带箭头的立方二次贝塞尔曲线

// Usage:
var p0={x:50,y:100};
var p1={x:100,y:0};
var p2={x:200,y:200};
var p3={x:300,y:100};

cubicCurveArrowHeads(p0, p1, p2, p3, 15, true, true);

quadraticCurveArrowHeads(p0, p1, p2, 15, true, true);

// or use defaults true for both ends with arrow heads
cubicCurveArrowHeads(p0, p1, p2, p3, 15);

quadraticCurveArrowHeads(p0, p1, p2, 15);

// draws both cubic and quadratic bezier
function bezWithArrowheads(p0, p1, p2, p3, arrowLength, hasStartArrow, hasEndArrow) {
    var x, y, norm, ex, ey;
    function pointsToNormalisedVec(p,pp){
        var len;
        norm.y = pp.x - p.x;
        norm.x = -(pp.y - p.y);
        len = Math.sqrt(norm.x * norm.x + norm.y * norm.y);
        norm.x /= len;
        norm.y /= len;
        return norm;
    }
        
    var arrowWidth = arrowLength / 2;
    norm = {};
    // defaults to true for both arrows if arguments not included
    hasStartArrow = hasStartArrow === undefined || hasStartArrow === null ? true : hasStartArrow;
    hasEndArrow = hasEndArrow === undefined || hasEndArrow === null ? true : hasEndArrow;
    ctx.beginPath();
    ctx.moveTo(p0.x, p0.y);
    if (p3 === undefined) {
        ctx.quadraticCurveTo(p1.x, p1.y, p2.x, p2.y);
        ex = p2.x;  // get end point
        ey = p2.y;
        norm = pointsToNormalisedVec(p1,p2);
    } else {
        ctx.bezierCurveTo(p1.x, p1.y, p2.x, p2.y, p3.x, p3.y)
        ex = p3.x; // get end point
        ey = p3.y;
        norm = pointsToNormalisedVec(p2,p3);
    }
    if (hasEndArrow) {
        x = arrowWidth * norm.x + arrowLength * -norm.y;
        y = arrowWidth * norm.y + arrowLength * norm.x;
        ctx.moveTo(ex + x, ey + y);
        ctx.lineTo(ex, ey);
        x = arrowWidth * -norm.x + arrowLength * -norm.y;
        y = arrowWidth * -norm.y + arrowLength * norm.x;
        ctx.lineTo(ex + x, ey + y);
    }
    if (hasStartArrow) {
        norm = pointsToNormalisedVec(p0,p1);
        x = arrowWidth * norm.x - arrowLength * -norm.y;
        y = arrowWidth * norm.y - arrowLength * norm.x;
        ctx.moveTo(p0.x + x, p0.y + y);
        ctx.lineTo(p0.x, p0.y);
        x = arrowWidth * -norm.x - arrowLength * -norm.y;
        y = arrowWidth * -norm.y - arrowLength * norm.x;
        ctx.lineTo(p0.x + x, p0.y + y);
    }

    ctx.stroke();
}

function cubicCurveArrowHeads(p0, p1, p2, p3, arrowLength, hasStartArrow, hasEndArrow) {
    bezWithArrowheads(p0, p1, p2, p3, arrowLength, hasStartArrow, hasEndArrow);
}
function quadraticCurveArrowHeads(p0, p1, p2, arrowLength, hasStartArrow, hasEndArrow) {
    bezWithArrowheads(p0, p1, p2, undefined, arrowLength, hasStartArrow, hasEndArrow);
}