Home > Articles > Web Development > HTML/CSS

An Introduction to the HTML5 Canvas

  • Print
  • + Share This
Larry Aronson, author of HTML Manual of Style: A Clear, Concise Reference for Hypertext Markup Language (including HTML5), Fourth Edition, explores a fundamental challenge to Flash as a platform for graphical Web applications -- the HTML5 canvas. Larry shows how easy it is to create useful web applications using only a simple text editor.
Like this article? We recommend

HTML5 is the World Wide Web Consortium’s (W3C’s) proposed specification for the next generation of the web. It’s the operating manual covering the workings of browsers, search robots, and other web software. After 10 years of relative stagnation in web evolution, HTML5 is a remarkable achievement with many new document elements, attributes and features. The canvas element is one of these.

As its name suggests, the canvas element creates a rectangular drawing surface on a web page. Its associated JavaScript application programming interface (API) provides methods for drawing lines and curves, filling shapes with color, positioning text, loading in external images and applying scaling, rotations, and other transformations.

The Canvas Challenge to Flash

The HTML5 canvas provides an open platform for graphical applications within the context of the web page. Because the canvas API is built into an HTML5 browser, canvas applications should perform better than plugin-based technologies, such as Adobe's Flash, especially on devices that can take advantage of hardware graphics acceleration.

Unlike a Flash application, a canvas-based app is a true HTML element. It can be styled with CSS and can interact with other page elements and scripts, including the HTML5 audio and video elements. Multiple canvases can be superimposed in switchable layers.

What's nice about the HTML5 canvas is how easy it is to use. Here's a quick (but complete) example that creates a simple interactive bar graph. The following code can be copied and saved in a text file, then opened locally in an HTML5 browser. It works with my current versions of Safari, Chrome, Opera, and the Firefox 4 beta on my desktop computer, as well as on the iPhone, iPad, and Android browsers. (Note: HTML and JavaScript comments are in boldface type.)

<!DOCTYPE html>
<html>
<head><title>Canvas Bar Graph</title>
<style type="text/css">
 canvas { border-bottom: thin solid; }
</style>
</head>
<body> 

<!-- Define a 400 x 400 pixel drawing canvas with id = c1 -->
<!-- give it a thin bottom border for the x axis          -->
<canvas id="c1" width="400" height="400">
Sadly, your browser doesn't support the <em>canvas</em> element.
</canvas>

<!-- Define an input field for numbers from 0 to 100    -->
<!-- The form returns without submitting anything       -->
<!-- the draw function is called when the input changes -->
<form onsubmit="return false;">
  <input type="text" size="3"          onchange="drawBar(this.value); this.value = '';" />
       enter a number from zero to 100 and hit return.
</form>

<!-- JavaScript for drawing vertical bars on the canvas -->
<script type="text/javascript">

// get the canvas DOM object and set a 2-dimensional context 
  var canvas = document.getElementById("c1");
  var c1 = canvas.getContext("2d");
  var offset = 5;              // initial x position

// our draw function.
  function drawBar(val) {  
      val *= 4;                // scale the input to pixels
      c1.fillStyle = 'blue';         // set the fill color

// draw a filled rectangle 30 pixels wide with user input height
    c1.fillRect(offset, 400 - val, 30, val);

// set the text color to white and write the value
    c1.fillStyle = 'white';  
    c1.font = 'bold 15px arial,sans-serif';
    c1.fillText(val/4, offset + 5, 420 - val, 26);
    offset += 40;            // offset to the next bar
  }
</script>
</body>
</html>

The canvas element is simple, having no required attributes. It can be given a height and width to override the default of 300x150 pixels. In the canvas element shown above, an id attribute is set that will be used to reference it in the script. The canvas element has a start and end tag; the content inside is fallback markup for older browsers, search robots, and other user agents. The fallback content could include the object, embed, and param elements for a Flash application that would play in legacy browsers.

In the previous example, a CSS rule is used to render the canvas with a thin bottom border to serve as the x-axis of the bar graph. Following the HTML code for the canvas is the markup to create an input form for entering the height of each successive bar. The input element has an onchange attribute that calls a handler function to draw the bar each time the input value changes.

The script block at the end of the example contains JavaScript code to initialize the canvas 2D drawing context and define the drawBar function. The drawBar function scales the input value to pixels, uses the fillStyle and fillRect methods to draw a vertical bar and labels it using the font and fillText methods. Finally, the offset variable is incremented to move the x-axis position for the next bar.

Figure 1 shows what this example looks like in Safari after a half-dozen numbers have been entered. It’s meant to be used with the number and enter keys on a keypad. The graphs can be screen-captured as images for use on blogs and in presentations. You can try out a working demo on my site.

Figure 1 A fast bargraph generator using an HTML5 Canvas element.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020