How to get the MouseEvent coordinates for an element that has CSS3 Transform?

I want to detect where a MouseEvent has occurred, in coordinates relative to the clicked element. Why? Because I want to add an absolutely positioned child element at the clicked location.

I know how to detect it when no CSS3 transformations exist (see description below). However, when I add a CSS3 Transform, then my algorithm breaks, and I don't know how to fix it.

I'm not using any JavaScript library, and I want to understand how things work in plain JavaScript. So, please, don't answer with "just use jQuery".

By the way, I want a solution that works for all MouseEvents, not just "click". Not that it matters, because I believe all mouse events share the same properties, thus the same solution should work for all of them.


Background information

According to DOM Level 2 specification, a MouseEvent has few properties related to getting the event coordinates:

  • screenX and screenY return the screen coordinates (the origin is the top-left corner of user's monitor)
  • clientX and clientY return the coordinates relative the document viewport.

Thus, in order to find the position of the MouseEvent relative to the clicked element content, I must do this math:

ev.clientX - this.getBoundingClientRect().left - this.clientLeft + this.scrollLeft
  • ev.clientX is the coordinate relative to the document viewport
  • this.getBoundingClientRect().left is the position of the element relative to the document viewport
  • this.clientLeft is the amount of border (and scrollbar) between the element boundary and the inner coordinates
  • this.scrollLeft is the amount of scrolling inside the element

getBoundingClientRect(), clientLeft and scrollLeft are specified at CSSOM View Module.

Experiment without CSS Transform (it works)

Confusing? Try the following piece of JavaScript and HTML. Upon clicking, a red dot should appear exactly where the click has happened. This version is "quite simple" and works as expected.

function click_handler(ev) {
    var rect = this.getBoundingClientRect();
    var left = ev.clientX - rect.left - this.clientLeft + this.scrollLeft;
    var top = ev.clientY - rect.top - this.clientTop + this.scrollTop;

    var dot = document.createElement('div');
    dot.setAttribute('style', 'position:absolute; width: 2px; height: 2px; top: '+top+'px; left: '+left+'px; background: red;');
    this.appendChild(dot);
}

document.getElementById("experiment").addEventListener('click', click_handler, false);

<div id="experiment" style="border: 5px inset #AAA; background: #CCC; height: 400px; position: relative; overflow: auto;">
    <div style="width: 900px; height: 2px;"></div> 
    <div style="height: 900px; width: 2px;"></div>
</div>
Experiment adding a CSS Transform (it fails)

Now, try adding a CSS transform:

#experiment {
    transform: scale(0.5);
    -moz-transform: scale(0.5);
    -o-transform: scale(0.5);
    -webkit-transform: scale(0.5);
    /* Note that this is a very simple transformation. */
    /* Remember to also think about more complex ones, as described below. */
}

The algorithm doesn't know about the transformations, and thus calculates a wrong position. What's more, the results are different between Firefox 3.6 and Chrome 12. Opera 11.50 behaves just like Chrome.

In this example, the only transformation was scaling, so I could multiply the scaling factor to calculate the correct coordinate. However, if we think about arbitrary transformations (scale, rotate, skew, translate, matrix), and even nested transformations (a transformed element inside another transformed element), then we really need a better way to calculate the coordinates.

Answers


The behaviour you are experiencing is correct, and your algorithm isn't breaking. Firstly CSS3 Transforms are designed not to interfere with the box model.

To try and explain...

When you apply a CSS3 Transform on an element. the Element assumes a kind of relative positioning. In that the surrounding elements are not effected by the transformed element.

e.g. imagine three div's in a horizontal row. If you apply a scale transform to decrease the size of the centre div. The surrounding div's will not move inwards to occupy the space that was once occupied the transformed element.

example: http://jsfiddle.net/AshMokhberi/bWwkC/

So in the box model, the element does not actually change size. Only it's rendered size changes.

You also have to keep in mind that you are applying a scale Transform, so your elements "real" size is actually the same as it's original size. You are only changing it's perceived size.

To explain..

Imagine you create a div with a width of 1000px and scale it down to 1/2 the size. The internal size of the div is still 1000px, not 500px.

So the position of your dots are correct relative to the div's "real" size.

I modified your example to illustrate.

Instructions

  1. Click the div and keep you mouse in the same position.
  2. Find the dot in the wrong position.
  3. Press Q, the div will become the correct size.
  4. Move your mouse to find the dot in the correct position to where you clicked.

http://jsfiddle.net/AshMokhberi/EwQLX/

So in order to make the mouse clicks co-ordinates match the visible location on the div, you need to understand that the mouse is giving back co-ordinates based on the window, and your div offsets are also based on its "real" size.

As your object size is relative to the window the only solution is to scale the offset co-ordinates by the same scale value as your div.

However this can get tricky based on where you set the Transform-origin property of your div. As that is going to effect the offsets.

See here.

http://jsfiddle.net/AshMokhberi/KmDxj/

Hope this helps.


if element is container and positioned absolute or relative, you can place inside of it element, position it relative to parent and width = 1px, height = 1px, and move to inside of container, and after each move use document.elementFromPoint(event.clientX, event.clientY) =))))

You can use binary search to make it faster. looks terrible, but it works

http://jsfiddle.net/3VT5N/3/ - demo


another way is place 3 divs in corners of that element, than find transform matrix ... but is also works only for positioned containerable elements – 4esn0k

demo: http://jsfiddle.net/dAwfF/3/


Also, for Webkit webkitConvertPointFromPageToNode method can be used:

var div = document.createElement('div'), scale, point;
div.style.cssText = 'position:absolute;left:-1000px;top:-1000px';
document.body.appendChild(div);
scale = webkitConvertPointFromNodeToPage(div, new WebKitPoint(0, 0));
div.parentNode.removeChild(div);
scale.x = -scale.x / 1000;
scale.y = -scale.y / 1000;
point = webkitConvertPointFromPageToNode(element, new WebKitPoint(event.pageX * scale.x, event.pageY * scale.y));
point.x = point.x / scale.x;
point.y = point.y / scale.x;

BY FAR the fastest. The accepted answer takes about 40-70 ms on my 3d transforms site, this usually takes less than 20 (fiddle):

function getOffset(event,elt){
    var st=new Date().getTime();
    var iterations=0;
    //if we have webkit, then use webkitConvertPointFromPageToNode instead
    if(webkitConvertPointFromPageToNode){
        var webkitPoint=webkitConvertPointFromPageToNode(elt,new WebKitPoint(event.clientX,event.clientY));
        //if it is off-element, return null
        if(webkitPoint.x<0||webkitPoint.y<0)
            return null;
        return {
            x: webkitPoint.x,
            y: webkitPoint.y,
            time: new Date().getTime()-st
        }
    }
    //make full-size element on top of specified element
    var cover=document.createElement('div');
    //add styling
    cover.style.cssText='height:100%;width:100%;opacity:0;position:absolute;z-index:5000;';
    //and add it to the document
    elt.appendChild(cover);
    //make sure the event is in the element given
    if(document.elementFromPoint(event.clientX,event.clientY)!==cover){
        //remove the cover
        cover.parentNode.removeChild(cover);
        //we've got nothing to show, so return null
        return null;
    }
    //array of all places for rects
    var rectPlaces=['topleft','topcenter','topright','centerleft','centercenter','centerright','bottomleft','bottomcenter','bottomright'];
    //function that adds 9 rects to element
    function addChildren(elt){
        iterations++;
        //loop through all places for rects
        rectPlaces.forEach(function(curRect){
            //create the element for this rect
            var curElt=document.createElement('div');
            //add class and id
            curElt.setAttribute('class','offsetrect');
            curElt.setAttribute('id',curRect+'offset');
            //add it to element
            elt.appendChild(curElt);
        });
        //get the element form point and its styling
        var eltFromPoint=document.elementFromPoint(event.clientX,event.clientY);
        var eltFromPointStyle=getComputedStyle(eltFromPoint);
        //Either return the element smaller than 1 pixel that the event was in, or recurse until we do find it, and return the result of the recursement
        return Math.max(parseFloat(eltFromPointStyle.getPropertyValue('height')),parseFloat(eltFromPointStyle.getPropertyValue('width')))<=1?eltFromPoint:addChildren(eltFromPoint);
    }
    //this is the innermost element
    var correctElt=addChildren(cover);
    //find the element's top and left value by going through all of its parents and adding up the values, as top and left are relative to the parent but we want relative to teh wall
    for(var curElt=correctElt,correctTop=0,correctLeft=0;curElt!==cover;curElt=curElt.parentNode){
        //get the style for the current element
        var curEltStyle=getComputedStyle(curElt);
        //add the top and left for the current element to the total
        correctTop+=parseFloat(curEltStyle.getPropertyValue('top'));
        correctLeft+=parseFloat(curEltStyle.getPropertyValue('left'));
    }
    //remove all of the elements used for testing
    cover.parentNode.removeChild(cover);
    //the returned object
    var returnObj={
        x: correctLeft,
        y: correctTop,
        time: new Date().getTime()-st,
        iterations: iterations
    }
    return returnObj;
}

and also include the following CSS in the same page:

.offsetrect{
    position: absolute;
    opacity: 0;
    height: 33.333%;
    width: 33.333%;
}
#topleftoffset{
    top: 0;
    left: 0;
}
#topcenteroffset{
    top: 0;
    left: 33.333%;
}
#toprightoffset{
    top: 0;
    left: 66.666%;
}
#centerleftoffset{
    top: 33.333%;
    left: 0;
}
#centercenteroffset{
    top: 33.333%;
    left: 33.333%;
}
#centerrightoffset{
    top: 33.333%;
    left: 66.666%;
}
#bottomleftoffset{
    top: 66.666%;
    left: 0;
}
#bottomcenteroffset{
    top: 66.666%;
    left: 33.333%;
}
#bottomrightoffset{
    top: 66.666%;
    left: 66.666%;
}

It essentially splits the element into 9 squares, determines which one the click was in via document.elementFromPoint. It then splits that into 9 smaller squares, etc until it is accurate to within a pixel. I know I over-commented it. The accepted answer is several times slower than this.

EDIT: It is now even faster, and if the user is in Chrome or Safari it will use a native function designed for this instead of the 9 sectors thingy and can do it consistently in LESS THAN 2 MILLISECONDS!


To get the coordinates of a MouseEvent relative to the clicked element, use offsetX / layerX.

Have you tried using ev.layerX or ev.offsetX?

var offsetX = (typeof ev.offsetX == "number") ? ev.offsetX : ev.layerX || 0;

See also:


This seems to work really well for me

var elementNewXPosition = (event.offsetX != null) ? event.offsetX : event.originalEvent.layerX;
var elementNewYPosition = (event.offsetY != null) ? event.offsetY : event.originalEvent.layerY; 

Works fine whether relative or absolute :) simple solution
var p = $( '.divName' );
var position = p.position();  
var left = (position.left  / 0.5);
var top =  (position.top  / 0.5);

EDIT: my answer is untested, WIP, I will update when I get it working.

I'm implementing a polyfill of the geomtetry-interfaces. The DOMPoint.matrixTransform method I will make next, which means we should be able to write something like the following in order to map a click coordinate onto a transformed (possiblly nested) DOM element:

// target is the element nested somewhere inside the scene.
function multiply(target) {
    let result = new DOMMatrix;

    while (target && /* insert your criteria for knowing when you arrive at the root node of the 3D scene*/) {
        const m = new DOMMatrix(target.style.transform)
        result.preMultiplySelf(m) // see w3c DOMMatrix (geometry-interfaces)
        target = target.parentNode
    }

    return result
}

// inside click handler
// traverse from nested node to root node and multiply on the way
const matrix = multiply(node)
const relativePoint = DOMPoint(clickX, clickY, 0, 800).matrixTransform(matrix)

relativePoint will be the point relative to the element's surface that you clicked on.

A w3c DOMMatrix can be constructed with a CSS transform string argument, which makes it super easy to use in JavaScript.

Unfortunately, this isn't working yet (only Firefox has a geometry-interfaces implementation, and my polyfill does not yet accept a CSS transform string). See: https://github.com/trusktr/geometry-interfaces/blob/7872f1f78a44e6384529e22505e6ca0ba9b30a4d/src/DOMMatrix.js#L15-L18

I will update this once I implement that and have a working example. Pull requests welcome!

EDIT: the value 800 is the scene's perspective, I'm not sure if this is what the fourth value for the DOMPoint constructor should be when we intend to do something like this. Also, I'm not sure if I should use preMultiplySelf or postMultiplySelf. I'll find out once I get it at least working (values may be incorrect at first) and will update my answer.


I am working on a polyfill to transfrom DOM coordinates. The GeometryUtils api is not available yet (@see https://drafts.csswg.org/cssom-view/). I created a "simple" code in 2014 to transform coordinates, like localToGlobal, globalToLocal and localToLocal. Its not finished yet, but its working :) I think I will finish it in the coming months (05.07.2017), so if you still need a API to accomplish coordinate transformation give it a try: https://github.com/jsidea/jsidea jsidea core library. Its not stable yet (pre alpha). You can use it like that:

Create your transform instance:

var transformer = jsidea.geom.Transform.create(yourElement);

The box model you want to transform to (default:"border", will be replaced by ENUM's later on):

var toBoxModel = "border";

The box model where your input coordinates coming from (default:"border"):

var fromBoxModel = "border";

Transform your global coordinates (here {x:50, y:100, z: 0}) to local space. The resulting point has 4 components: x, y, z and w.

var local = transformer.globalToLocal(50, 100, 0, toBoxModel, fromBoxModel);

I have implemented some other functions like localToGlobal and localToLocal. If you want to give a try, just download the release build and use the jsidea.min.js.

Download the first release here: Download TypeScript code

Feel free to change the code, I never put it under any license :)


I have this issue and started trying to compute the matrix. I started a library around it: https://github.com/ombr/referentiel

$('.referentiel').each ->
  ref = new Referentiel(this)
  $(this).on 'click', (e)->
    input = [e.pageX, e.pageY]
    p = ref.global_to_local(input)
    $pointer = $('.pointer', this)
    $pointer.css('left', p[0])
    $pointer.css('top', p[1])

What do you think ?


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