Gunnar Peipman's ASP.NET blog

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Generating radial indicator images using C#

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.

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.

Alex Peta said:

Really nice example!

But, wouldn't this feel more natural with SVG / Canvas :D, maybe Raphael.js and let the client side do all the "burden" ?

# October 18, 2012 10:16 AM

DigiMortal said:

Thanks for ideas, Alex. Client-side is also option but it needs more testing on mobile devices and mobile browsers as their support for JavaScript and CSS is varying.

# October 18, 2012 10:28 AM

Sandy said:

Thanks for the sharing the nice example here. It helps me a lot!!

# October 19, 2012 1:23 AM

slaneyrw said:

You can avoid the FillEllipse by use DrawArc instead of FillPie.  Use a Pen with a suitable width for the arc and offset the rectangle as appropriate.

If you do this then you could make the rest transparent and overlay the arc+text over a separate dial image.

As an aside, depending on your resolution of percentages you could use a sprite approach and generate all versions of the image in one go and lay them out in a grid.  Then use CSS positioning to "move" the 100x100 window.  Static image, no round trip... just change the css class

# October 24, 2012 12:40 AM