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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.
The last transform we're going to take a look at is the custom transform. Now in addition to the transforms that are built into the canvas, you can also define your own. Transforms are defined as a matrix. A matrix has the format that you see here. There are six variables. There is a, b, c, d, e, and f. And to use a transform, a new point is found by applying that transformation matrix to each point in the image that's being transformed. Now, you don't need to panic.
I'm not going to make you learn matrix math, but this does come in handy when you want to apply a transformation that is not built into the canvas. So, for example, canvas gives you transforms for rotation, scaling, and translating, but there are plenty of other transforms out there that you might want to use, and since they are not built in, you would have to use a matrix like this to define one. So we'll see examples of that in a moment, but the way that you apply a custom transform is by using one of the two transform functions. The first one is simply called transform, and it takes these six arguments, which correspond to the variables you see in this matrix here.
The transform function takes the transform you're defining here and adds it to whatever the current transform of the canvas currently is. So if the canvas already has a transform, like a rotation or a scale or a resulting transform from a series of transforms that came before it, then this one will simply be added to that one, because remember, transforms are additive. The second one is called setTransform, and it takes the same six arguments. The main difference between setTransform and transform is that setTransform resets the canvas to be what's called the identity transform, which is a transform that just doesn't do anything.
So in other words, it erases all the previous transforms and then applies the one you're defining here. So this one is additive. This one is not additive. This one resets the canvas and then apply the transform that you're defining. Let's take a look in the code to see how this stuff works. So here I am in the code. I've got my snippets file open, and I'm at the custom transform area, so let's open up our custom transform example. It's called transform_start.
Let's do something simple that we've already seen before. So, let's first copy these two lines, and we'll paste those in. Now, you've seen this already. Basically, we just take a blue rectangle and put it on the canvas, so let's just make sure that that works, and it does work. That's good. All right, so let's go back to the code. What we're going to do now is recreate the translate transform using our custom transform. So we'll copy this and we'll paste it over, and I'll explain how it works.
So a translation matrix for a translate transform looks like this. Those six variables are right here. And the first row is 1 0 and then the amount you want to translate the x by, and then the second row is 0 1 and the amount you want to translate y by. So remember, the variables go in the order of a, b, c, d, e, and f. So let's define that transform. We're going to make the new rectangle red, so that's the fillStyle there. Just like in the translate example, we're going to translate the rectangle to the center of the canvas.
So tx is going to be the width of the canvas divided by 2. The y will be the canvas height divided by 2-- that gives us the middle of the canvas. And then instead of using the translate function, we're just going to call the transform function, and we pass in 1, 0-- that's these two right here--then 0, 1--that's these two right here--and then tx and then ty. That will add this translate transform to the canvas, then we'll fill the rectangle, and then we'll call the reverse transform, which translates us back to the upper left of the canvas--just in case we had any drawing that came after this.
So we save and then let's go to the browser and let's refresh. And you can see that the blue rectangle is drawn at 0, 0. The red rectangle is also drawn at 0, 0, but the new origin has now been translated to right there. So that seems to have worked fine. Let's go back to the code. So you can see right there, even though the rectangle is placed at 0, 0, because of the translation, the origin was moved to the middle of the canvas. Let's create a new transform. So now we're going to go back to snippets, and we're going to copy this example.
Okay, so we copy and we'll come over here and we'll paste. In this example, we're going to create what's called a skew transform. Now a skew transform--sometimes it's called a shear transform--and in this case we're going to have a green rectangle and we're going to shear the objects using various directions x and y. So, a skew transform is defined by the number 1 and then the amount you want to skew in the y direction, then the amount you want to skew in the x direction, the number 1 here, and then these two are left at 0.
So we're going to use a green rectangle for this one. And the skewing factor is defined by a multiplication factor, so I'm going to multiply by 0.2 in the x direction and I'm going to leave the y direction alone. Now in this case, I'm going to use the setTransform function, and remember, this resets whatever the transform is that's currently on the canvas. So, I don't have to worry about any transforms that came before me. And then we're going to fill the rectangle at this location, the point 250 by 20, and it will be 100 wide and 50 high.
So let's save, and let's go back to the browser and let's refresh. You can see now that I've drawn a green rectangle that's been skewed in the X axis direction, so it's been sheared a little bit. Let's make some changes and see how that works. Let's go back to the code. I can increase the amount of shear. I can say this is 0.4, and let's refresh the browser now. You can see that the shear amount got a little bit more. All right, let's try it in the y direction. So I'll set this to 0, and we'll make this 0.3 and we'll save and we'll refresh.
And you can see that this time the rectangle got sheared in the Y axis. Now what we have done here is create a new transform that is not built into the canvas. And just for good measure, let's make sure it works in other browsers. So we'll bring this up in Firefox, and you can see, the effect is the same. Let's bring it up in Chrome, and you can see the effect is the same there, too. So what we have learned here is how to use the custom transform matrix to recreate a transform we already know how to use, but we also saw how to create a new transform that's not built into the canvas.
You can look up all kinds of transform matrices on the Internet and just plug in the values into that transform function. And doing so, you can create transforms that are not built into the canvas that you can use in your own drawings.
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