Sami Sami - 1 year ago 116
Javascript Question

Move object around circle javascript

enter image description here

Above is just image to get a link for fiddle its role is nothing in question.

I am trying to move an object along circle (actually a rectangular canvas with border-radius=100% so behaving as circle). But only able to move it and not able to resrict the movemment in the circle)

Here is my JS Fiddle Link. Note: Please scroll down in js code and view the only code written under following comments

//---- Code of interest begins-----//
to //---code of interest ends---//

My desired output will be like this wheel farbtastic. Though it is open source yet i have been unable to use its mouse move event code in my colour wheel

While trying to search I found Canvas move object in circle
but I can not use it because i do not know
and do not know what is
. So unable to get theta with

Answer Source

The easiest way I think is to calculate the angle and apply css3 rotate transform to an element.

I let you the example here..

Read the comments, specially on the rotate() function


        <div id="circle">
        <div id="circle-in"></div>
        <div id="picker">
        <div id="picker-circle"></div>


        position: relative;
        width: 300px;
        height: 300px;
        border-radius: 50%;
        background: #000;

        position: absolute;
        top: 35px;
        left: 35px;
        width: 230px;
        height: 230px;
        border-radius: 50%;
        background: #fff;

        position: absolute;
        top: 50%;
        left: 50%;
        height: 30px;
        margin-top: -15px;
        width: 50%;

        /* important: sets the transform origin to the center of the circle */
        transform-origin: center left;

        width: 30px;
        height: 30px;
        border-radius: 50%;
        background: #fff;
        margin: 0 3px 0 auto;
        cursor: move;


document.addEventListener('DOMContentLoaded', function(){
    var circle = document.getElementById('circle'),
        picker = document.getElementById('picker'),
        pickerCircle = picker.firstElementChild,
        rect = circle.getBoundingClientRect(),

        center = {
            x: rect.left + rect.width / 2,
            y: + rect.height / 2

        rotate = function(x, y){
            var deltaX = x - center.x,
                deltaY = y - center.y,

            // The atan2 method returns a numeric value between -pi and pi representing the angle theta of an (x,y) point.
            // This is the counterclockwise angle, measured in radians, between the positive X axis, and the point (x,y).
            // Note that the arguments to this function pass the y-coordinate first and the x-coordinate second.
            // atan2 is passed separate x and y arguments, and atan is passed the ratio of those two arguments.
            // * from Mozilla's MDN

            // Basically you give it an [y, x] difference of two points and it give you back an angle
            // The 0 point of the angle is right (the initial position of the picker is also right)

                angle = Math.atan2(deltaY, deltaX) * 180 / Math.PI

            // Math.atan2(deltaY, deltaX) => [-PI +PI]
            // We must convert it to deg so...
            // / Math.PI => [-1 +1]
            // * 180 => [-180 +180]

            return angle

        // DRAGSTART
        mousedown = function(event){
   = 'move'
            document.addEventListener('mousemove', mousemove)
            document.addEventListener('mouseup', mouseup)

        // DRAG
        mousemove = function(event){
   = 'rotate(' + rotate(event.x, event.y) + 'deg)'

        // DRAGEND
        mouseup = function(){
   = null;
            document.removeEventListener('mouseup', mouseup)
            document.removeEventListener('mousemove', mousemove)

    pickerCircle.addEventListener('mousedown', mousedown)

    circle.addEventListener('mousedown', function(event){
        if( == this) mousedown(event)
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