html5-canvas Navigating along a Path Find point on curve

Example

This example finds a point on a bezier or cubic curve at position where position is he unit distance on the curve 0 <= position <= 1. The position is clamped to the range thus if values < 0 or > 1 are passed they will be set 0,1 respectively.

Pass the function 6 coordinates for quadratic bezier or 8 for cubic.

The last optional argument is the returned vector (point). If not given it will be created.

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Example usage

var p1 = {x : 10 , y : 100};
var p2 = {x : 100, y : 200};
var p3 = {x : 200, y : 0};
var p4 = {x : 300, y : 100};
var point = {x : null, y : null};

// for cubic beziers
point = getPointOnCurve(0.5, p1.x, p1.y, p2.x, p2.y, p3.x, p3.y, p4.x, p4.y, point);
// or No need to set point as it is a referance and will be set
getPointOnCurve(0.5, p1.x, p1.y, p2.x, p2.y, p3.x, p3.y, p4.x, p4.y, point);
// or to create a new point
var point1 = getPointOnCurve(0.5, p1.x, p1.y, p2.x, p2.y, p3.x, p3.y, p4.x, p4.y);

// for quadratic beziers
point = getPointOnCurve(0.5, p1.x, p1.y, p2.x, p2.y, p3.x, p3.y, null, null, point);
// or No need to set point as it is a referance and will be set
getPointOnCurve(0.5, p1.x, p1.y, p2.x, p2.y, p3.x, p3.y, null, null, point);
// or to create a new point
var point1 = getPointOnCurve(0.5, p1.x, p1.y, p2.x, p2.y, p3.x, p3.y);

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The function

getPointOnCurve = function(position, x1, y1, x2, y2, x3, y3, [x4, y4], [vec])

Note: Arguments inside [x4, y4] are optional.

Note: x4,y4 if null, or undefined means that the curve is a quadratic bezier. vec is optional and will hold the returned point if supplied. If not it will be created.

var getPointOnCurve = function(position, x1, y1, x2, y2, x3, y3, x4, y4, vec){ 
    var vec, quad;
    quad = false;
    if(vec === undefined){        
        vec = {};
    }
    
    if(x4 === undefined || x4 === null){
        quad = true;
        x4 = x3;
        y4 = y3;
    }
        
    if(position <= 0){
        vec.x = x1;
        vec.y = y1;
        return vec;
    }
    if(position >= 1){
        vec.x = x4;
        vec.y = y4;
        return vec;
    }
    c = position;
    if(quad){
        x1 += (x2 - x1) * c;
        y1 += (y2 - y1) * c;
        x2 += (x3 - x2) * c;
        y2 += (y3 - y2) * c;
        vec.x = x1 + (x2 - x1) * c;
        vec.y = y1 + (y2 - y1) * c;
        return vec;
    }
    x1 += (x2 - x1) * c;
    y1 += (y2 - y1) * c;
    x2 += (x3 - x2) * c;
    y2 += (y3 - y2) * c;
    x3 += (x4 - x3) * c;
    y3 += (y4 - y3) * c;
    x1 += (x2 - x1) * c;
    y1 += (y2 - y1) * c;
    x2 += (x3 - x2) * c;
    y2 += (y3 - y2) * c;
    vec.x = x1 + (x2 - x1) * c;
    vec.y = y1 + (y2 - y1) * c;
    return vec;     
}