The Tool Desk
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The basic composition pattern
html2canvas() is asynchronous. Each call returns a Promise that resolves to a <canvas> element, so composition must happen after both captures finish. Promise.all() starts independent captures together and gives you both results in their original order.
const [first, second] = await Promise.all([
html2canvas(document.querySelector('#first')),
html2canvas(document.querySelector('#second')),
]);
const combined = document.createElement('canvas');
combined.width = Math.max(first.width, second.width);
combined.height = first.height + second.height;
const context = combined.getContext('2d');
context.drawImage(first, 0, 0);
context.drawImage(second, 0, first.height);
document.body.appendChild(combined);The first image is placed at the destination origin. The second starts at a y-coordinate equal to the first bitmap’s height, so it cannot overlap it. The calculation uses canvas.width and canvas.height, not CSS dimensions.
Complete browser example
This page contains two capture targets, a button, and a PNG download. Load html2canvas before the application script, or install it through your normal package workflow.
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<button id="capture" type="button">Combine captures</button>
<div id="first" class="panel">
<h2>First panel</h2>
<p>This content becomes the top part of the output.</p>
</div>
<div id="second" class="panel">
<h2>Second panel</h2>
<p>This content becomes the lower part of the output.</p>
</div>
<img id="preview" alt="Combined capture preview" hidden>
<script src="https://cdn.jsdelivr.net/npm/html2canvas@latest/dist/html2canvas.min.js"></script>
<script>
const button = document.querySelector('#capture');
const preview = document.querySelector('#preview');
button.addEventListener('click', async () => {
button.disabled = true;
try {
const [first, second] = await Promise.all([
html2canvas(document.querySelector('#first')),
html2canvas(document.querySelector('#second')),
]);
const combined = document.createElement('canvas');
combined.width = Math.max(first.width, second.width);
combined.height = first.height + second.height;
const context = combined.getContext('2d');
if (!context) throw new Error('Canvas 2D context is unavailable');
context.drawImage(first, 0, 0);
context.drawImage(second, 0, first.height);
preview.src = combined.toDataURL('image/png');
preview.hidden = false;
combined.toBlob((blob) => {
if (!blob) throw new Error('PNG encoding failed');
const link = document.createElement('a');
link.href = URL.createObjectURL(blob);
link.download = 'combined-capture.png';
link.click();
URL.revokeObjectURL(link.href);
}, 'image/png');
} catch (error) {
console.error('Capture failed:', error);
alert('The captures could not be combined. Check the console for details.');
} finally {
button.disabled = false;
}
});
</script>For production code, prefer toBlob() for downloads and uploads. toDataURL() is convenient for a small preview but stores the entire encoded image in a JavaScript string and can consume considerably more memory.
Choose the output layout
Vertical stacking
Use this for a long report, receipt, or page assembled from sections. The destination width is the larger source width and the destination height is the sum of both heights.
const gap = 24;
combined.width = Math.max(first.width, second.width);
combined.height = first.height + gap + second.height;
context.fillStyle = '#ffffff';
context.fillRect(0, 0, combined.width, combined.height);
context.drawImage(first, 0, 0);
context.drawImage(second, 0, first.height + gap);The background fill is important when you want a predictable color instead of transparent gaps.
Side-by-side placement
For a comparison image, add the source widths and use the taller source height. The second bitmap begins at the first bitmap’s width.
const combined = document.createElement('canvas');
combined.width = first.width + second.width;
combined.height = Math.max(first.height, second.height);
const context = combined.getContext('2d');
context.drawImage(first, 0, 0);
context.drawImage(second, first.width, 0);If the panels have different heights, the shorter one leaves transparent pixels below it unless you paint a background first.
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Overlap or layered composition
Set the destination dimensions for the design, draw the background or lower layer first, and then draw the foreground. Later calls are painted above earlier pixels.
combined.width = 1200;
combined.height = 800;
context.fillStyle = '#111827';
context.fillRect(0, 0, combined.width, combined.height);
context.drawImage(first, 40, 40, 700, 500);
context.drawImage(second, 360, 220, 700, 500);The four-argument form of drawImage() deliberately resizes a source. Use it only when the design calls for resampling; otherwise, draw at native bitmap dimensions.
Resolution, scale, and coordinate choices
html2canvas defaults its scale option to window.devicePixelRatio. A high-density display can therefore produce a bitmap several times larger than the element’s CSS box. That is why a 600 CSS-pixel element may have a canvas width of 1,200 or more.
- Preserve capture resolution: calculate positions and output dimensions from each source’s
widthandheight. This avoids accidental resampling. - Create a CSS-sized result: choose one scale, divide or resize source dimensions consistently, and use matching destination coordinates. Mixing CSS pixels with bitmap pixels causes gaps, cropping, or unexpected size.
- Control scale at capture time: pass an explicit
scaleto both html2canvas calls when predictable output size matters. Use the same value unless different resolutions are intentional.
Keep the destination dimensions as integers. Before allocating a very large canvas, estimate memory: an RGBA bitmap requires roughly four bytes per pixel, before temporary buffers and encoded output are included.
Capture in parallel or sequentially?
Parallel captures with Promise.all()
Parallel scheduling is concise and normally reduces total waiting time when the targets are independent. Both render operations and their temporary bitmaps exist at once, so peak memory is higher.
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Sequential captures
const first = await html2canvas(document.querySelector('#first'));
// Release or process first here if your application needs to limit memory.
const second = await html2canvas(document.querySelector('#second'));Use sequential work when the page is memory-constrained, when the second target depends on state established by the first, or when you intentionally want to limit rendering load. It increases elapsed time.
Export and lifecycle management
- PNG: lossless and supports transparency; use
combined.toBlob(callback, 'image/png'). - JPEG: smaller for photographic content, but transparency is discarded; pass a quality value such as
0.9. - WebP: available in browsers that support that encoder; verify support if files are user-critical.
Revoke every object URL after the download or image has loaded. Remove references to source and destination canvases when finished so garbage collection can reclaim their backing stores. If you repeatedly capture, reuse a destination canvas only when its dimensions and background policy are known; otherwise, stale pixels can remain.
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Cross-origin images and tainted canvases
html2canvas follows browser security rules. A remote image, font, video frame, or other resource without appropriate CORS permission can taint a source canvas. Drawing that source into a new canvas does not remove the taint; export methods such as toBlob() and toDataURL() can then throw a security error.
Practical remedies
- Serve the resource from the same origin as the page.
- Configure the remote server to send an appropriate
Access-Control-Allow-Originheader and use html2canvas withuseCORS: true. - Route the asset through a server-side, same-origin proxy that you control.
allowTaint does not make a tainted canvas safe to read or export. It changes rendering behavior, not the browser’s origin-clean requirement.
Canvas size and browser limits
Maximum canvas dimensions vary by browser, operating system, graphics stack, and device. Oversized canvases may be blank, partially rendered, or fail without a useful exception. The limits documented by html2canvas are approximate guides, not guarantees.
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- Test the actual output size on the browsers and devices you support.
- Split a long page into several captures instead of creating one enormous bitmap.
- Reduce html2canvas
scaleor resize each source before composition when full device-pixel resolution is unnecessary. - Check for a zero-sized target before capture; hidden elements and collapsed containers can produce empty results.
Troubleshooting common failures
The second image is missing or appears at the wrong location
Check that the y-coordinate is first.height for vertical stacking, or the x-coordinate is first.width for side-by-side output. Do not use offsetHeight or CSS width when the source bitmap has a device-pixel scale.
The result is blank
Confirm that both selectors resolve to visible, laid-out elements and that the destination dimensions are positive. A destination that exceeds a device’s canvas limit can also render blank. Try a lower explicit capture scale and a smaller test region.
toDataURL() throws a security error
A source is probably tainted by cross-origin content. Apply the CORS or same-origin remedies above, then capture again; compositing an already tainted canvas cannot repair it.
Text or CSS differs from the page
html2canvas is a DOM renderer, not a native browser screenshot API. It reconstructs the image from DOM and style information and does not implement every CSS property. Verify each source capture independently before debugging the composition code.
Images are missing inside a capture
Wait until images and fonts are loaded before calling html2canvas, and ensure remote assets permit CORS when they are not same-origin. Lazy content may also need to be made visible or loaded before capture.
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The browser becomes unresponsive
Reduce concurrent work by capturing sequentially, lower the scale, capture smaller elements, and avoid holding multiple large data URLs. Prefer blobs and release object URLs promptly.
Or skip the browser setup
ScreenshotNeo provides a website screenshot API and MCP server when you need a rendered URL rather than a DOM-only browser script. A single request returns PNG, JPEG, WebP, or PDF. It accepts cookie and consent banners like a visitor, then removes more than 60 known consent platforms along with newsletter popups and chat widgets before capture. Bot checks, CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers identify the page verdict and billing result.
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(`Screenshot failed: ${res.status}`);
const data = Buffer.from(await res.arrayBuffer());
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Frequently Asked Questions
Can I combine more than two html2canvas results?
Yes. Allocate a destination large enough for the chosen layout and call drawImage() once for each source, updating each x and y offset as you place it.
Why are my source canvases different sizes on the same page?
html2canvas can follow the display’s device-pixel ratio, and the elements may also have different CSS dimensions. Inspect each bitmap’s width and height before calculating the destination.
Should I use an image editor instead of a canvas?
Canvas is appropriate for a browser-side result that you can immediately display, download, or upload. Use a server-side image pipeline when files are extremely large or must be processed reliably outside a browser.
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