Question

I have the code that draw a point inside a polygon. Each time I change value on textbox for x & y, it will draw a new point and still keep old points on my polygon, so I want to ask how can I clear all old points ?

I already try to remove old tag and create new each time draw a new point but it’s not ok. If you know about this, pls help. Thanks

Canvas = function(){ //v1.0
    var o = this;
    (o.penPos = {x: 0, y: 0}, o.pixelSize = 10, o.pen = {style: "solid",
        size: 1, color: "#000"}, o.brush = {style: "solid", color: "#000"});
};
with({p: Canvas.prototype}){
    p.pixel = function(x, y, color) {
        var o = this, s = document.body.appendChild(document.createElement("div")).style;
//alert ("top"+(y * o.pixelSize) + "px");
        return (s.position = "absolute", s.width = (o.pen.size * o.pixelSize) + "px",
            s.height = (o.pen.size * o.pixelSize) + "px", s.fontSize = "1px",
            s.left = (x * o.pixelSize) + "px", s.top = (y * o.pixelSize) + "px",
            s.backgroundColor = color || o.pen.color, o);
    };
    p.line = function(x1, y1, x2, y2){
        if(Math.abs(x1 - x2) < Math.abs(y1 - y2))
            for(y = Math.min(y1, y2) - 1, x = Math.max(y1, y2); ++y <= x;
                this.pixel((y * (x1 - x2) - x1 * y2 + y1 * x2) / (y1 - y2), y));
        else
            for(x = Math.min(x1, x2) - 1, y = Math.max(x1, x2); ++x <= y;
                this.pixel(x, (x * (y1 - y2) - y1 * x2 + x1 * y2) / (x1 - x2)));
        return this;
    };
    p.arc = function(x, y, raio, startAngle, degrees) {
        for(degrees += startAngle; degrees --> startAngle;
            this.pixel(Math.cos(degrees * Math.PI / 180) * raio + x,
                Math.sin(degrees * Math.PI / 180) * raio + y)); return this;
    };
    p.rectangle = function(x, y, width, height, rotation){
        return this.moveTo(x, y).lineBy(0, height).lineBy(width, 0).lineBy(0, -height).lineBy(-width, 0);
    };
    p.moveTo = function(x, y){var o = this; return (o.penPos.x = x, o.penPos.y = y, o);};
    p.moveBy = function(x, y){var o = this; return o.moveTo(o.penPos.x + x, o.penPos.y + y);};
    p.lineTo = function(x, y){var o = this; return o.line(o.penPos.x, o.penPos.y, x, y).moveTo(x, y);};
    p.lineBy = function(x, y){var o = this; return o.lineTo(o.penPos.x + x, o.penPos.y + y);};
    p.curveTo = function(cx, cy, x, y){};
    p.polyBezier = function(points){};
    p.path = function(points){};
}
function isPointInPoly(poly, pt){
    for(var c = false, i = -1, l = poly.length, j = l - 1; ++i < l; j = i)
        ((poly[i].y <= pt.y && pt.y < poly[j].y) || (poly[j].y <= pt.y && pt.y < poly[i].y))
        && (pt.x < (poly[j].x - poly[i].x) * (pt.y - poly[i].y) / (poly[j].y - poly[i].y) + poly[i].x)
        && (c = !c);
    return c;
}
var length = 50,
points = [
    {x: 35, y:10422},
    {x: 36, y:32752},
    {x: 40, y:35752},
    {x: 55, y:27216},
    {x: 59, y:29319},
    {x: 58, y:10411}
];

var canvas = new Canvas;
canvas.pen.color = "#f00";
canvas.pixelSize = 1;

canvas.moveTo(getx(points[points.length-1].x) , gety(points[points.length-1].y));
for(var i = points.length; i--; canvas.lineTo(getx( points[i].x), gety(points[i].y)));

function draw(){
    var x=38; var y=10433;
    canvas.pixel(getx(x),gety(y));
    alert(isPointInPoly(points, {x: x,y: y}) ? "In" : "Out");
}
function getx(x){
    return Math.round(x*10);
}
function gety(y){
    return Math.round(400-y/250);
}
//]]>

Answer 1

You can clear a rectangular section of the canvas by using context.clearRect(x,y,width,height). For your application, I would imagine every time you draw a polygon, you want to clear our your canvas and start over. To do this, simply, call context.clearRect(0,0,canvas.width,canvas.height).

There is also a trick to clearing any polygon shaped region outlined in this SO question.