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Draw every inner canvas first, then wait for the renderer and at least one browser paint frame before calling html2canvas(). When the returned Promise resolves, export its canvas with toDataURL() or toBlob(). A blank nested region is usually a timing problem: html2canvas cloned the container before the chart, game, or custom renderer had finished putting pixels into the child canvas.
The reliable sequence
Keep the capture target on the element that contains the nested canvas. Your application—not html2canvas—must know when the inner renderer is complete.
- Start the chart, game, or custom draw operation.
- Await its completion Promise (or finish the synchronous draw).
- Wait for a browser frame so painting can catch up.
- Call
html2canvas()on the containing element. - Await the returned Promise.
- Export the resulting canvas.
async function captureContainer() {
// Application code: resolve when the nested canvas contains its final pixels.
await drawNestedCanvas();
await new Promise(requestAnimationFrame);
const output = await html2canvas(document.querySelector('#capture'), {
useCORS: true,
backgroundColor: null,
scale: window.devicePixelRatio,
});
const pngUrl = output.toDataURL('image/png');
return pngUrl;
}drawNestedCanvas() is deliberately an application function. html2canvas cannot infer when Chart.js, a WebGL scene, a game loop, or another third-party renderer has completed. If drawing is synchronous, the animation-frame wait still gives the browser a chance to present the new pixels before the DOM is cloned.
What html2canvas captures—and what it does not
html2canvas does not ask the browser for a native screenshot. The project describes it as building a representation from DOM information, cloning and traversing the tree, then painting supported properties to an output canvas (project documentation). That reconstruction can differ from the visible screen when CSS, browser features, or content outside the accessible DOM is unsupported.
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A child <canvas> is therefore safest when it already contains its pixels before cloning begins. Capture the actual element that contains the canvas; a wrapper with different dimensions or an inaccessible frame document can produce a result that looks blank even though the live page looks correct.
Does html2canvas copy an existing canvas?
It can render an accessible canvas element as part of the reconstructed DOM, but it is not a general-purpose pixel copier for every browser boundary. The destination option named canvas only supplies an existing output canvas for html2canvas to draw into; it does not import a nested canvas automatically. Treat the child canvas as input that must be fully rendered and readable first.
A complete browser example
The following example draws a nested canvas, captures its parent, and downloads a PNG. Replace the drawing function with your own renderer or chart library’s completion hook.
<div id="capture">
<h2>Revenue</h2>
<canvas id="chart" width="900" height="450"></canvas>
</div>
<button id="save">Save PNG</button>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/html2canvas.min.js"></script>
<script>
const chart = document.querySelector('#chart');
function drawNestedCanvas() {
const ctx = chart.getContext('2d');
ctx.clearRect(0, 0, chart.width, chart.height);
ctx.fillStyle = '#1769aa';
ctx.fillRect(80, 80, 500, 180);
ctx.fillStyle = '#111';
ctx.font = '28px sans-serif';
ctx.fillText('Quarterly revenue', 80, fifty = fifty = fifty = fifty = fifty = fifty = 50);
return Promise.resolve();
}
async function captureContainer() {
await drawNestedCanvas();
await new Promise(requestAnimationFrame);
const output = await html2canvas(document.querySelector('#capture'), {
useCORS: true,
backgroundColor: null,
scale: window.devicePixelRatio,
});
return output;
}
document.querySelector('#save').addEventListener('click', async () => {
const output = await captureContainer();
const link = document.createElement('a');
link.download = 'capture.png';
link.href = output.toDataURL('image/png');
link.click();
});
</script>In production, correct the sample’s text baseline to your desired coordinate and use your real renderer. The important part is the ordering, not the chart geometry.
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Options that matter for nested canvases
scale: output sharpness
Set scale to window.devicePixelRatio for a high-DPI result, as shown in the official configuration examples (configuration). Larger values increase output dimensions and memory use; use a lower fixed value when screenshots are too large.
Cropping with x, y, width, and height
When only the chart area is needed, provide crop coordinates in the target element’s coordinate system. First make the complete content appear, then tune the crop. Cropping too early can hide the real timing or layout failure.
Cross-origin images: useCORS, proxy, and allowTaint
useCORS: true asks the browser to request images with CORS. It works only when the remote server returns permitting CORS headers. If direct loading is impossible, proxy can route resources through a proxy that your application controls. The html2canvas project states that images must be same-origin unless CORS or a proxy makes them available (FAQ).
allowTaint controls whether cross-origin images that would taint the output are allowed into rendering. It does not make a tainted canvas readable for a later toDataURL() call. If the nested canvas draws an image or video from another origin without successful CORS, redraw it from an accessible source or fix the response headers.
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foreignObjectRendering
With foreignObjectRendering: true, html2canvas can use its alternate ForeignObjectRenderer where the browser supports it. This may improve fidelity for some DOM/CSS combinations, but it changes browser compatibility and is not a universal fix for a blank nested canvas. Try the default renderer first.
onclone and canvas
Use onclone to make capture-only changes in the cloned document—for example, reveal a print label without altering the live page. The canvas option supplies the destination canvas. Neither option replaces the need to finish the nested draw.
Charts, animation, WebGL, and asynchronous data
Chart libraries
Prefer the library’s documented “render complete” callback or Promise. Do not assume that the constructor returning means the final pixels are present; fonts, data, and animations may still be pending. Disable animation for deterministic exports when your library supports that setting, then await its completion callback.
Animations and games
Pause the loop at a known frame, draw that frame, resolve your capture Promise, and resume afterward. Capturing during a continuously changing animation can produce a valid but inconsistent frame.
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WebGL canvases
Finish GPU commands and present the intended frame before capture. A security or context configuration that prevents reading pixels cannot be repaired by html2canvas; use an application-supported export path or render the scene in a readable canvas.
Same-origin rules, tainting, and iframes
Once a canvas has been tainted by a cross-origin image or video, browser security prevents safe pixel reads. html2canvas reports the same limitation: an already tainted canvas cannot be read by the script (documentation). CORS must be valid at the time the resource is loaded; adding useCORS after the fact does not clean existing pixels.
Same-origin iframe
A same-origin iframe can be rendered recursively because the parent can access its document.
Cross-origin or sandboxed iframe
Cross-origin frames, and sandboxed frames without allow-same-origin, are blocked by browser security. The outer page cannot inspect their contentDocument. Run html2canvas inside the frame with cooperation from that document, or redesign the integration so the content is available to the capturing origin. A parent-page option cannot bypass this boundary.
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Debugging a blank nested region
- Inspect the inner canvas. Confirm its width and height are non-zero and that it contains pixels before capture. A quick test is to call
innerCanvas.toDataURL()before html2canvas; a security exception indicates tainting. - Verify the target. Pass the element that actually contains the canvas, not an ancestor with hidden dimensions and not an inaccessible frame document.
- Prove sequencing. Log immediately before and after the draw Promise, then await
requestAnimationFramebefore html2canvas. - Enable logging. html2canvas logging is enabled by default; inspect the browser console while reproducing the issue (configuration).
- Read security errors. Fix image or video origins, response headers, or proxy configuration before changing rendering options.
- Compare renderers. Test the default renderer first; compare
foreignObjectRendering: trueonly in a browser combination where it is supported. - Tune geometry last. After content appears, adjust
scaleand crop coordinates. - Export after resolution. Never call
toDataURL()on the Promise returned by html2canvas; await it first.
Common errors and fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Child canvas is white or transparent | Capture ran before drawing completed | Await the renderer, then wait one animation frame. |
SecurityError from toDataURL() |
The canvas is tainted by a cross-origin resource | Use same-origin assets or valid CORS/proxy support; redraw after fixing access. |
| Images missing but DOM text appears | Remote image lacks CORS headers or loaded after cloning | Preload it, enable useCORS when permitted, or configure a proxy. |
| Iframe content missing | Cross-origin or sandbox isolation | Capture inside the frame or obtain cooperation from its origin. |
| Screenshot is sharp but enormous | Device-pixel scale multiplies dimensions | Use a controlled numeric scale and crop with width/height. |
| Layout differs from the screen | DOM reconstruction does not support a CSS feature exactly | Adjust capture-only markup in onclone, or use a native browser capture. |
When html2canvas is the right tool
Use it when the target DOM, nested canvas, and image resources are accessible to the page and you want a client-side image. It gives direct control over crop geometry, output scale, and a capture-only clone. Choose native browser automation or server-side rendering when you need a pixel-level browser screenshot, must capture cross-origin frames without cooperation, or cannot change inaccessible resources.
Evaluate alternatives on five questions: can the renderer read the content; how closely must CSS match; does the inner renderer expose a completion signal; what output dimensions are required; and can any iframe origin cooperate? These constraints determine the tool more than a single option flag.
Or skip the browser setup
For a server-side screenshot or PDF, ScreenshotNeo accepts one GET request and returns PNG, JPEG, WebP, or PDF. It removes cookie and consent banners, newsletter popups, and chat widgets before capture; bot checks, blank pages, failed loads, timeouts, and cache hits are not billed, with the result identified by X-Page-Verdict and X-Billed headers. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools to Claude, Cursor, and other MCP clients.
Basic cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
r.raise_for_status()
open("shot.webp", "wb").write(r.content)Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://stripe.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`${res.status} ${await res.text()}`);
const data = Buffer.from(await res.arrayBuffer());
require('node:fs').writeFileSync('shot.webp', data);See the full parameter list and options in the ScreenshotNeo documentation. You can select full-page or element captures, dark mode, device presets, retina scale, custom CSS and JavaScript, waits, request blocking, headers, cookies, geolocation, transparent backgrounds, resizing, caching, signed links, asynchronous webhooks, bulk capture, and PDF settings. Every plan includes every feature. The Free plan includes 1,000 shots per month with no card; paid plans start at $5 for 3,000 shots. Create a free ScreenshotNeo account.
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Can I capture only the nested canvas instead of its parent?
Yes. Pass the canvas element itself to html2canvas when you do not need surrounding DOM, but the same draw-completion, accessibility, and tainting rules still apply.
Why does waiting with setTimeout sometimes help but remain unreliable?
A fixed delay guesses at renderer timing. A completion Promise or callback identifies the actual end of drawing; use a frame wait afterward only to coordinate browser painting.
Will html2canvas capture a cross-origin iframe if the page has CORS enabled?
No. Image CORS and iframe document access are separate controls. The frame must be same-origin or cooperate by running capture inside itself.
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