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HTML5 Canvas - Aqua Gauge

, 23 Dec 2011
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Gauge control using HTML5 Canvas

Introduction

We all know HTML5 is picking up fast and it has already almost swallowed Silverlight and Flash. So, I started to learn HTML5 and to find out what was inside that makes it interesting. Having read it, now, I find HTML5 is fun, easy to learn and also feel it is the future of client applications. I started to learn by developing a UI control. As an example, I chose to write a gauge control which I developed few years agon in .NET 2.0 (Aqua Gauge).

Through this application, I have tried to explain some of the HTML5 canvas APIs and a full implementation of Gauge control.

Sample Image

How to Get Started?

All we need is a basic knowledge on HTML, JavaScript, CSS and a text editor tool and, of course, a HTML5 compatible browser. HTML5 has many new features for creating rich internet applications. Here, I’ll be concentrating on "canvas" element. To get started, open a text editor, enter the following HTML code and save it as "index.html".

<!DOCTYPE html>
<html>
<head><title>My first HTML5 App</title></head>
<body>
	<canvas width="300" height="200" style="border: 1px solid red;">
		Your browse doesn't support canvas :-(
	</canvas>
</body>
</html>

Open the index.html file in browser. If you see a red rectangle, then your browser supports HTML5 canvas. Otherwise, you need to upgrade your browser to continue.

Drawing on the Canvas

Now let’s add a few lines of JavaScript code to index.html to draw a rectangle on canvas.

<!DOCTYPE html>
<html>
<head><title>My first HTML5 App</title>
<script language="javascript">
	function drawSomething() {
		var canvas = document.getElementById('drawingBoard');
		var context = canvas.getContext('2d');
		context.fillStyle = "blue";
		context.fillRect(10, 10, 200, 100);
	}
</script>
</head>
<body onload="drawSomething()">
	<canvas id="drawingBoard" width="300" height="150">
		Your browse doesn't support canvas :-(
	</canvas>
</body>
</html>

Draw a Rectangle with Gradient

function drawSomething() {
    var canvas = document.getElementById('drawingBoard');
    var context = canvas.getContext('2d');

    var gradientBrush = context.createLinearGradient(0, 0, 0, canvas.width);
    gradientBrush.addColorStop(0, 'lightblue');
    gradientBrush.addColorStop(1, 'blue');
    context.fillStyle = gradientBrush;
    context.fillRect(0, 0, canvas.width, canvas.height);
}

Gradient Rectange

Draw Text Inside the Rectangle

context.fillStyle = "White";
context.font = "bold 30pt Arial";
var text = "HTML5 Canvas";
var txtWidth = context.measureText(text).width;
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
context.fillText(text, centerX-txtWidth/2, centerY);
context.restore();

Gradient Rectange with text

Add Code to Rotate the Text

context.save();
context.fillStyle = "White";
context.font = "bold 30pt Arial";
var text = "HTML5 Canvas";
var txtWidth = context.measureText(text).width;
var centerX = canvas.width / 2;
var centerY = canvas.height / 2;
context.translate(centerX, centerY);
context.rotate(-20 * Math.PI / 180);
context.fillText(text, -txtWidth/2, 0);
context.restore();

Gradient Rectange with rotated text

Draw a Translucent Circle using arc()

context.save();
//Text drawing code should go here. Below restore() will reset translate() 
//and rotate() operations.
context.restore();

context.fillStyle = "rgba(255, 255, 0 , 0.5)";
context.beginPath();
var x = canvas.width * 0.75;	
var y = canvas.height * 0.50;
var radius = 60;
var startAngle = 0;
var endAngle = 360 * Math.PI / 180;
context.arc(x, y, radius, startAngle, endAngle, true);
context.closePath();
context.fill(); 

Gradient Rectange with rotated text and translucent circle

Aqua Gauge Code

Now we know how to use the canvas element! The complete API specification for canvas is available here. The source code archive contains the following files:

  • Index.html – HTML file to host the canvas element
  • js\AquaGauge.js – contains the gauge implementation
  • js\Helper.js – contains utility methods to update gauge object properties with values from html controls
  • css\Styles.css – style sheet to layout the page. It uses few CSS3 constructs.
  • All other files in the archive are jQuery related files to enable slider controls.

You are now ready to explore the aqua gauge source code. Following are the properties of aqua gauge object.

Property Name Type Description
backgroundColor Color Overall canvas background color
dialColor Color Circular dial area background color
dialGradientColor Color Circular dial area inner background color for gradient
dialTitle String Dial title text. This will be placed at top
dialTitleTextFont Font Dial title text font style
dialTitleTextColor Color Dial title text foreground color
dialSubTitle String Dial sub title text. This will be placed at bottom
dialSubTitleTextFont Font Dial sub title text font style
dialSubTitleTextColor Color Dial sub title text foreground color
dialValueTextFont Font Current value text font style
dialValueTextColor Color Current value text foreground color
showGlossiness Bool Shows or hides glossiness
minValue Int Minimum value
maxValue Int Maximum value
noOfDivisions Int No of graduations/divisions on the scale
noOfSubDivisions Int No of sub graduations/divisions on the scale
majorDivisionColor Color Major scale graduation color
minorDivisionColor Color Minor scale graduation color
rimColor Color Rim arc color
rimWidth Int Rim arc line width/thickness
rangeSegments Array Defines the color ranges on the rim. Should be list of {start: [int], end: [int], color: [color] }
rangeSegments[].start Int Range arc start value
rangeSegments[].end Int Range arc end value
rangeSegments[].color Color Range arc color
dialScaleFont Font Major scale text font style
dialScaleTextColor Color Major scale text color
dialSubScaleFont Font Minor scale text font style
dialSubScaleTextColor Color Minor scale text color
dialScaleTextShadowColor Color Scale text shadow color
showMinorScaleValue Bool Shows or hides minor scale value text
pointerColor Color Pointer color
pointerGradientColor Color Pointer gradient color
shadowColor Color Pointer shadow color
currentValue float Current value the pointer points to

Using the Code

Create an HTML page like how we did in the above sample, then add AquaGauge.js reference and initialize the control as shown below. Use aGauge.refresh(value) to update the gauge current value.

<!DOCTYPE html>
<html>
<head>
    <title>Aqua Gauge</title>
    <script src="AquaGauge.js" type="text/javascript"></script>
    <script language="javascript" type="text/javascript">
        function showGauge() {
            var aGauge = new AquaGauge('gauge');
            aGauge.props.minValue = 0;
            aGauge.props.maxValue = 100;
            aGauge.refresh(45);
        }
    </script>
</head>
<body onload="showGauge()">
    <canvas id="gauge" width="300" height="300">Browser not supported.</canvas>
</body>
</html>

Conclusion

I have just started learning HTML5 and its related components. I’m excited that I was able to write this within a day! However, the code contains no proper validation and it is not tested properly. Hope you liked the article.

History

  • 23rd December, 2011: Initial version

License

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

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About the Author

Ambalavanar Thirugnanam
Architect BNY Mellon
India India
Ambalavanar, working as a .NET Solutions Architect at BNY Mellon (iNautix), Chennai. Enjoys designing and developing UI & highly scalable apps.

Comments and Discussions

 
QuestionVote for 5 Pinmemberjslr0728-Dec-11 2:23 
AnswerRe: Vote for 5 PinmemberAmbalavanar Thirugnanam20-Jan-12 4:39 

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