I'm actually using a similar browser based curve implementation (I'm using the bezier rendering thing instead of a quadratic) -- https://github.com/ezl/signature-pad/blob/master/jquery.sign...
The first pad is the original that just draws lines between the sampled points.
The second 2 actually take points and compute a cubic spline that runs through all the sampled points. The problem I have though is that it takes 4 points to compute a bezier curve, which means there are at least 4 sampled points worth of lag in determining what curve to render on the canvas.
For example, 4 sampled points on a line should render a totally linear segment between the points, but then if the 5th points goes off the line, it actually impacts the preceding points -- so you have to lag even further to make draw the right curve.
Even if you wait every 3 points to determine a quadratic that canvas's built-in can render, it creates some lag between the current mouse position and the rendered curve.
If you keep rendering quadratics every 3 points, you'll get funny cusps where you join each of the quadratics (solution seemed to me to compute the spline for the fully determined segments of the curve).
My janky solution is "draw linear segments until I have enough data, then restroke it with pretty curves". I didn't really understand the full implementation under the "responsiveness" section on Square's blog post here http://corner.squareup.com/2012/07/smoother-signatures.html so I glossed over it since it wasn't super critical.
But then when I saw this link, I couldn't help but think -- oh man, that's sexy...
