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One of the most exciting additions that HTML5 offers to designers is the ability to draw free-form graphics on a drawing surface known as the Canvas. In this course, author Joe Marini introduces the technical concepts behind Canvas and shows how to perform drawing operations directly in a web page. The course covers drawing basic and complex shapes, setting colors and styles, adding shadows, patterns, and gradients, more advanced techniques such as scaling, rotating, and compositing objects, and how to incorporate Canvas elements in a slideshow and an animation.
In this example, we're going to see how to incorporate a canvas into a real web page. And to do that, we are going to use the Explore California example site that comes with your exercise files. So, here I've got the ExploreCalifornia exercise files ready to go. Let me first show you the page that we are going to build. So, if I bring up the index page, this is the Explore California web site that we've been working with. And I am going to click on the TOURS section, and you can see down here there is a list of tours.
What we are going to do is we are going to build a canvas-based tour finder, and here is what we are going to do. Let's go to the TOUR FINDER page. So, right now, you can see this example is empty, but let me show you the finished version so you can see what it is going to look like. Okay, it's going to look like this. So, we will have a canvas which displays a graph of some attributes of a given tour, and it will show us things like the duration, elevation, environment, and overall difficulty of a given tour.
So you see, as we choose different tours, the graph is changing to show us what the overall difficulty is. In this case, if it's easy, it turns green and if it happens to be a moderate tour, it turns yellow. And in the case of really difficult tours, you can see that the difficulty bar turns red. And you notice how as I am choosing each one of these tours, the graph is changing right here in the page. So, to do this without using canvas, you would have a couple of choices. You would either have to use some other dynamic media solution, such as a plug-in like either Silverlight or Flash, or you would have to build each one of these as a separate graphic image and then swap out the image whenever a new tour was chosen.
However, each time you added a new tour or if you changed some of the data associated with the particular tour, you would then have to either make a new image or go back and change the existing one, which is kind of labor intensive and is not really well suited to data intensive examples. Because again, let's suppose somebody goes and does some work on this particular tour and makes it easier. In the case of using the canvas, all you have to do is change one little data point and then the canvas graph is going to update automatically. To do this with an image or some other complex dynamic media solution, there would be a lot more work involved.
So, this is what we are going to build, so let's go build it. So, we are going to close this. Let's go to the code. So, here are the snippets for the Explore California example here in chapter 08. And I am going to go open up the starting point file, and here we are in ExploreCalifornia. So, open up the tourcompare page, and we're going to go ahead and scroll on down, and I'll show you where we are going to put this. So, we are going to put this right down here where it says tourDescriptions.
So, let's go back to our snippets and paste in the content we are going to need to let the user choose a tour, and that's down here. So, to do that, we are going to need this snippet of code, and I will paste it in and explain what it does. And we are going to put that right there. So, what I have done is I've pasted in some HTML code that has a select list in it, which is called tourSelector. And you can see that there's a series of options that allow me to choose the different tours. And then below that, I have my canvas, and you can see that my canvas has an id on it, and it's got a width and a height. And I put a little one- pixel solid border around it so we can see it in the page.
So just doing that, let me just save and let's go back to the browser and see how that looks. So, let's just go ahead and bring the tourcompare page up. And you can see that what I have done so far is here is the canvas and here is that select list. So, we are already pretty well on our way. All you do now is draw the canvas and then respond to changes in this menu such that when the menu changes, we redraw the canvas. So, let's go back and do that. Here we are back in the code. So, let's scroll up to the script portion, because that's where the rest of this is going to take place, and go back over to the snippets.
So, let's begin by copying the first few lines of code. And I will just copy this and explain it. And I'll put the closing script tag on there. So, what I have here is an array of tourdata, and inside that array, there is a series of objects, and there are five of them for the five tours. There is a few properties associated with each object. There is the tour name.
There is the tour duration. And I just chose these numbers arbitrarily on some scale, so these are just going to happen to work out to the size of the bars on the chart, but if you had some other prior existing data, you could put whatever you wanted in a chart like this and then just do whatever conversion you need to do to make the chart draw the right size bars. In this case, just to make the example simple, I have just got a 1:1 relationship between these numbers. So, for example, a duration of 200 means two days and so on and so forth. So, we have got duration.
You got values for elevation, value for how extreme the environment is, and then an overall difficulty rating. So, there is five objects, and then I have got a couple of variables here that are going to hold a reference to the tour canvas and the context for that canvas. So, let's go back to the snippets. Now, let's copy over the drawChart function. This is the function that's going to get called every time the select list index changes. So, this will take care of drawing the chart backgrounds, drawing the labels, and drawing the data.
So, let's copy over those. Let's copy over drawBackground first. So, I will copy this function over and we'll paste it. And for the time being, what we will do is we'll fill in little empty stubs for draw labels and draw tour data. So, we'll say function drawLabels (forWhichTour) and we will have a function drawTourData again (forWhichTour).
So, now we have those functions stubbed out and we can go ahead and take a look at the code. Actually, before we do that, let me do one more thing. Let's copy over the initialization code that sets up the chart when the page first loads, and that's this code right down here. So, here I'm using the jQuery code to create a function that runs when the page loads, and for the moment, let's go ahead and comment this one out.
What it's doing is it gets reference to the canvas, then gets the canvas's drawing context, and then it calls drawChart with 0 to initialize the chart to the first set of tour data. So, let's go ahead and save, and let's review this in the browser. Let's refresh. And you can see that right now we've got the canvas background drawing and the drawing area, so let's go to the code and see how that works. So, up here in the drawBackground, a couple of things happened. First, I save off the canvas context.
Again, I just do that because it's a nice practice in case any drawing came before this function got called. So, the first thing we do is fill in the chart background. So, we set the fillStyle to that brownish color, and then we call fillRect starting from the upper left and doing it for the entire width and height of the canvas. That's pretty simple. Then we create the graph area. So, we change the strokeStyle to a somewhat darkish gray color, and then we call strokeRect, and that's what gives us the outline of the area where we are going to draw the bars. Then we need to draw the chart lines. So, we change the strokeStyle to a slightly lighter gray color, and then we have a loop that starts at a certain point on the canvas, at the x point, goes to a certain x point, and increments every 20 pixels.
So, inside that loop, we begin a path, and then we just draw each line and stroke it, and that just creates that little bars across the drawing area, and then we are done. We restore the canvas context, and the function exits. So, now let's fill in the drawLabels function. The drawLabels function is responsible for two things: it draws the tour name and it draws the labels. So, let's copy that code and paste it in. So, again, we save and restore the context each time.
Again, that's a nice little practice-- not mandatory in this case, but we are doing it anyway. So, now we draw the tour name, so we set the font to be 24pt Arial, and we set the fillStyle to be the blue text color, and then we just call the fillText routine. FillText takes the constant string tour name plus the tour data's name property for whatever the currently selected tour is and draws that text at this location on the canvas, so that's that part. And for the labels, we change it to 20pt Arial and the fillStyle for that color, and then we draw the labels for duration, elevation, environment, and difficulty.
Again, we just use the fillText function at various points on the canvas. Let's save and let's refresh. So, we can see this being built up slowly over time. So, you can see that here's the labels that we drew on the canvas using the text function. Here's the tour name for the currently selected tour. And it starts out to 0, so that's the first data object. Now, what we need to do is draw the bars for the various data points and respond to changes in this menu. So, back in the code, let's go ahead and uncomment this function right here.
So, what we're doing is we are using jQuery to listen for the change event on the select list. Remember, we had the id of tourSelector. So, we are listening to the change event, and whenever the change event fires, we are simply calling a function that calls drawChart and we are getting the selectedIndex of the select list. So, this will go from 0 up to 4 because that's how many options there are in the select list. So, now that we've got that working, we can save.
And before we go ahead and draw the data, let's just go ahead and see the effects of that. So, we refresh and now you can see that as I change the menu, it's changing the currently selected tour, so the title is changing. All right, so now let's go back to the code, and now let's fill in the part where we actually draw the data and that's down here. So, what we are going to do is copy this, and we are going to paste it in to the drawTourData part.
So, let's go ahead and refresh the browser and see what the effects of this was. So, now we are getting the data drawn as well, and you can see that each time we are changing the selection, the data changes. So, let's go back to the code and explain what happened. As you may have guessed, drawing each one of those bars on the chart is just a rectangle. So to do that, recall that we set the strokeStyle and the fillStyle for the rectangle, and then it is just a matter of drawing each bar for the four properties. And remember, the forWhichTour index gets passed in from the function for drawing the chart, so we use that as an index into the tourData array.
So, we get each object and we just get the duration, elevation, environment, and then the difficulty for that particular tour. So, you can see that that's just a simple matter of calling fillRect and strokeRect for each one of those data points. At a particular point on the canvas starting here, they get this width. The value of that data field in that object just serves as the width, and each one of those bars is 30 pixels high. So, we fill each rectangle, and then we stroke it with a grayish color to give it an outline. So, we do that for each one of the data points.
Now, notice for the other properties, it's always yellow. But for difficulty, we are actually going to draw different colors based upon whether the tour is easy or hard or moderate. So, we've chosen a green color for easy yellow, yellow for moderate, and red for hard. So, that's just a simple matter of changing the fill style based upon what the difficulty rating is. So, we check the difficulty rating, and if it's less than or equal to 150, we choose the green color. If it's between 150 and 300, we choose the yellow color. And if the difficulty is more than 300, we choose the red color. And then we just fill and stroke the difficulty rect and draw it appropriately.
So, we save, refresh. And you can see now that as we choose each tour, we are getting a real-time chart drawn in the page. And let's just go ahead and try this in another browser. I am going to switch over to Firefox. Okay, here we are in Firefox. You can see, again, the same appearance, same behavior, right. So, we have built an interactive tour finder using canvas. So, let's just recap really quickly.
So, using the canvas, we built an area here in the page and we hooked up a select list whose change event we used to get the selected index to choose which tour data we are going to draw. We used rectangles, text, we used some paths in or to draw the canvas, and that's a real live example of incorporating campus into a real web page.
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