How to draw non-scalable circle in SVG with Javascript

Question

I\'m developing a map, in Javascript using SVG to draw the lines.

I would like to add a feature where you can search for a road, and if the road is found, a circle a

Answer 1

It took me a while, but I finally got the math clean. This solution requires three things:

1. Include this script in your page (along with the SVGPan.js script), e.g.
   
2. Identify the items you want not to scale (e.g. place them in a group with a special class or ID, or put a particular class on each element) and then tell the script how to find those items, e.g.
   unscaleEach("g.non-scaling > *, circle.non-scaling");
3. Use transform="translate(…,…)" to place each element on the diagram, not cx="…" cy="…".

With just those steps, zooming and panning using SVGPan will not affect the scale (or rotation, or skew) of marked elements.

Demo: http://phrogz.net/svg/scale-independent-elements.svg

Library

// Copyright 2012 © Gavin Kistner, !@phrogz.net
// License: http://phrogz.net/JS/_ReuseLicense.txt

// Undo the scaling to selected elements inside an SVGPan viewport
function unscaleEach(selector){
  if (!selector) selector = "g.non-scaling > *";
  window.addEventListener('mousewheel',     unzoom, false);
  window.addEventListener('DOMMouseScroll', unzoom, false);
  function unzoom(evt){
    // getRoot is a global function exposed by SVGPan
    var r = getRoot(evt.target.ownerDocument);
    [].forEach.call(r.querySelectorAll(selector), unscale);
  }
}

// Counteract all transforms applied above an element.
// Apply a translation to the element to have it remain at a local position
function unscale(el){
  var svg = el.ownerSVGElement;
  var xf = el.scaleIndependentXForm;
  if (!xf){
    // Keep a single transform matrix in the stack for fighting transformations
    // Be sure to apply this transform after existing transforms (translate)
    xf = el.scaleIndependentXForm = svg.createSVGTransform();
    el.transform.baseVal.appendItem(xf);
  }
  var m = svg.getTransformToElement(el.parentNode);
  m.e = m.f = 0; // Ignore (preserve) any translations done up to this point
  xf.setMatrix(m);
}

Demo Code