title: Antialiasing and updating a path lead to thicker lines than expected
body: <p>The problem is pretty complicated to explain but here goes: </p>
  
  <p>I'm making a paint program that draws paths onto a canvas with textured background. Each stroke is stored as a path that updates as the user moves the stylus across the screen. When the path is updated I call drawpath on the canvas. The problem is that on each move event, the path is drawn over the existing line on the canvas, so the antialiasing on it darkens the existing line and make it appear thicker and jaggier than expected.</p>
  
  <p>I had a solution where I store the older canvas (the one without the active path) and keep another transparent canvas on top of that. I would clear the top canvas and redraw the path on each move event, and then draw both canvases together. BUT that makes the program so slow that the paths look terrible - you can tell the drawing is lagging way behind the stylus movements.</p>
  
  <p>Is there any way to make either A) drawing / clearing multiple canvases faster  or B) make antialiasing not mess up on multiple redraws?</p>