Convert SVG to PNG and maintain CSS integrity

问题:

I am currently using canvg() and Canvas2Image to copy my SVG to a canvas and then convert the canvas to PNG. I would like to maintain the image format and not use PDF.

How can I maintain the CSS integrity? Chart is made using NVD3.js.

downloadPhoto: function() {
  var chartArea = document.getElementsByTagName('svg')[0].parentNode;
  var svg = chartArea.innerHTML;
  var canvas = document.createElement('canvas');
  canvas.setAttribute('width', chartArea.offsetWidth);
  canvas.setAttribute('height', chartArea.offsetHeight);
  canvas.setAttribute('display', 'none');

  canvas.setAttribute(
    'style',
    'position: absolute; ' +
    'top: ' + (-chartArea.offsetHeight * 2) + 'px;' +
    'left: ' + (-chartArea.offsetWidth * 2) + 'px;');
  document.body.appendChild(canvas);
  canvg(canvas, svg);
  Canvas2Image.saveAsPNG(canvas);
  canvas.parentNode.removeChild(canvas);
}

回答1:

Style definitions for svg elements defined in stylesheets are not applied to the generated canvas. This can be patched by adding style definitions to the svg elements before calling canvg.

Inspired on this article, I've created this:

function generateStyleDefs(svgDomElement) {
  var styleDefs = "";
  var sheets = document.styleSheets;
  for (var i = 0; i < sheets.length; i++) {
    var rules = sheets[i].cssRules;
    for (var j = 0; j < rules.length; j++) {
      var rule = rules[j];
      if (rule.style) {
        var selectorText = rule.selectorText;
        var elems = svgDomElement.querySelectorAll(selectorText);

        if (elems.length) {
          styleDefs += selectorText + " { " + rule.style.cssText + " }\n";
        }
      }
    }
  }

  var s = document.createElement('style');
  s.setAttribute('type', 'text/css');
  s.innerHTML = "<![CDATA[\n" + styleDefs + "\n]]>";
  //somehow cdata section doesn't always work; you could use this instead:
  //s.innerHTML = styleDefs;

  var defs = document.createElement('defs');
  defs.appendChild(s);
  svgDomElement.insertBefore(defs, svgDomElement.firstChild);
}

// generate style definitions on the svg element(s)
generateStyleDefs(document.getElementById('svgElementId'));

回答2:

The key thing here is that all the style rules need to be part of the SVG, not in external style files. So you would need to go through all the CSS for NVD3 and set all of those attributes in the code. Anything that is set via an external stylesheet will be ignored.

回答3:

just to make @Lars Kotthoff's answer more concrete. "example of how to export a png directly from an svg" has a working example. the code snippet/gist tries to first apply all css to the svg inline and then draw the image on the canvas and export the data as png. (internally it adopted svg-crowbar code). and i apply the technique in my project and it works smoothly - a download button that can download the svg image rendered using nvd3.