Render a single SVG element to canvas or image-CodePudding

How can I render a single SVG element (as opposed to the whole SVG document or parts of it) to an image or a canvas?

I have an SVG document with a lot of nodes. If I render the whole SVG or a part of it, the resulting image will contain other pieces of graphics I'm not interested in. I am interested in one specific SVG element and would like to render all of it without anything else.

Example:

<!DOCTYPE html>
<html>
<body>

  <svg height="150" width="150">
    <circle cx="50" cy="50" r="25" stroke="green" stroke-width="3" fill="gray" />
    <circle id="IWantToRenderThis" cx="75" cy="75" r="25" stroke="red" stroke-width="3" fill="white" />
    <circle cx="100" cy="100" r="25" stroke="blue" stroke-width="3" fill="black" />
  </svg>

  <script>
    const target = document.getElementById("IWantToRenderThis");
    // how can I render *target* into a texture?
  </script>

</body>
</html>

How can I render target alone? I would like to obtain an image which has no traces of the other two circles, a transparent background, and the right size to fit target perfectly.

CodePudding user response：

You could remove all elements but the one you want to render. To fit the SVG to the remaining element, you need to calculate a new position and size. For your example the code could look like this:

<!DOCTYPE html>
<html>
<body>

  <svg height="150" width="150" id="svg">
    <circle cx="50" cy="50" r="25" stroke="green" stroke-width="3" fill="gray" />
    <circle id="IWantToRenderThis" cx="75" cy="75" r="25" stroke="red" stroke-width="3" fill="white" />
    <circle cx="100" cy="100" r="25" stroke="blue" stroke-width="3" fill="black" />
  </svg>

  <script>
    const svg = document.getElementById("svg");
    const target = document.getElementById("IWantToRenderThis");
    const children = svg.children;
    
    // Remove all child elements but the target
    for(let index = 0; index < children.length; index  ) {
      const child = children[index];
            if(child.id !== 'IWantToRenderThis') {
        child.remove()
      }
    }
    
    // Recalculate element position and svg size
    const targetSize = parseInt(target.getAttribute('r'))    
    const targetStroke = parseInt(target.getAttribute('stroke-width'))
    target.setAttribute('cx', targetSize   (targetStroke/2))
    target.setAttribute('cy', targetSize   (targetStroke/2))
    
    svg.setAttribute('width', targetSize*2   targetStroke)
    svg.setAttribute('height', targetSize*2   targetStroke)
  </script>

</body>
</html>

Note that you have to include the stroke width of your element to properly calculate its new position and the SVG's new size.

CodePudding user response：

Here the target element is just copied using outerHTML into a new data URL representing the new SVG, loaded into an image object, drawn in a canvas and exported as a PNG image.

let img = document.getElementById('img');
let target = document.getElementById("IWantToRenderThis");
let svg = target.closest('svg');
let width = svg.attributes['width'].value;
let height = svg.attributes['height'].value;
let image = new Image();
let canvas = document.getElementById('canvas');
canvas.height = height;
canvas.width = width;
let ctx = canvas.getContext('2d');

image.addEventListener('load', e => {
  ctx.drawImage(e.target, 0, 0, e.target.width, e.target.height);
  img.src = canvas.toDataURL("image/png");
});
image.src = `data:image/svg xml,<svg xmlns="http://www.w3.org/2000/svg"
             width="${width}" height="${height}">${target.outerHTML}</svg>`;

<p>Original SVG:</p>
<svg id="svg" height="150" width="150">
  <circle cx="50" cy="50" r="25" stroke="green" stroke-width="3" fill="gray" />
  <circle id="IWantToRenderThis" cx="75" cy="75" r="25" stroke="red" stroke-width="3" fill="white" />
  <circle cx="100" cy="100" r="25" stroke="blue" stroke-width="3" fill="black" />
</svg>
<p>SVG rendered in canvas:</p>
<canvas id="canvas"></canvas>
<p>PNG image based on canvas:</p>
<img id="img" />