Bitwise Moo
Fluid curves with Smooth.js and HTML5 canvas

To show off the sort of stuff you can do with the Smooth.js interpolation library, I’ve put together a demo showing how it can be used to perform highly customizable interpolation of two dimensional points. The interface code depends on jQuery and jQuery UI. You can check out the CoffeeScript source for this demo.

There’s a lot going on in there, and most of it is code for handling the UI, but I want to talk specifically about the drawing code.

The main function driving the drawing is the `addCurveSegment` function, which takes the set of points, and adds the segment from `points[i]` to `points[i+1]` to the passed context. I’ll talk further down this post about the reason for handling each curve segment separately like this, but for now, let’s see how `addCurveSegment` works.

So first we do the obvious; we turn the array into the function `s` (since Smooth.js uses lazy evaluation, recreating `s` each time is inexpensive).

The next chunk of code needs some explanation. Basically what we want to do is turn the parametric function `s` into a sequence of connected line segments approximating the function.

The most obvious way to do that is to vary `t` from `i` to `i+1`, stepping by, say 0.1. Unfortunately, that’s not good enough for longer curve segments, because you need the lines to be short enough that the curve appears smooth.

So the next obvious step is to estimate the length of the curve (by breaking it into a few segments via the previous method), and then divide that into the desired line length to get your step size for `t`. That gets us closer, but it’s still not good enough. The problem is that the distance traveled as you increase `t` is not constant, so you’ll get some short segments and some long segments, resulting in hard corners.

Ideally, we’d like a solution where we can figure out exactly how much to increase `t` to travel x distance. For some kinds of curves, you can use calculus to get a closed form solution for this. For others, you can use more expensive numerical methods.

Instead, we’ll exploit the fact that for sufficiently small intervals along `t`, distance traveled does remain approximately constant as `t` increases. In fact, for our purposes, simply splitting the curve into two pieces does the trick. Within each of those pieces we once again estimate the length of the piece and then divide that into the desired line length, and we get a nice smooth result.

### redraw

The redraw function is straightforward and there’s not much point in pasting the code here. In short, it clears out the canvas, calls `addCurveSegment` for each segment, and then strokes the path. Simple.

### hitTest

Now we get to the question of why we are handling each segment separately.

The goal of the `hitTest` function is to determine if a double-click was on or near a curve segment. One way we could do that is to iterate along the smoothed function, checking point distances along the way. That would work, but advancing by small steps is even more crucial in that case than when drawing. It’s tends to be pretty ugly, it’s hard to debug, and it involves repeating, with slight changes, a lot of the code we already have for drawing.

There is an easier way, however:

Drawing code in a hit test function? What’s going on here?

This is a trick that’s good to know if you’re ever doing hit testing on a graphical element that can’t be approximated by a simple rectangle or other primitive shape.

What we’re doing is drawing each curve segment as before into an invisible canvas, and then seeing if the pixel under the mouse has been drawn into. If it has, then that segment was clicked. If it hasn’t, then that segment was not clicked. Note that we don’t have to clear the canvas after each segment.

By varying the line width, we can change the precision with which the user must click on the path. A line width of 20 gives the user a nice 10 pixel radius to click on. A line width of 1 would require the user to click exactly on the line.

A side note: the above implementation of the hit test is not very efficient. You can achieve the same result with a 1x1 pixel canvas by using transforms. That code is harder to understand though, and for a small canvas like the one in this demo, even my six-year-old MacBook handles it without blinking.

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