How do I get an internal SVG element's positio

Question

Say I have an SVG element containing some stuff:

<div style="margin-left:50px; width: 100%; min-height: 400px;">
  <svg>
    <g transform="translate(34.34,47.5) scale(0.345)" height="100%" width="100%">
      <svg x="20" y ="50" style="overflow: visible">
        <circle cx="0" cy="0" r="35" stroke="red" fill="blue">
        <text>a bunch of text</text>
      </svg>
      <line />
    </g>
  <svg>
<div>

I'm trying to find the center position of the <g> relative to the viewport of the outer <svg> element, so that I can translate the <g> to be centered within the outer <svg>, and scale it to fit.

I was able to get it working using getBoundingClientRect() and adjusting for the transform scale, but this does not work in Firefox because the <svg> elements inside the <g> container are not constrained to the bounding box of the displayed section of their contents (rather it's the same size as the outer <svg>, with some scaling).

There is probably a solution using createSVGPoint() and getScreenCTM() or getCTM() but frankly I'm not sure what I should be using.

Answer 1

An SVG without a viewBox attribute has a width of 300px and a height of 150px. I've added a viewBox="0 0 300 150". You can remove it.

Also I've added a rectangle to be able to see the position and the size of the <g> element. You can remove it as well.

How I would center the <g> element: Since the <g> element is transformed the easiest way to get it's size and position would be wrapping the <g> element in another one, in this case <g id="wrap"> Next I can get the bounding box of the wrap: wrap.getBBox()

In order to center the wrap I need to know the center of the main svg canvas: x = 300/2; y=150/2. Now I can translate the wrap into the center

let c = {x:150,y:75}//the center of the main svg element
let bb = wrap.getBBox()//the bounding box of the wrap

let transformation = `translate(${c.x - bb.x - bb.width/2},
                                ${c.y - bb.y - bb.height/2})`

wrap.setAttributeNS(null,"transform",transformation)

svg{border:1px solid;width:100vh;}
text{fill:black;}
path{fill:none;stroke:black}

<div style="margin-left:50px; width: 100%; min-height: 400px;">
  <svg id="main" viewBox="0 0 300 150" >
    <g id="wrap">
      <rect x="29.165" y="47.5" width="45.03" height="29.325" fill="gold" fill-opacity=".5" />
      <g transform="translate(34.34,47.5) scale(0.345)" height="100%" width="100%">
      <svg x="20" y ="50" style="overflow: visible">
        <circle cx="0" cy="0" r="35" stroke="red" fill="blue"/>
        <text>a bunch of text</text>
      </svg>
      <line />
    </g>
    </g>
    
    <path d="M0,0L300,150M0,150L300,0" />
  <svg>
<div>

I hope this is what you were asking.

Answer 2

I managed to figure out a solution using one of the d3.zoom transform methods (we're using d3.zoom to manage the translate/scale transform), and SVGElement.getBBox(). I originally was using this method but had messed up the calculation; this way it works though.

const selection = d3.select(group);
const zoomBehavior = d3.zoom().on('zoom', () => {
  selectionTransform = d3.event.transform;
});
selection.call(zoomBehavior);

const scaleAndTransformTo = () => {
  selection.call(zoomBehavior.translateBy, Math.random() * 100, Math.random() * 150);

  group.setAttribute("transform", selectionTransform.toString());
}
scaleAndTransformTo();

reset.addEventListener('click', scaleAndTransformTo);
run.addEventListener('click', () => {
  const { width: containerWidth, height: containerHeight } = container.getBoundingClientRect();
  const containerCenter = [containerWidth / 2, containerHeight / 2];

  const { height, width, x, y } = group.getBBox();
  const nodeBBoxCenter = [x + (width / 2), y + (height / 2)];

  // Apply the current interpolated translate/scale to the BBox center to get the actual position
  const groupCenterCoords = selectionTransform.apply(nodeBBoxCenter);

  const translationOffset = [
    (containerCenter[0] - groupCenterCoords[0]) / selectionTransform.k,
    (containerCenter[1] - groupCenterCoords[1]) / selectionTransform.k,
  ];

  selection.call(zoomBehavior.translateBy, ...translationOffset);

  group.setAttribute("transform", selectionTransform.toString());
});

#page {
  display: flex;
  flex-direction: column;
  position: relative;
  align-items: stretch;
  margin-left: 100px;
}

#container {
  background-color: grey;
  flex-grow: 1;
  flex-shrink: 0;
  min-height: 500px;
  border: 1px solid red;
}

#group > svg {
  overflow: visible;
}

#group > svg > circle {
  overflow: visible;
}

text {
  fill: black;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/5.7.0/d3.min.js"></script>
<div id="page">
  <div>
    <button id="run">Run</button>
    <button id="reset">Reset</button>
  </div>
  <svg id="container">
    <g x="0" y="0" id="group" width="100%" height="100%">
      <line x1="20" y1="50" x2="150" y2="150" stroke="brown" />
      <svg x="20" y ="50">
        <circle cx="0" cy="0" r="35" stroke="red" fill="blue">
        <text x="35" y="0" height="100%" width="100%">a bunch of text</text>
      </svg>
      <line x1="100" y1="350" x2="160" y2="340" stroke="brown" />
      <svg x="100" y ="350">
        <circle cx="0" cy="0" r="35" stroke="red" fill="blue">
        <text x="35" y="0" height="100%" width="100%">a bunch of text 3</text>
      </svg>
    </g>
  <svg>
<div>