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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.
There are three ways to draw images. The first one is the simplest. drawImage takes a source image, and the source image is either an image, a video, or a canvas, and it draws that image onto the destination canvas at the point specified by dx and dy. So that's the destination point where the image will be drawn. The second version drawImage again takes the source image that you want to draw and it draws it onto the destination canvas at the location point dx and dy, but it scales the image to fit into the width specified by dw and the height by dh.
The last version of drawImage takes a source image but also takes two sets of arguments that specify how the source and how the destination should be drawn. It takes a source x and source y as well as a source width and source height. So you can actually draw only a portion of the source image onto the destination canvas at dx, dy, and the given width and the given height. So that sounds pretty complex. It's probably a lot easier to see an illustration of how each of these work. So let's take a look.
The simple version of this takes the image and dx and dy. So you have the source image, and then you have the destination canvas, and this version just simply takes the source image and copies it directly onto the destination canvas at the dx, dy point. The next version up from there takes the image and draws at the location, but optionally scales it. So once again, you have the source image. You have the destination canvas. But in this case I've drawn the image onto the canvas at the destination point. But in this case it's been scaled to be a little bit larger.
You can also scale it to be smaller. Now, the last example is a little bit more complex. So it takes the source image and optionally a portion thereof and draws it onto the destination canvas. So here you have the source image and the destination canvas right there. But now, you've got only a portion of the image that you are copying. So you copy that portion onto the destination canvas specified by the arguments that you pass. Let's take a look at these in action and see how they work in the real world.
So here I am in my example. This is the snippets, and I've got my images_start file open. So the first example I'm going to show is just drawing an image directly onto the canvas. So I'm going to copy, and I'm going to come in here and I'm going to paste. So what I'm going to do is get the image element by using getElementById and then just simply draw that image right onto the canvas. So let's scroll down here in the document, and you can see that I've got this image down here as well as a video, and this is the source for that image.
And you can see that I've got both of these elements are set to not display, so they don't show up in the web page when we have them in the browser. So let's take a look at each one of these. So, here are the images. So this is the image I'm going to be using-- that's a nice little shot of Lake Tahoe--and this video right here. So I'm going to draw this video onto the canvas, and it's just a basic video. So, that's that. So let's take a look at the first example. Scroll back up. Again, pretty straight forward. Just get the image and draw it onto the canvas.
So we'll save and you can see it works. So the image is being copied and then pasted onto the canvas. Let's take a look at another example. So for the next example, I'm going to draw the image scaled down onto the canvas using the second version of the drawing routines. So I'm going to get rid of this one to avoid any competition. There we go! Let's get rid of that comment. So now this is the second version of the drawImage function. So, same idea; draw out in a location, but scale it down.
In this case, it looks like I am going to scale it down by about half. So I'll save and refresh. We can see that that worked, and in fact let's try it in another browser. Let's open that in Firefox. Yup! So that worked as well. Let's go back to the code. All right! So for the next example, now we're going to draw just a portion of the source image onto the destination canvas. So I'm going to copy that, go back to the sample, and paste.
So what we're going to do is specify a rectangle that starts in the source image--so we have the source image right here. We're going to copy from this portion of the source onto this part of the destination canvas. So, let's save. What we're going to do is just get a copy right here. This little island is going to be what's taken out of the source image and drawn. So we refresh. We can see that that's what happened. Let's go back to Firefox, do the same thing, Refresh.
You can see that in both cases a portion of the source image was essentially cropped and drawn into the destination canvas. And then for the last example, let's actually draw some video. Now, drawing video is a little bit challenging because you can't just simply say, hey, draw this video. The drawImage function is going to draw whatever the current playback frame happens to be. So here's how we need to do this. First, let's scroll down.
So, here's the video. The video tag is right here. And you can see that it's set to loop, and it's currently hidden. It's also not playing. There's no auto-play in here. So what we are going to do is get the video element, tell the video element to go ahead and play, and then we're going to set a interval function, which is going to be called every 16 milliseconds. So every 16 milliseconds, we're going to get the canvas, get the drawing context, and get the video element, and then we're just going to draw the image from the source video directly onto the canvas.
So this will grab each frame as it's being played, every 16 milliseconds, and draw the video. So let's save. Let's refresh. So what's happening is the video element is actually down here, and this is the canvas. But the interval function is being called every 16 milliseconds which gives us the frame rate that's needed to play the video back. So let's try it out in Safari because we haven't picked on Safari yet, and we can see it's working in Safari as well. So, what have we seen? We've seen ways to draw images onto the canvas, and we've seen ways of drawing source portions of the image.
We've seen how to resize images, and we even saw how to take video content from a video element and play it back on a canvas.
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