If you have an angle of 0 and a radius > 0, the mac and win port handle arcTo different. The mac port just adds (x1,y1) to the current path and draws a line of (x0,y0) to (x1,y1). http://philip.html5.org/demos/canvas/arcto-inf.png The win port draws to parallel lines to the infity. One throug (x1,y1)->(x0,y0) and the second parallel to this line with the distance of the radius.