Why is my Canvas `drawImage()` returning empty data or failing to render, and how can I fix it using `onload` and CORS?

CanvasContext2D drawImage() Conundrum: Onload and CORS

In an attempt to paint an image onto a canvas before retrieving its dataURL, users may encounter an issue where the returned data appears empty, resembling a string filled with "A" characters. Furthermore, appending the canvas to the document may fail to render the image, while no errors are reported in the console.

Problem Resolution

The root of the issue lies in the asynchronous nature of image loading. To resolve it, one must utilize the onload event handler of the element to execute the painting operation only after the image has finished loading.

// Create a new image
var img = new Image();

// Define a function to execute after the image has loaded
img.onload = function() {
  var canvas = document.createElement('canvas');
  canvas.width = img.width;
  canvas.height = img.height;
  var context = canvas.getContext('2d');
  context.drawImage(img, 0, 0);

  // Now you can retrieve the dataURL
  var dataURL = canvas.toDataURL();
  doSomething(dataURL);
};

// Set the image source
img.src = "http://somerandomWebsite/picture.png";

CORS and Data Retrieval

For context.toDataURL() and context.getImageData methods to function properly, the image resource must be retrieved in a cross-origin-compliant manner. Otherwise, the canvas becomes "tainted," blocking any attempt to retrieve pixel data.

If the image originates from the same server as your script, there are no issues. However, if the image resides on an external server, it is crucial to ensure that the server allows cross-origin access by setting appropriate CORS headers. Additionally, the img.crossOrigin property should be set to either "use-credentials" for authenticated requests or "anonymous" for anonymous requests.

It is important to note that CORS headers are set on the server side. The cross-origin attribute only indicates the client's intention to access the image data using CORS. Circumventing CORS restrictions is not possible if the server configuration prohibits it.

In certain scenarios, some images may be hosted on your server while others may require CORS. To handle this, consider using the onerror event handler, which is triggered when the cross-origin attribute is set to "anonymous" on a server that does not support CORS.

function corsError() {
  this.crossOrigin = "";
  this.src = "";
  this.removeEventListener("error", corsError, false);
}

img.addEventListener("error", corsError, false);

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