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Scaling objects with the scale transformation

From: HTML5: Graphics and Animation with Canvas

Video: Scaling objects with the scale transformation

The scale transformation causes drawing operations to be multiplied by a scale factor in the X and Y directions. And you can scale operations in the X and Y directions independently. You can provide a scaling factor for X and a scaling factor for Y. So the way that scale transformation works is pretty simple. So suppose I had a rectangle that was 20 pixels wide and 10 pixels high. Well, if you put that through a scale transform and I pass in say a factor of 2 for both the X and the Y direction, the result of that would be, if I call the same function, even though I said fill the rectangle with 20 pixels wide and 10 high, because of the scale factor, this would be 40 pixels wide and 20 pixels high. So let's take a look at that in code and see how it works.

Scaling objects with the scale transformation

The scale transformation causes drawing operations to be multiplied by a scale factor in the X and Y directions. And you can scale operations in the X and Y directions independently. You can provide a scaling factor for X and a scaling factor for Y. So the way that scale transformation works is pretty simple. So suppose I had a rectangle that was 20 pixels wide and 10 pixels high. Well, if you put that through a scale transform and I pass in say a factor of 2 for both the X and the Y direction, the result of that would be, if I call the same function, even though I said fill the rectangle with 20 pixels wide and 10 high, because of the scale factor, this would be 40 pixels wide and 20 pixels high. So let's take a look at that in code and see how it works.

So here I am in the code editor, and I've got my Scale Transform section of my snippets. So let's open up our scale example. So here is our scaling code. Let's go back to the snippets. So I'm going to copy this guy over and paste. So, a couple of things I want to point out. First, I'm using the blue fill style to fill the rectangle at the 0,0 location.

So what I'm going to do now is perform a scale transform and draw the same size rectangle again, only at a different location. So here you can see the width and the height of the same. I'm just offsetting this rectangle so we can see it. I'm also using the save and restore in order to save the drawing context of the canvas and restore it after doing any drawing. So let's save. Let's go out to the scale_start. So you can see, here is the rectangle, right. Here is the first one. Here is the second one.

Now they're both being told to draw at the same size, but because of the scaling factor, this one is larger. So let's take a look at where save and restore canvas state come in handy. So suppose these weren't here, and suppose I had another call to scale, only in this case I want to scale things down. So I would say, instead of 1.50.5-- and let's change this one to 0.5 as well, and I'll do the same thing with the fillRect, and in fact I'll offset this one a little bit--325--so we can see it.

Now let's go over to the canvas and see what happened. We'll refresh. Actually, I should probably move it a little bit more. So you can see that the scale factor is not really affecting this the way that we'd expect it to, because what's happening is I'm first doing a scale-up and then a scale back down. If I put save and restore around this, watch how that changes things.

Now we save, and then we go back out. Now let's refresh. See, now things are really being shrunk down. So what was happening was, since I was calling this scale function after this one--remember, transforms are additive. So I go from a scale factor of 1 to a scale factor of 1.5 and then 2. Then I go to scale factor of a half. Well, that only cuts this scale factor in half, not the original one. But by putting the save and restore around it, this scale function operates on the drawing context's regional scale factor, which is 1 and 1 in both the X and Y directions.

So now we've seen how to use a scale transform and we've even seen how to use save and restore to save the canvas's drawing context so that transforms don't interfere with each other.

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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 · 18690 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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