Maintain consistent font text regardless of viewBox scaling

Question

I have an interactive SVG diagram that is almost completely different from this simple demo ...

Demo: http://jsfiddle.net/CphxH/6/

... except that:

• It has pile elements <text>,
• dynamically adjusts viewBoxand
• I would like the font size of the text displayed on the screen to be the same no matter how large the viewBox is.

Ideally, I would like to get the equivalent vector-effect:non-scaling-stroke(at least on Webkit), but for font-size. Is there such a thing? If not, is it best to calculate the size of the viewBox, combine it with aspectRatioto determine the major axis, and then use JS to control the CSS rules for the property font-size?

Answer 1

Here is a JavaScript based solution. You call recordSVGFontSizes()once at the beginning, passing it an SVG element and a StyleSheet. It calculates the viewing area for the SVG and scans the stylesheet and finds all the rules that mention font-sizeand record the original font size.

, viewBox , normalizeSVGFontSizes(). .

, Firefox , script .

: http://jsfiddle.net/CphxH/8/

// Do this once at startup
var recording = recordSVGFontSizes( mySVG, document.styleSheets[0] );

// Do this each time the viewport changes
normalizeSVGFontSizes(recording);

function recordSVGFontSizes(svgElement,styleSheet){
  var rules = [];
  var viewport = calculateViewport(svgElement);
  for (var i=styleSheet.cssRules.length;i--;){
    var rule = styleSheet.cssRules[i];
    if (rule.style.fontSize){
      var parts = rule.style.fontSize.split(/(\D+)/);
      rules.push({ rule:rule, units:parts[1], perpx:parts[0] / viewport.width });
    }
  }
  return {rules:rules, svg:svgElement};
}

function normalizeSVGFontSizes(fontSizeRecording){
  var viewport = calculateViewport(fontSizeRecording.svg);
  for (var i=fontSizeRecording.rules.length;i--;){
    var record = fontSizeRecording.rules[i];
    record.rule.style.fontSize = record.perpx*viewport.width + record.units;
  }
}

// Given an <svg> element, returns an object with the visible bounds
// expressed in local viewBox units, e.g.
// { x:-50, y:-50, width:100, height:100 }
function calculateViewport(svg){ // http://phrogz.net/JS/_ReuseLicense.txt
  var outer    = svg.getBoundingClientRect();
  var aspect   = svg.preserveAspectRatio.baseVal,
      viewBox  = svg.viewBox.baseVal,
      width    = viewBox && viewBox.width  || outer.width,
      height   = viewBox && viewBox.height || outer.height,
      x        = viewBox ? viewBox.x : 0,
      y        = viewBox ? viewBox.y : 0;
  if (!width || !height || !outer.width) return;
  if (aspect.align==aspect.SVG_PRESERVEASPECTRATIO_NONE || !viewBox || !viewBox.height){
    return {x:x,y:y,width:width,height:height};
  }else{
    var inRatio  = viewBox.width / viewBox.height,
        outRatio = outer.width / outer.height;
    var meetFlag = aspect.meetOrSlice != aspect.SVG_MEETORSLICE_SLICE;
    var fillAxis = outRatio>inRatio ? (meetFlag?'y':'x') : (meetFlag?'x':'y');
    if (fillAxis=='x'){
      height = width/outRatio;
      var diff = viewBox.height - height;
      switch (aspect.align){
        case aspect.SVG_PRESERVEASPECTRATIO_UNKNOWN: 
        case aspect.SVG_PRESERVEASPECTRATIO_XMINYMID:
        case aspect.SVG_PRESERVEASPECTRATIO_XMIDYMID:
        case aspect.SVG_PRESERVEASPECTRATIO_XMAXYMID:
          y += diff/2;
        break;
        case aspect.SVG_PRESERVEASPECTRATIO_XMINYMAX:
        case aspect.SVG_PRESERVEASPECTRATIO_XMIDYMAX:
        case aspect.SVG_PRESERVEASPECTRATIO_XMAXYMAX:
          y += diff;
        break;
      }
    }
    else{
      width = height*outRatio;
      var diff = viewBox.width - width;
      switch (aspect.align){
        case aspect.SVG_PRESERVEASPECTRATIO_UNKNOWN: 
        case aspect.SVG_PRESERVEASPECTRATIO_XMIDYMIN:
        case aspect.SVG_PRESERVEASPECTRATIO_XMIDYMID:
        case aspect.SVG_PRESERVEASPECTRATIO_XMIDYMAX:
          x += diff/2;
        break;
        case aspect.SVG_PRESERVEASPECTRATIO_XMAXYMID:
        case aspect.SVG_PRESERVEASPECTRATIO_XMAXYMIN:
        case aspect.SVG_PRESERVEASPECTRATIO_XMAXYMAX:
          x += diff;
        break;
      }
    }
    return {x:x,y:y,width:width,height:height};
  }
}

Answer 2

, , viewBox. Javascript, .