How to Solve CanvasContext2D drawImage() Issues with Onload and CORS?

CanvasContext2D drawImage() Issue: Onload and CORS

When attempting to draw an image onto a canvas prior to obtaining its dataURL, the returned information may appear deficient, filled with numerous "A" characters. This issue arises due to the necessity of waiting for the image to complete loading before it can be painted on the canvas. To rectify this, utilize the load event handler for the image element.

To address this issue, implement the following steps:

1. Wait for Image Loading: Include a onload event handler for the image element. This function will execute once the image has fully loaded.
2. Draw Image on Canvas: Within the onload handler, create the canvas and draw the image onto it using the drawImage method.
3. Obtain DataURL: After drawing the image, utilize toDataURL to retrieve the dataURL.

A potential hindrance is related to CORS (Cross-Origin Resource Sharing). For context.toDataURL() and context.getImageData to operate seamlessly, the image resource must be obtained in a cross-origin compatible manner. Otherwise, the canvas will be marked as "tainted," preventing access to pixel data. To circumvent this, adhere to the following guidelines:

1. Same-Origin Images: Images from the same server pose no issue.
2. External Images - Cross-Origin Header: Ensure that the external server hosting the image enables CORS in its headers. Additionally, set img.crossOrigin to "use-credentials."
3. External Images - Anonymous Requests: If the server permits anonymous requests, set img.crossOrigin to "anonymous."

It is crucial to note that CORS headers originate from the server. The cross-origin attribute solely signifies your intention to leverage CORS for data retrieval. If the server is not configured correctly, no workaround exists.

In rare cases involving images from mixed sources (own server and CORS-compliant servers), consider employing the onerror event handler. This handler will activate when the cross-origin attribute is set to "anonymous" on a non-CORS server.

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