Set the image’s src to a complete Base64 data URL, wait for that replacement image to finish loading, and only then call html2canvas(). jQuery can change the DOM element, but it cannot bypass browser same-origin rules. If the original image is on another domain, obtain its bytes through a CORS-enabled response or an authorized same-origin proxy first.
Working replacement pattern
This example replaces #target-image and captures the element containing it. The dataUrl value must include the media type and the base64, marker, not just the encoded payload.
const dataUrl = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAA...';
const $image = $('#target-image');
$image.one('load error', function (event) {
if (event.type === 'error') {
console.error('The replacement image could not be decoded.');
return;
}
html2canvas(document.querySelector('#capture'))
.then(function (canvas) {
document.body.appendChild(canvas);
})
.catch(function (error) {
console.error('html2canvas failed:', error);
});
});
$image.attr('src', dataUrl);The html2canvas call returns a Promise that resolves to the output canvas. Waiting for the image event prevents a capture from racing the replacement. The error branch makes a failed conversion visible instead of silently capturing a broken image.
Handle cached or already-complete images
A browser can finish loading a data URL immediately, before a listener is attached in more complex code. Check the underlying DOM image after assigning src, and verify naturalWidth:
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const image = document.querySelector('#target-image');
const dataUrl = 'data:image/jpeg;base64,/9j/4AAQSkZJRgABAQ...';
function capture() {
html2canvas(document.querySelector('#capture'))
.then(canvas => {
const outputUrl = canvas.toDataURL('image/png');
document.querySelector('#download-link').href = outputUrl;
})
.catch(console.error);
}
image.addEventListener('load', capture, { once: true });
image.addEventListener('error', function () {
console.error('Invalid or unsupported Base64 image.');
}, { once: true });
image.src = dataUrl;
if (image.complete) {
if (image.naturalWidth > 0) {
capture();
} else {
console.error('The image completed with a decoding error.');
}
}Guard against invoking capture() twice if your surrounding application can fire both the event and the completion check; a boolean flag or a one-shot helper is sufficient.
What a valid Base64 image URL contains
Use this structure:
data:<MIME type>;base64,<Base64 bytes>- PNG:
data:image/png;base64,... - JPEG:
data:image/jpeg;base64,... - WebP:
data:image/webp;base64,... - GIF:
data:image/gif;base64,...
Do not put a file path, a URL-encoded image URL, or a bare Base64 string in src. The browser needs the MIME type to decode the data. Base64 also increases the byte representation by roughly one third, so very large images consume more HTML and memory than a normal URL.
Converting an image to a data URL
If an image is already same-origin, fetch it as a Blob and use FileReader. This obtains the bytes before jQuery performs the replacement:
function imageUrlToDataUrl(url) {
return fetch(url)
.then(function (response) {
if (!response.ok) {
throw new Error('Image request failed: ' + response.status);
}
return response.blob();
})
.then(function (blob) {
return new Promise(function (resolve, reject) {
const reader = new FileReader();
reader.onload = function () { resolve(reader.result); };
reader.onerror = reject;
reader.readAsDataURL(blob);
});
});
}
imageUrlToDataUrl('/assets/logo.png')
.then(function (dataUrl) {
$('#target-image').attr('src', dataUrl);
// Attach the load/error handling shown earlier before capture.
})
.catch(console.error);For a cross-origin URL, this fetch succeeds only when the server permits your origin with CORS headers. A server response that lacks the required Access-Control-Allow-Origin header cannot be made readable by changing jQuery code.
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html2canvas reconstructs a canvas from DOM and style information; it is not a pixel-level browser screenshot. A foreign image can taint the canvas, after which reading it with toDataURL() or similar APIs fails. The project’s FAQ, limitations documentation and MDN CORS guidance describe this boundary.
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Same-origin image
If the page and image share an origin (scheme, host and port), use the direct URL or convert it locally. No CORS option is needed.
Remote host you control
Configure the image server to return an appropriate Access-Control-Allow-Origin value, then load the image with CORS enabled:
html2canvas(document.querySelector('#capture'), {
useCORS: true
}).then(function (canvas) {
document.body.appendChild(canvas);
});useCORS asks the browser to perform a CORS image load; it does not grant permission when the server omits the header.
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Use a controlled, same-origin proxy that fetches an allowlisted remote image and returns it to your page with your origin. The html2canvas configuration supports a proxy option. Do not expose an unrestricted “fetch any URL” endpoint: validate destinations, restrict protocols and private-network addresses, limit response size, verify content type, and apply timeouts and rate limits. The project documents the proxy mechanism, not a particular proxy provider.
Why allowTaint is not a solution
The documented allowTaint option permits rendering conditions that may taint the canvas; it does not make a tainted canvas exportable. If you need a readable PNG or data URL, solve access with same-origin delivery, CORS, or a proxy.
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Waiting for several replacement images
When a capture contains multiple images, wait for every replacement to settle. Decide whether one failure should abort the operation or leave that image out:
function waitForImage(image, dataUrl) {
return new Promise(function (resolve, reject) {
let settled = false;
function done(fn, value) {
if (!settled) {
settled = true;
fn(value);
}
}
image.addEventListener('load', function () { done(resolve, image); }, { once: true });
image.addEventListener('error', function () {
done(reject, new Error('Image failed to load: ' + image.src.slice(0, 80)));
}, { once: true });
image.src = dataUrl;
if (image.complete) {
image.naturalWidth > 0 ? done(resolve, image) : done(reject, new Error('Image decode failed'));
}
});
}
const replacements = [
{ element: document.querySelector('#hero'), dataUrl: heroDataUrl },
{ element: document.querySelector('#avatar'), dataUrl: avatarDataUrl }
];
Promise.all(replacements.map(function (item) {
return waitForImage(item.element, item.dataUrl);
}))
.then(function () {
return html2canvas(document.querySelector('#capture'));
})
.then(function (canvas) {
document.body.appendChild(canvas);
})
.catch(function (error) {
console.error('Capture cancelled:', error);
});Useful html2canvas configuration
Verify defaults against the version installed in your project; documentation and defaults can change. The documented options most relevant to this problem are:
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| Option | Purpose | Important behavior |
|---|---|---|
useCORS |
Attempt CORS image loading | Default is false; server headers are still required. |
proxy |
Route resource loading through a proxy | Default is null; operate a restricted proxy. |
allowTaint |
Allow potentially tainting resources | Does not make export APIs work on a tainted canvas. |
imageTimeout |
Maximum image wait | Documented default is 15,000 milliseconds. |
onclone |
Modify the cloned document before rendering | Useful for temporary style or content changes. |
See the project’s configuration reference for the release-specific list.
Exporting the captured canvas
The Base64 assigned to the source image and the output data URL are separate values. After html2canvas resolves, export the resulting canvas:
html2canvas(document.querySelector('#capture')).then(function (canvas) {
const pngDataUrl = canvas.toDataURL('image/png');
const link = document.createElement('a');
link.href = pngDataUrl;
link.download = 'capture.png';
link.click();
});This follows the output pattern shown in the official examples.
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Troubleshooting checklist
The image area is blank
- Confirm the value starts with
data:image/...and contains;base64,. - Inspect
naturalWidth; zero indicates an invalid or undecodable payload. - Start html2canvas only after the replacement’s
loadevent. - Check that CSS does not hide the element or give it zero dimensions.
SecurityError appears during export
A cross-origin resource tainted the canvas. Enable CORS only when the remote server sends the matching header, or deliver the image through a restricted same-origin proxy. Replacing the URL string with jQuery does not alter browser permissions.
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useCORS: true changes nothing
Inspect the image response in browser developer tools. If Access-Control-Allow-Origin is absent or incompatible, the browser will still block readable access. Correct the server configuration or use a proxy.
The capture times out
Large images, slow hosts and a low imageTimeout can delay rendering. Reduce source dimensions, preload images, or set a timeout appropriate for your application while retaining an explicit failure path.
Only some images render
Use a per-image load/error promise and log the failing URL. Choose an all-or-nothing policy with Promise.all, or use a settled-results approach if missing images are acceptable.
Performance and security considerations
- Base64 enlarges image data and duplicates it in memory when both the original and replacement remain referenced. Revoke unnecessary object URLs and release temporary data after capture.
- Resize oversized source images before encoding; html2canvas must process the rendered pixel area.
- Keep proxy allowlists narrow and reject redirects to untrusted destinations. Treat remote image bytes as untrusted input.
- Do not put private credentials in a browser-visible image URL or data URL. Fetch protected assets through an authorized backend route.
- Run capture work after the page has applied the final layout. Fonts, animations and lazy-loaded content can otherwise produce inconsistent output.
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Frequently Asked Questions
Can I assign only the Base64 characters to src?
No. Include the complete data URL prefix, such as data:image/png;base64,, before the encoded bytes.
Does html2canvas capture browser pixels exactly?
No. It rebuilds a canvas from DOM and style information and supports only the features it understands.
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Should I use a proxy for every remote image?
No. Use direct same-origin delivery when possible, CORS when the image host allows it, and a restricted proxy only when the host cannot be changed.
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