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Building an image slideshow control

From: HTML5: Graphics and Animation with Canvas

Video: Building an image slideshow control

So we have reached the point now where we can use what we've learned on the HTML5 canvas to start building some real live useful stuff and one of the first things that we are going to build is a slideshow. So let me show you what the slideshow looks like when it's finished. So this is a fairly typical slideshow. You can see the images are rotating. There are four of them. So when we reach the last one, after a few seconds it goes back to the first one. These images are all being drawn on a canvas, and there is a few-seconds delay between each one.

Building an image slideshow control

So we have reached the point now where we can use what we've learned on the HTML5 canvas to start building some real live useful stuff and one of the first things that we are going to build is a slideshow. So let me show you what the slideshow looks like when it's finished. So this is a fairly typical slideshow. You can see the images are rotating. There are four of them. So when we reach the last one, after a few seconds it goes back to the first one. These images are all being drawn on a canvas, and there is a few-seconds delay between each one.

So let's see how to go and build this. So I'll close this, and we will open up the start version in my editor. So this is SlideShow_start. Let's go over to the snippets. You can see this is way the snippets are. I am under the Slideshow section. So let's get started building the slideshow. So I have got an empty document here with a placeholder for the script. So I am going to copy these first few lines and paste them in.

So this is just some initialization and setup code. So let's just take a quick look at it. I have an array here that has paths to all the images that I am going to show in the slideshow. I have a couple variables that refer to the slideshow canvas and its context, and I am creating a new image element using the DOM, and I have a little indexer here that keeps track of what the current image is. So that's just the setup stuff. Then we have a function, and we will copy this over.

So now we are setting a function to run when the window loads. And the first thing we do is get the reference to the canvas. We've done this before throughout the title, and here we have the reference to the canvas's context. The first thing we do is set the width and height of the newly created image element up here to be 600 x 400, which is the size of the images. Then we have a function call to switchImage, which we will write in second, and then we have an interval which we are going to use to switch the image every 3,000 milliseconds, or three seconds.

So let's go ahead and copy that over. So now we have the switchImage function. So the switch image function simply sets the source of the image object to whatever the current image indexer is looking at in the array of image paths. So it's going to cycle throughout. Here is the path. It's going to start at 0 and then go to 1 and then go to 2 so on and so forth.

So the switchImage is responsible for doing the heavy lifting of the example. It displays the current image and looks at the image in the array that's currently being indexed by the currentImage counter. So up here we have the image paths 0, 1, 2, and so on. So the first thing it does is sets the source attribute on the image object to whatever path is being looked at by the currentImage counter. When that image loads we have a function that runs, and it checks to see if the current image is greater than the length of the image path's array, because if it is, we need to be reset it back to 0 so it starts at the beginning again.

Then we simply call the drawImage function on the canvas context to draw the image at the location on the canvas, starting at the upper-left corner and with the given width and height. And then, because of this interval, the switchImage function is going to be called each time the interval timer goes off, which we've set to three seconds. So, every three seconds the image counter gets incremented, we look at the next image, and we set the canvas context to draw whatever the current image is. So let's save, and let's go back to the browser.

If everything goes right then, yup, you can see every three seconds that image is going to change. This is the last one here. So, after three seconds it should go back to the first one, and it does. That's a pretty simple example of using an interval timer and the canvas's drawImage function to create a slideshow.

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This video is part of

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

37 video lessons · 18339 viewers

Joe Marini
Author

 
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  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

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