html5-canvas Finding points along a quadratic curve


Example

This example finds an array of approximately evenly spaced points along a quadratic curve.

It decomposes Path segments created with context.quadraticCurveTo into points along that curve.

// Return: an array of approximately evenly spaced points along a Quadratic curve
//
// Attribution: Stackoverflow's @Blindman67
// Cite: http://stackoverflow.com/questions/36637211/drawing-a-curved-line-in-css-or-canvas-and-moving-circle-along-it/36827074#36827074
// As modified from the above citation
//
// ptCount: sample this many points at interval along the curve
// pxTolerance: approximate spacing allowed between points
// Ax,Ay,Bx,By,Cx,Cy: control points defining the curve
//
function plotQBez(ptCount,pxTolerance,Ax,Ay,Bx,By,Cx,Cy){
    var deltaBAx=Bx-Ax;
    var deltaCBx=Cx-Bx;
    var deltaBAy=By-Ay;
    var deltaCBy=Cy-By;
    var ax,ay;
    var lastX=-10000;
    var lastY=-10000;
    var pts=[{x:Ax,y:Ay}];
    for(var i=1;i<ptCount;i++){
        var t=i/ptCount;
        ax=Ax+deltaBAx*t;
        ay=Ay+deltaBAy*t;
        var x=ax+((Bx+deltaCBx*t)-ax)*t;
        var y=ay+((By+deltaCBy*t)-ay)*t;
        var dx=x-lastX;
        var dy=y-lastY;
        if(dx*dx+dy*dy>pxTolerance){
            pts.push({x:x,y:y});
            lastX=x;
            lastY=y;
        }
    }
    pts.push({x:Cx,y:Cy});
    return(pts);
}