Programmatically rotate shapes using coordinates

If I have some shapes defined using arrays of coordinates e.g.

[[-30, -30], [-30, 30], [30, 30], [30, -30]]

and edges defined using:

[[0,1],[0,3],[1,2],[2,3]]

to make a square.

How do I programmatically tell the shape to rotate *at the center against an angle of 0->359 in javascript?

*Or better yet, is there a function which allows me to choose the center of rotation?

** Currently, I've managed to get the shape to circle the canvas using :

var x_rotate = Math.sin(angle * Math.PI /180);
var y_rotate = Math.cos(angle * Math.PI /180);

// for each coordinate
//add x_rotate and y_rotate if coordinate > 0 or subtract if coordinate < 0 

3 answers

  • answered 2017-08-12 09:40 Rajesh Meena

    use css transition and change transform by javascript

    https://www.w3schools.com/cssref/css3_pr_transform.asp

    https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Transitions/Using_CSS_transitions

  • answered 2017-08-12 09:40 Oliver Ni

    Here you go, rotates point x, y around point centerx, centery by degrees degrees. Returns floating values, round if you need to.

    To rotate a shape, rotate all the points. Calculate the center if you need to, this does not do it.

    Desmos link with x as a, y as b, centerx as p, centery as q, and degrees as s

    function rotatePoint(x, y, centerx, centery, degrees) {
        var newx = (x - centerx) * Math.cos(degrees * Math.PI / 180) - (y - centery) * Math.sin(degrees * math.PI / 180) + centerx;
        var newy = (x - centerx) * Math.sin(degrees * Math.PI / 180) + (y - centery) * Math.cos(degrees * math.PI / 180) + centery;
        return [newx, newy];
    }
    

  • answered 2017-08-12 09:40 Nina Scholz

    You could use a formular for rotating the point around the origin.

    function rotate(array, angle) {
        return array.map(function (p) {
            function r2d(a) { return a * Math.PI / 180; }
            return [
                Math.cos(r2d(angle)) * p[0] - Math.sin(r2d(angle)) * p[1],
                Math.sin(r2d(angle)) * p[0] - Math.cos(r2d(angle)) * p[1],
            ];
        });
    }
    console.log(rotate([[-30, -30], [-30, 30], [30, 30], [30, -30]], 45));
    console.log(rotate([[-30, -30], [-30, 30], [30, 30], [30, -30]], 90));
    .as-console-wrapper { max-height: 100% !important; top: 0; }