In one of my projects I needed to draw radial indicators for processes measured in percent. Simple images like the one shown on right. I solved the problem by creating images in C# and saving them on server hard disc so if image is once generated then it is returned from disc next time. I am not master of graphics or geometrics but here is the code I wrote.

Drawing radial indicator

To get things done quick’n’easy way – later may some of younger developers be the one who may need to changes things – I divided my indicator drawing process to four steps shown below.

1. Fill pie	2. Draw circles	3. Fill inner circle	4. Draw text

Drawing image

Here is the code to draw indicators.

private static void SaveRadialIndicator(int percent, string filePath)
{
    using (Bitmap bitmap = new Bitmap(100, 100))
    using (Graphics objGraphics = Graphics.FromImage(bitmap))
    {
        // Initialize graphics
        objGraphics.Clear(Color.White);
        objGraphics.SmoothingMode = SmoothingMode.AntiAlias;
        objGraphics.TextRenderingHint = TextRenderingHint.ClearTypeGridFit;
 
        // Fill pie
        // Degrees are taken clockwise, 0 is parallel with x
        // For sweep angle we must convert percent to degrees (90/25 = 18/5)
        float startAngle = -90.0F;               
        float sweepAngle = (18.0F / 5) * percent;

        Rectangle rectangle = new Rectangle(5, 5, 90, 90);
        objGraphics.FillPie(Brushes.Orange, rectangle, startAngle, sweepAngle);

        // Draw circles
        rectangle = new Rectangle(5, 5, 90, 90);
        objGraphics.DrawEllipse(Pens.LightGray, rectangle);
        rectangle = new Rectangle(20, 20, 60, 60);
        objGraphics.DrawEllipse(Pens.LightGray, rectangle);

        // Fill inner circle with white
        rectangle = new Rectangle(21, 21, 58, 58);
        objGraphics.FillEllipse(Brushes.White, rectangle);

        // Draw text on image
        // Use rectangle for text and align text to center of rectangle
        var font = new Font("Arial", 13, FontStyle.Bold);
        StringFormat stringFormat = new StringFormat();
        stringFormat.Alignment = StringAlignment.Center;
        stringFormat.LineAlignment = StringAlignment.Center;

 
        rectangle = new Rectangle(20, 40, 62, 20);
        objGraphics.DrawString(percent + "%", font, Brushes.DarkGray, rectangle, stringFormat);

        // Save indicator to file
        objGraphics.Flush();
        if (File.Exists(filePath))
            File.Delete(filePath);

        bitmap.Save(filePath, ImageFormat.Png);
    }       
}

Using indicators on web page

To show indicators on your web page you can use the following code on page that outputs indicator images:

protected void Page_Load(object sender, EventArgs e)
{
    var percentString = Request.QueryString["percent"];
    var percent = 0;
    if(!int.TryParse(percentString, out percent))
        return;
    if(percent < 0 || percent > 100)
        return;

    var file = Server.MapPath("~/images/percent/" + percent + ".png");
    if(!File.Exists(file))
        SaveImage(percent, file);

    Response.Clear();
    Response.ContentType = "image/png";
    Response.WriteFile(file);
    Response.End();
}

Om your pages where you need indicator you can set image source to Indicator.aspx (if you named your indicator handling file like this) and add percent as query string:

<img src="Indicator.aspx?percent=30" />

That’s it! If somebody knows simpler way how to generate indicators like this I am interested in your feedback.