image quality.

Rendering the control starts with drawing a solid circle with our background color. Then we draw a radial gradient over it with our highlight color, transitioning to transparent. To draw the reflection, we draw another radial gradient, from white to transparent, but in a smaller rectangle shifted up and left. To prevent the white gradient from showing up outside the bulb, we set the clip parameter to the bounds of our original circle.

// Fill in the background circle 
g.FillEllipse(new SolidBrush(darkColor), drawRectangle);

// Draw the glow gradient
GraphicsPath path = new GraphicsPath();
path.AddEllipse(drawRectangle);
PathGradientBrush pathBrush = new PathGradientBrush(path);
pathBrush.CenterColor = lightColor;
pathBrush.SurroundColors = new Color[] { Color.FromArgb(0, lightColor) };
g.FillEllipse(pathBrush, drawRectangle);

// Set the clip boundary to the edge of the ellipse
GraphicsPath gp = new GraphicsPath();
gp.AddEllipse(drawRectangle);
g.SetClip(gp);

// Draw the white reflection gradient
GraphicsPath path1 = new GraphicsPath();
path1.AddEllipse(whiteRectangle); // a smaller rectangle set to the top left
PathGradientBrush pathBrush1 = new PathGradientBrush(path);
pathBrush1.CenterColor = Color.FromArgb(180, 255, 255, 255);
pathBrush1.SurroundColors = new Color[] { Color.FromArgb(0, 255, 255, 255) };
g.FillEllipse(pathBrush1, whiteRect);

Transparent Background

It is important that this control has a transparent background. To do this, we need to override a few methods in our control.

protected override CreateParams CreateParams {
    get {
        CreateParams cp = base.CreateParams;
        cp.ExStyle |= 0x20;
        return cp;
    }
}

protected override void OnMove(EventArgs e) {
    RecreateHandle();
}
		
protected override void OnPaintBackground(PaintEventArgs e) {
    // Do nothing
}

Customizable Colors

You can customize the color of the LED display by setting the Color property. Setting this property also automatically calculates two other Properties ColorDark and ColorDarkDark which are used for gradients when the control is drawn. These colors are calculated using the using the ControlPaint class, a Control helper class in the System.Windows.Forms namespace.

this.DarkColor = ControlPaint.Dark(_this.Color);
this.DarkDarkColor = ControlPaint.DarkDark(_this.Color); 

Points of Interest

This control uses double-buffering by drawing its image to an off-screen bitmap first, then sending the final rendering to the client. Double buffering creates a smoother drawing of the control and avoids flicker when moving or resizing. The code to accomplish this is below. This technique works great on most controls, however the transparent background of this control means that the parent is responsible for drawing the background of our control so we still get a flicker when resizing. Fortunately the LEDBulb probably won’t need to be actively resized, but any suggestions on how to solve this would be appreciated.

protected override void OnPaint(PaintEventArgs e){
	// Create an offscreen graphics object for double buffering
	Bitmap offScreenBmp = new Bitmap(this.ClientRectangle.Width, 
				this.ClientRectangle.Height);
	System.Drawing.Graphics g = Graphics.FromImage(offScreenBmp);
	g.SmoothingMode = SmoothingMode.HighQuality;

	// Render the control to the off-screen bitmap
	drawControl(g);

	// Draw the image to the screen
	e.Graphics.DrawImageUnscaled(offScreenBmp, 0, 0);	
}  

Links

• Double Buffering
• Making Transparent Controls using GDI+ and C#
• Lighten and Darken Colors in .NET

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)