Change svg fill color and then draw to canvas

Question

What I want to do is load an svg, change its fill color to a random value and then draw it on the canvas. This has proved to be much more difficult than I would have thought. Here's the code I have at the moment.

var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");

//images
var bottomLeftTop = new Image();
var bottomRightTop = new Image();
var fullTop= new Image();
var leftMidSide = new Image();
var leftSide = new Image();
var rightMidSide = new Image();
var rightSide = new Image();
var topLeftTop = new Image();
var topRightTop = new Image();

bottomLeftTop.src = "img/bottomLeftTop.svg";
bottomRightTop.src = "img/bottomRightTop.svg";
fullTop.src = "img/fullTop.svg";
leftMidSide.src = "img/leftMidSide.svg";
leftSide.src = "img/leftSide.svg";
rightMidSide.src = "img/rightMidSide.svg";
rightSide.src = "img/rightSide.svg";
topLeftTop.src = "img/topLeftTop.svg";
topRightTop.src = "img/topRightTop.svg";

//draw
context.drawImage(fullTop,50,50);

I'm currently loading my svg's as Image objects, which works fine for just drawing but won't allow me to change the fill color.

I did try converting my svg to canvas commands, which allows me to change the fill but requires a lot of work to get scaled and positioned properly, and just isn't feasible with the amount of images I'm working with.

Is there any other way I can do this while still working with the canvas?

Answer 1

The trick is to load your svg as XML via XHR and manipulate it any way you want, then create your image out of it using data:image format.

E.g.

$.get('img/bottomLeftTop.svg', function(svgXml) {
  var img = new Image();
  var coloredSvgXml = svgXml.replace(/#3080d0/g,'#e05030');
  img.onload = () => context.drawImage(img,0,0);
  img.src = "data:image/svg+xml;charset=utf-8,"+coloredSvgXml;    
});

Here is a snippet I created to demonstrate the manipulation principle. It uses in-html hidden svg node to draw on 2d canvas, then changes the color via regexp and draws on the same canvas again:

var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");

var svg = document.getElementById('tmpSvg')
var blueCircle = (new XMLSerializer).serializeToString(svg);

var img = new Image();
img.onload = () => context.drawImage(img, 0, 0, 160, 120);
img.src = "data:image/svg+xml;charset=utf-8;base64, " + btoa(blueCircle);

redCircle = blueCircle.replace(/#3080d0/g, '#e05030');
var img2 = new Image();
img2.onload = () => context.drawImage(img2, 10, 10, 160, 120);
img2.src = "data:image/svg+xml;charset=utf-8;base64, " + btoa(redCircle);

.wrapper {
  display: none;
}
#canvas {
  width: 400px;
  height: 300px;
}

<canvas id="canvas"></canvas>

<div class="wrapper">
  <svg id="tmpSvg" xmlns="http://www.w3.org/2000/svg" width="200" height="200">
    <style>
      circle {
        fill-opacity: 0.5;
        stroke-width: 4;
        fill: #3080d0;
        stroke: #3080d0;
      }
    </style>
    <circle id="my-circle" cx="50" cy="50" r="30" />
  </svg>
</div>

Of course, nothing prevents you from using JavaScript built-in XML parser and XPath-based node manipulation. But in this particular case and for particular colors, regexp is probably more efficient.

Answer 2

One way is to create a temporary canvas for each image, get the image data and loop through it. In the loop, if pixel has color data, change that to the desired value.

var canvas = document.getElementById("c");
var ctx = canvas.getContext("2d");

canvas.width = 600;
canvas.height = 400;

var svg = new Image();

svg.onload = function () {
    canvas.width = svg.width;
    canvas.height = svg.height;
    // create temporary canvas
    var svgCanvas = document.createElement("canvas");
    var svgCtx = svgCanvas.getContext("2d");
    svgCanvas.width = svg.width;
    svgCanvas.height = svg.height;
    // draw the actual svg image to temporary canvas
    svgCtx.drawImage(svg, 0, 0);
    // get ImageData object
    var svgData = svgCtx.getImageData(0, 0, svgCanvas.width, svgCanvas.height);
    // get pixel data
    var data = svgData.data;
    // loop through data
    for (var i = 0; i < data.length; i += 4) {
        // check if pixel alpha value is not 0, then change the data
        if (data[i + 3] !== 0) {
            data[i] = 255; // pixel red value
            data[i + 1] = 0; // pixel green value
            data[i + 2] = 0; // pixel blue value
        }
    }
    // put the data back to the temporary svg canvas
    svgCtx.putImageData(svgData, 0, 0);
    // draw temporary canvas to the real canvas
    ctx.drawImage(svgCanvas, 0, 0);
}
// look out for CORS, the svg needs to be on the same origin.
svg.src = "https://upload.wikimedia.org/wikipedia/commons/thumb/3/38/Maki-alcohol-shop-15.svg/1024px-Maki-alcohol-shop-15.svg.png"; 

Reference to Pixel manipulation with canvas