Question

Shapes

Write a program that draws the following shapes on a canvas:

1. A trapezoid (a rectangle that is wider on one side)

2. A red diamond (a rectangle rotated 45 degrees or ¼π radians)

3. A zigzagging line

4. A spiral made up of 100 straight line segments

5. A yellow star

When drawing the last two, you may want to refer to the explanation of Math.cos and Math.sin in Chapter 13 , which describes how to get coordinates on a circle using these functions.

I recommend creating a function for each shape. Pass the position, and optionally other properties, such as the size or the number of points, as parameters. The alternative, which is to hard-code numbers all over your code, tends to make the code needlessly hard to read and modify.

The pie chart

Earlier in the chapter, we saw an example program that drew a pie chart. Modify this program so that the name of each category is shown next to the slice that represents it. Try to find a pleasing-looking way to automatically position this text, which would work for other data sets as well. You may assume that categories are no smaller than 5 percent (that is, there won’t be a bunch of tiny ones next to each other).

You might again need Math.sin and Math.cos , as described in the previous exercise.

A bouncing ball

Use the requestAnimationFrame technique that we saw in Chapter 13 and Chapter 15 to draw a box with a bouncing ball in it. The ball moves at a constant speed and bounces off the box’s sides when it hits them.

Precomputed mirroring

One unfortunate thing about transformations is that they slow down drawing of bitmaps. For vector graphics, the effect is less serious since only a few points (for example, the center of a circle) need to be transformed, after which drawing can happen as normal. For a bitmap image, the position of each pixel has to be transformed, and though it is possible that browsers will get more clever about this in the future, this currently causes a measurable increase in the time it takes to draw a bitmap.

In a game like ours, where we are drawing only a single transformed sprite, this is a nonissue. But imagine that we need to draw hundreds of characters or thousands of rotating particles from an explosion.

Think of a way to allow us to draw an inverted character without loading additional image files and without having to make transformed drawImage calls every frame.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.