var width = 50;
var height = 50;
Next, using the context.translate() function call, we must change the origin of the
canvas to be the center of the red square we want to rotate and draw. This function
moves the origin of the canvas to the accepted x and y locations. The center of our red
square will now be the desired top-left corner x location for our object (100), plus half
the width of our object. Using the variables we created to hold attributes of the red
square, this would look like:
Next, we must find the y location for the origin translation. This time, we use the y value
of the top-left corner of our shape and the height of the shape:
The translate() function call looks like this:
Now that we have translated the canvas to the correct point, we can do our rotation.
The code has not changed:
Finally, we need to draw our shape. We cannot simply reuse the same values from
Example 2-8 because the canvas origin point has moved to the center of the location
where we want to draw our object. You can now consider 125,125 as the starting point
for all draw operations. We get 125 for x by taking the upper-left corner of the square
(100) and adding half its width (25). We do the same for the y origin position. The
translate() method call accomplishes this.
We will need to draw the object starting with the correct upper-left coordinates for x
and y. We do this by subtracting half the width of our object from the origin x, and half
the height of our object from the origin y:
context.fillRect(-0.5*width,-0.5*height, width, height);
Why do we do this? Figure 2-14 illustrates the situation.
Consider that we want to draw our square starting at the top-left corner. If our origin
point is at 125,125, the top left is actually 100,100. However, we have translated our
origin so that the canvas now considers 125,125 to be 0,0. To start our box drawing at
the nontranslated canvas, we have to start at −25,−25 on the “translated” canvas.
This forces us to draw our box as though the origin is at 0,0, not 125,125. Therefore,
when we do the actual drawing of the box, we must use these coordinates, as shown in
Simple Canvas Transformations | 53