Question

I wrote a little drawing script (canvas) for this website: http://scri.ch/

When you click on the document, every mousemove event basically executes the following:
– Get coordinates.
– context.lineTo() between this point and the previous one
– context.stroke() the line

As you can see, if you move the cursor very fast, the event isn’t triggering enough (depending on your CPU / Browser / etc.), and a straight line is traced.

In pseudocode:

window.addEventListener('mousemove', function(e){
  myContext.lineTo(e.pageX, e.pageY);
  myContext.stroke();
}, false);

This is a known problem, and the solution is fine, but I would like to optimize that.

So instead of stroke() each time a mousemove event is triggered, I put the new coordinates inside an array queue, and regularly draw / empty it with a timer.

In pseudocode:

var coordsQueue = [];

window.addEventListener('mousemove', function(e){
  coordsQueue.push([e.pageX, e.pageY]);
}, false);

function drawLoop(){
  window.setTimeout(function(){
    var coords;
    while (coords = coordsQueue.shift()) {
      myContext.lineTo(coords[0], coords[1]);
    }
    myContext.stroke();
    drawLoop();
  }, 1000); // For testing purposes
}

But it did not improve the line. So I tried to only draw a point on mousemove. Same result: too much space between the points.

It made me realize that the first code block is efficient enough, it is just the mousemove event that is triggering too slowly.

So, after having myself spent some time to implement a useless optimization, it’s your turn: is there a way to optimize the mousemove triggering speed in DOM scripting?

Is it possible to “request” the mouse position at any time?

Thanks for your advices!

Answer 1

If you want to increase the reporting frequency, I’m afraid you’re out of luck. Mice only report their position to the operating system n times per second, and I think n is usually less than 100. (If anyone can confirm this with actual specs, feel free to add them!)

So in order to get a smooth line, you’ll have to come up with some sort of interpolation scheme. There’s a whole lot of literature on the topic; I recommend monotone cubic interpolation because it’s local, simple to implement, and very stable (no overshoot).

Then, once you’ve computed the spline, you can approximate it with line segments short enough so that it looks smooth, or you can go all-out and write your own Bresenham algorithm to draw it.

If all this is worth it for a simple drawing application… that’s for you to decide, of course.