Easy-to-follow video tutorials help you learn software, creative, and business skills.Become a member

Using the Canvas drawing context

From: HTML5: Graphics and Animation with Canvas

Video: Using the Canvas drawing context

To actually draw things onto the canvas, we need to use the canvas's 2D drawing API and to do that, let's take a look at how it works. Now drawing on a canvas requires a series of steps. And in step one, we need to retrieve a reference to the canvas; in other words, we have to actually get the canvas element out of the page. Once we've got a reference to the canvas element, we get the drawing context from it using the getContext () method that we talked about a little bit earlier. Now if the result of the getContext () call is not null, then you know you have the drawing API and you can go ahead and start using it.

Using the Canvas drawing context

To actually draw things onto the canvas, we need to use the canvas's 2D drawing API and to do that, let's take a look at how it works. Now drawing on a canvas requires a series of steps. And in step one, we need to retrieve a reference to the canvas; in other words, we have to actually get the canvas element out of the page. Once we've got a reference to the canvas element, we get the drawing context from it using the getContext () method that we talked about a little bit earlier. Now if the result of the getContext () call is not null, then you know you have the drawing API and you can go ahead and start using it.

Now to do this in script, let's imagine we have the following example. We've the canvas tag that's defined. This is the same way we've been defining our canvas tags up until now. We've got an ID, width, height, and some fallback content. So to draw in script, this is how it goes. Typically, you would do something like define a function. You would call getElementById to get the canvas element. Once you've got the canvas element, you then ask for its context. So here we are saying canvas element.getContext, and we're passing in 2D, because we want the 2D drawing API. And if that result is not equal to null, then we can go ahead and draw, and then you close the function and off you go.

So the canvas drawing API in 2D is pretty comprehensive. It's broken up into three groups. The first group has to do with shapes. There are rectangles, lines, arcs, paths, colors, and styles. There are different kinds of curves, such as Bezier and Quadratic, and then there is text. The next group has to do with how things are drawn. There are various compositing properties we can use, patterns and gradients. You can create shadows. You can also create clipping paths, and we'll investigate all of these later. And then finally there are things like transforms, images and video, and you can even access the raw pixels of the canvas in order to do some pretty interesting effects. And throughout the rest of this course, we'll see how to do each one of these.

So let's take a really simple example to begin with. Let's actually draw on the canvas using the 2D drawing API. So I'm over here back in my code editor again, and I'm going to open up my example. And in this case, I'm going to use the contextdraw_start example, under chapter 04, so I open that. So here is our file. There is our canvas. Here is the code we're going to write. I can go copy these lines from the snippets under ContextDraw. So I'll copy, and we'll go back to the source code and we'll paste it in here.

So what we're going to do now is use the 2D drawing API to put that light gray background that we saw earlier into the canvas. Now, I don't expect you to necessarily understand all this right away. It's just a simple example to show you how this works. So first, we get a reference to the canvas element by calling getElementById canvas. Then we say hey, do we have the canvas object, and is there a method named getContext? And if there is, we then call the getContext method using 2D as the argument.

That gets us the 2D drawing API. And then finally, if all that was successful, on the context, we say set the fill style to be light gray, and then we're going to call a function called fillRect and fillRect takes a couple of arguments. There is the origin to start drawing at, 0,0, and then the width and height of the rectangle you want to draw--in this case, the width and height of the canvas. So you've probably guessed by now what this does. It's going to fill the entire canvas with a light gray background. So let's go ahead and save. All right, let's go over to the browser.

So I am going to bring this up in the browser, and you can see that's exactly what happens. So now the canvas has been filled in. Let's go ahead and view this in another browser to make sure it is working correctly. I'll open this in Firefox, and there is the same result. So that's a pretty simple example of using the 2D drawing API to fill a rectangle with a gray color, but it gets a lot better than that.

Show transcript

This video is part of

Image for HTML5: Graphics and Animation with Canvas
HTML5: Graphics and Animation with Canvas

37 video lessons · 18346 viewers

Joe Marini
Author

 
Expand all | Collapse all
  1. 4m 54s
    1. Welcome
      1m 3s
    2. Using the exercise files
      2m 3s
    3. Using the HTML5 Canvas element in the real world
      1m 48s
  2. 10m 31s
    1. Real-world example: CanvasMol
      2m 9s
    2. Real-world example: Raphaël-JavaScript Library
      1m 47s
    3. Real-world example: The Wilderness Downtown
      4m 1s
    4. Real-world example: Sketchpad
      1m 10s
    5. Real-world example: Pirates Love Daisies
      1m 24s
  3. 3m 28s
    1. Installing the tools
      1m 29s
    2. Exploring the Canvas examples used in this course
      1m 59s
  4. 8m 58s
    1. Introducing the Canvas tag
      6m 30s
    2. Understanding the differences between Canvas and SVG
      2m 28s
  5. 5m 36s
    1. Identifying the Canvas element's methods and properties
      1m 40s
    2. Using the Canvas drawing context
      3m 56s
  6. 43m 14s
    1. Setting and using colors and styles
      3m 19s
    2. Drawing basic shapes: Rectangles and lines
      10m 21s
    3. Understanding the Canvas state
      5m 15s
    4. Drawing complex shapes: Arcs and paths
      9m 15s
    5. Drawing complex shapes: Bézier and quadratic curves
      5m 46s
    6. Rendering text
      9m 18s
  7. 32m 35s
    1. Creating shadows
      6m 41s
    2. Drawing with patterns
      7m 20s
    3. Drawing with gradients
      6m 18s
    4. Using clipping paths
      4m 46s
    5. Drawing images and video
      7m 30s
  8. 35m 42s
    1. Transforming objects using the translate tag
      4m 18s
    2. Scaling objects with the scale transformation
      4m 7s
    3. Rotating objects with the rotate transformation
      4m 33s
    4. Applying a custom transformation
      6m 58s
    5. Compositing in Canvas using globalAlpha
      6m 36s
    6. Manipulating raw pixels
      9m 10s
  9. 41m 23s
    1. Building an image slideshow control
      4m 24s
    2. Using smooth transitions in a slideshow
      4m 28s
    3. Creating a basic animation
      5m 42s
    4. Creating animation with double buffering
      13m 13s
    5. Incorporating Canvas into a real page
      13m 36s
  10. 48s
    1. Goodbye
      48s

Start learning today

Get unlimited access to all courses for just $25/month.

Become a member
Sometimes @lynda teaches me how to use a program and sometimes Lynda.com changes my life forever. @JosefShutter
@lynda lynda.com is an absolute life saver when it comes to learning todays software. Definitely recommend it! #higherlearning @Michael_Caraway
@lynda The best thing online! Your database of courses is great! To the mark and very helpful. Thanks! @ru22more
Got to create something yesterday I never thought I could do. #thanks @lynda @Ngventurella
I really do love @lynda as a learning platform. Never stop learning and developing, it’s probably our greatest gift as a species! @soundslikedavid
@lynda just subscribed to lynda.com all I can say its brilliant join now trust me @ButchSamurai
@lynda is an awesome resource. The membership is priceless if you take advantage of it. @diabetic_techie
One of the best decision I made this year. Buy a 1yr subscription to @lynda @cybercaptive
guys lynda.com (@lynda) is the best. So far I’ve learned Java, principles of OO programming, and now learning about MS project @lucasmitchell
Signed back up to @lynda dot com. I’ve missed it!! Proper geeking out right now! #timetolearn #geek @JayGodbold
Share a link to this course

What are exercise files?

Exercise files are the same files the author uses in the course. Save time by downloading the author's files instead of setting up your own files, and learn by following along with the instructor.

Can I take this course without the exercise files?

Yes! If you decide you would like the exercise files later, you can upgrade to a premium account any time.

Become a member Download sample files See plans and pricing

Please wait... please wait ...
Upgrade to get access to exercise files.

Exercise files video

How to use exercise files.

Learn by watching, listening, and doing, Exercise files are the same files the author uses in the course, so you can download them and follow along Premium memberships include access to all exercise files in the library.


Exercise files

Exercise files video

How to use exercise files.

For additional information on downloading and using exercise files, watch our instructional video or read the instructions in the FAQ.

This course includes free exercise files, so you can practice while you watch the course. To access all the exercise files in our library, become a Premium Member.

Are you sure you want to mark all the videos in this course as unwatched?

This will not affect your course history, your reports, or your certificates of completion for this course.


Mark all as unwatched Cancel

Congratulations

You have completed HTML5: Graphics and Animation with Canvas.

Return to your organization's learning portal to continue training, or close this page.


OK
Become a member to add this course to a playlist

Join today and get unlimited access to the entire library of video courses—and create as many playlists as you like.

Get started

Already a member?

Become a member to like this course.

Join today and get unlimited access to the entire library of video courses.

Get started

Already a member?

Exercise files

Learn by watching, listening, and doing! Exercise files are the same files the author uses in the course, so you can download them and follow along. Exercise files are available with all Premium memberships. Learn more

Get started

Already a Premium member?

Exercise files video

How to use exercise files.

Ask a question

Thanks for contacting us.
You’ll hear from our Customer Service team within 24 hours.

Please enter the text shown below:

The classic layout automatically defaults to the latest Flash Player.

To choose a different player, hold the cursor over your name at the top right of any lynda.com page and choose Site preferencesfrom the dropdown menu.

Continue to classic layout Stay on new layout
Exercise files

Access exercise files from a button right under the course name.

Mark videos as unwatched

Remove icons showing you already watched videos if you want to start over.

Control your viewing experience

Make the video wide, narrow, full-screen, or pop the player out of the page into its own window.

Interactive transcripts

Click on text in the transcript to jump to that spot in the video. As the video plays, the relevant spot in the transcript will be highlighted.

Are you sure you want to delete this note?

No

Your file was successfully uploaded.

Thanks for signing up.

We’ll send you a confirmation email shortly.


Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

Keep up with news, tips, and latest courses with emails from lynda.com.

Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

   
submit Lightbox submit clicked
Terms and conditions of use

We've updated our terms and conditions (now called terms of service).Go
Review and accept our updated terms of service.