问题

I have to apply some styles on <img> thanks to a CSS class.

Is it possible to get the dataURL of the <img> with the CSS style ?

$(function() {
  // Original
  const imgOriginal = document.getElementById('original');
  const c1 = document.getElementById('c1');
  let ctx = c1.getContext('2d');
  ctx.drawImage(imgOriginal, 100, 100);

  // Filtered
  const imgFiltered = document.getElementById('filtered');
  const c2 = document.getElementById('c2');
  ctx = c2.getContext('2d');
  ctx.drawImage(imgFiltered, 100, 100);

  // Same dataURL :(
  console.log(c1.toDataURL(), c2.toDataURL());
  console.log(c1.toDataURL() === c2.toDataURL());
})

.filter::before {
  display: block;
  height: 100%;
  left: 0;
  position: absolute;
  top: 0;
  width: 100%;
  z-index: 1;
  border: 1px solid red;
}

.filter {
  position: relative;
  -webkit-filter: sepia(.5) hue-rotate(-30deg) saturate(1.4);
  filter: sepia(.5) hue-rotate(-30deg) saturate(1.4);
}

canvas {
  display: block;
  width: 100px;
  height: 100px;
  border: 1px solid red;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>


<div>

  <img id="original" src="https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Brad_Pitt_Inglorious_Basterds_Berlin_premiere.jpg/170px-Brad_Pitt_Inglorious_Basterds_Berlin_premiere.jpg">
  <canvas id="c1"></canvas>

  <img id="filtered" class="filter" src="https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Brad_Pitt_Inglorious_Basterds_Berlin_premiere.jpg/170px-Brad_Pitt_Inglorious_Basterds_Berlin_premiere.jpg">
  <canvas id="c2"></canvas>

</div>

Maybe snippet is going to have a bug because of the <canvas> tag, the idea is there anyway.

EDIT :

If anyone has a suggestion with SVG or something else, I'm using fabricJS.

EDIT 2 (NOT RESOLVE BUT FIND OTHER WAY) :

Thanks to @KavianK. you could replicate CSS style with the canvas context, however to me it's boring because we have to store a different callback for each CSS class to get the dataURL. Working anyway!
Thanks to @Emeeus maybe a solution provide from your backend, not solution for me beacause i'm want to do this ONLY on the front-end. wkhtmltopdf
Thanks to @pegasuspect we can filter an image with SVG, I'm following this way and I replace fabricJS by svgjs, this librairie can replace easly a canvas and it's more easier to work with img and I dind't need the DataURL anymore !
Thanks to @Kaiido there is a way to take a snapshot of your HTML rendered with CSS style with html2canvas easy to get dataURL with this case. Unfortunataly some CSS styles are not supported yet like box-shadow or filter that's why it's not a solution for me

This topic is not resolve but with svgjs I don't need actually work with dataURL.

回答1:

CSS and DOM is a separate world from the bitmaps that are used for images and canvas. The bitmaps themselves are not affected by CSS, only the elements which acts as a looking-glass to the bitmap. So, CSS filters applied to the canvas will not be applied to the image that is produced. You either need to replicate the filters in canvas or rather re apply the same filters to the generated image.

Example:

There is a little known property on the context object, conveniently named filter. This will apply a filter on the context it self. The filter must be set before next draw operation.

var img = new Image();

img.crossOrigin = '';
img.src = document.getElementById( 'original' ).src;

img.onload = function() {
    var canvas = document.getElementById( 'canvas' ),
        ctx = canvas.getContext( '2d' );

    canvas.width = this.width;
    canvas.height = this.height;

    // filter
    if ( typeof ctx.filter !== 'undefined' ) {
        ctx.filter = "sepia(.5) hue-rotate(-30deg) saturate(1.4)";
        ctx.drawImage(this, 0, 0);
    } else {
        ctx.drawImage(this, 0, 0);
    }

    document.getElementById( 'filtered' ).src = canvas.toDataURL();
}

<img id="original" src="https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Brad_Pitt_Inglorious_Basterds_Berlin_premiere.jpg/170px-Brad_Pitt_Inglorious_Basterds_Berlin_premiere.jpg" />
<img id="filtered" />
<canvas id="canvas" style="display: none"></canvas>

回答2:

tldr;

You can do it with SVG. http://jsfiddle.net/1hambw93/91/

How to use SVG as a data Source for Images

It is explained here quite nicely. It basically says you can use svg element in src tag of an img.

How to use filters in SVG

It is explained here quite nicely as well: I could achieve the same filter effect with your code, using an SVG filter.

You can generate filter for svg from this site using Sepium to get the same filter as your css. You would have the following SVG so far:

<svg id="test">
    <image xlink:href='https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Brad_Pitt_Inglorious_Basterds_Berlin_premiere.jpg/170px-Brad_Pitt_Inglorious_Basterds_Berlin_premiere.jpg'
      x="0" y="0" height="170px" width="218px" 
      filter='url(#sepium-filter)' />
    <filter id="sepium-filter">
      <feColorMatrix type="matrix" 
        values="1.3 -0.3 1.1   0 0 
                  0  1.3 0.2   0 0 
                  0    0 0.8 0.2 0 
                  0    0   0   1 0">
      </feColorMatrix>
    </filter>
</svg>

Then I used a minimal javascript code to convert to base64 and write the result from the SVG to an object, to display in HTML.

回答3:

As I understand, You need the computed image, so you need the result of the work made by the browser. To accomplish that, you could use wkhtmltopdf specifically wkhtmltoimage. It uses Qt WebKit rendering engine (just like a browser). You have to install that in the server and run somethig like:

wkhtmltoimage  http://mysite/image-Plus-css-PLus-canvas.html myComputedImge.jpg

Where http://mysite/image-Plus-css-PLus-canvas.html is your image in the html with the css, javascript and whatever. All of this of course could be accomplished using ajax.

So, using this way you have a .jpg or .png file (myComputedImge.jpg in this case) that has all what you want computed, like a screenshot. If you want the base64 you could do the same as you do with .toDataURL() and the result is the base64 of the image with the css.

If you want to perform this using ajax, you could:

Send a request, with the file as parameter or the remote url.
If the file is remote the server must be able to reach it.
In server side, you create a .html file with the image, the css and javascript or whatever you want.
Run the code above detailed to create a .jpg or .png
Respond the location of the file created to the ajax request.
In the callback, you have the url of the new image with the filters computed.
Since you have this new image in server side you could respond the base64 directly instead of the location, then in client side you don't need .toDataURL().

回答4:

There is 1 work around; It might work for your use case using CSS:

img[src^="data:image/png;"] {
  background-color: #000;
  filter: sepia(50%);
}

This will work for all the images URLs starting with data:image/png

来源：https://stackoverflow.com/questions/50971937/dataurl-of-an-img-with-css-class

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fabricjs

DataURL of an img with CSS class

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