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If a Firebase Storage image appears on your page but disappears from an html2canvas capture or jsPDF file, first check the image response for CORS headers. Displaying an image does not prove the browser will let JavaScript read its pixels. Configure the bucket to allow the exact page origin and GET requests, set crossOrigin = "anonymous" before assigning the image URL, then capture with useCORS: true. Changing jsPDF settings cannot repair a response that the browser has made unreadable.
Why a Firebase image can display but fail in a canvas or PDF
A Firebase download URL can point to a different origin from the page displaying it. Browsers may show a cross-origin image in an <img>, while blocking script access to its pixel data unless the image response permits cross-origin access. Canvas export requires those pixels to be readable. If a canvas draws an image without the required permission, it can become tainted; reading it with toDataURL() then fails. Passing that canvas to jsPDF does not bypass this browser security boundary.
Firebase’s browser download guidance says: “To download data directly in the browser, you must configure your Cloud Storage bucket for cross-origin access (CORS).” Firebase Storage: Download files in web. Cloud Storage returns an Access-Control-Allow-Origin response header when the request origin matches the bucket’s CORS policy. Google Cloud Storage CORS
Diagnose the image request before changing PDF code
- Get the actual download URL. Use Firebase’s
getDownloadURL(ref(storage, path))and inspect the resulting URL locally. Avoid writing full download URLs to production logs if they expose access-bearing tokens. - Inspect the request from the deployed page. Open browser DevTools, select Network, reload the page, and inspect the Firebase image request. Check its status,
Content-Type, and whether the response includesAccess-Control-Allow-Originfor the exact deployed page origin. - Separate access failures from CORS failures. A 401 or 403, a missing object, or a response that is not an image must be resolved as an authentication, Storage Rules, path, or content issue. Confirm the current user is allowed to read the object. CORS does not grant permission to read a protected file.
- Test all relevant origins. Development and production often use different origins, such as localhost and a deployed domain. A policy for one does not automatically allow the other.
html2canvas exposes useCORS, which defaults to false, and allowTaint, which also defaults to false. Its default image timeout is 15,000 ms. html2canvas configuration Its FAQ explains that cross-origin images can taint a canvas and describes proxy loading when direct cross-origin access is unavailable. html2canvas FAQ
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Configure Firebase Storage CORS for the page origin
Set the bucket policy to allow the exact origins that render and export the page, including separate development and production origins as needed, and allow GET. Deploy the policy using the Google Cloud tooling and instructions for Cloud Storage CORS. Configure CORS on a bucket
Use the origin only, not an individual page path: an origin consists of the scheme, host, and optional port. For example, https://app.example.com and http://localhost:3000 are distinct origins. Do not use a wildcard merely to make an error disappear when the application needs a narrower allow-list. After changing the policy, repeat the Network check from the actual page origin and confirm the response includes the expected allow-origin header.
Load the image with CORS enabled, then capture and save
Set the image’s crossOrigin property before setting src; setting it afterward can be too late because the browser may already have started the request. Wait for image decoding before placing it in the page and capturing. The following ES module example fetches a Firebase download URL, inserts the image into an invoice, captures the invoice and adds the result to a PDF:
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import { getStorage, ref, getDownloadURL } from "firebase/storage";
import html2canvas from "html2canvas";
import { jsPDF } from "jspdf";
const storage = getStorage();
const url = await getDownloadURL(ref(storage, "images/photo.jpg"));
const img = new Image();
img.crossOrigin = "anonymous";
img.src = url;
await img.decode();
document.querySelector("#preview").replaceChildren(img);
const element = document.querySelector("#invoice");
const canvas = await html2canvas(element, {
useCORS: true,
allowTaint: false
});
const pdf = new jsPDF();
pdf.addImage(canvas.toDataURL("image/png"), "PNG", 10, 10, 190, 0);
pdf.save("invoice.pdf");Replace images/photo.jpg and the selectors with your object path and page elements. Ensure the image is part of the element being captured. If img.decode() rejects, handle that as an image-loading failure rather than proceeding to capture. Where you prefer an event-based load flow, wait for the image’s load event and handle its error event before calling html2canvas.
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What each setting does—and does not do
img.crossOrigin = "anonymous"requests the image in CORS mode without credentials. The server must still return an appropriate CORS header.useCORS: truetells html2canvas to try loading images with cross-origin support. It cannot add headers to Firebase’s response or change the browser’s access decision.allowTaint: falseavoids relying on drawing an image that would make the canvas unreadable. Setting it totrueis not a fix for exporting a tainted canvas.pdf.addImage(...)receives image data from the already-created canvas. If the canvas cannot be read, fix image access before adjusting PDF output.
Choose a fallback if direct bucket CORS is not an option
A controlled same-origin proxy can fetch the image and return it with a valid CORS policy; html2canvas has a proxy option for this use case. The proxy must be designed carefully: validate requested URLs or restrict it to known storage objects, enforce authorization, and avoid creating an open proxy. A server-side PDF renderer is another option when rendering should not depend on a browser client. The trade-offs differ:
| Approach | Storage policy control | Image URL exposure | Authentication | Operational complexity | Where rendering happens |
|---|---|---|---|---|---|
| Direct bucket CORS | Requires control of the bucket CORS policy. | The browser receives the Firebase download URL. | The browser must be able to access the object under the applicable Firebase access rules. | Lowest when you control the bucket and can deploy its CORS policy. | In the browser. |
| Same-origin proxy | The proxy owner controls its response policy; the bucket still needs to be reachable by the proxy. | The browser can request a same-origin proxy URL rather than the bucket URL directly. | The proxy must securely enforce whatever user or service authorization is needed. | Requires operating and securing a proxy. | In the browser after proxy delivery. |
| Server-side PDF rendering | Server code controls how it retrieves the source image; browser CORS is not the rendering path. | The browser need not receive the source image URL if the server returns the finished PDF. | The server must be authorized to fetch the image and the user must be authorized to request the PDF. | Requires a rendering service and its maintenance. | On the server. |
Common errors and practical fixes
The image is visible, but toDataURL() throws a security error
Likely cause: the image was loaded without a CORS-readable response, so drawing it tainted the canvas. Check the image response’s Access-Control-Allow-Origin, bucket policy and request origin. Set crossOrigin before src and use useCORS: true.
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The console reports a CORS error even though the policy was updated
Check that the policy is on the bucket serving this exact download URL, that it includes the page’s current origin and allows GET, and that the browser’s request is reaching the expected object. Development and production origins must be handled separately when they differ. Verify the response in Network rather than assuming a policy deployment took effect.
The image request returns 401, 403, or 404
This is not fixed by adding CORS alone. Confirm the object path, that it exists, and that Firebase Storage Rules allow the current user to read it. Resolve an authorization or missing-object failure before diagnosing canvas access.
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Make sure the image has decoded before capture and that the Firebase response is CORS-readable. Check the console and Network panel for a failed request. If loading is slow, consider the documented imageTimeout option; its default is 15,000 ms. A longer timeout can allow slow images more time, but it cannot fix an invalid URL, denied read, or missing CORS header.
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It works on localhost but fails after deployment
The deployed hostname is a different origin. Add the exact production origin to the bucket CORS policy, deploy it, then retest from that deployed page. Do not assume that permission for localhost covers a hosted domain.
A browser extension makes it work locally
Do not treat a CORS-disabling extension as a production solution. It changes enforcement only in that local browser; your users’ browsers and deployed application remain subject to the server response.
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Direct browser rendering avoids adding a separate proxy or PDF-rendering service, but it depends on the user’s browser being able to fetch and decode each image and on the page remaining available for capture. Larger images and full-page captures can take longer and use more browser memory. Wait for the specific images the document needs rather than assuming that the page’s initial load means every image is ready. The html2canvas timeout is a wait limit, not a guarantee of successful rendering.
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A proxy adds a network hop and an operational component; server-side rendering moves the work off the user’s browser but requires server capacity and secure access to source files. No universal cost or speed winner follows from the browser CORS documentation: it depends on image sizes, request volume, hosting and rendering setup. Avoid retry loops that repeatedly fetch a failing image without distinguishing a transient timeout from a persistent permission or CORS configuration error.
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curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo documentation for setup and options. ScreenshotNeo accepts cookie or consent banners and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each step can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers identify the page verdict and billing status. Its MCP server offers screenshot and PDF tools for AI agents. The free plan includes 1,000 shots per month with no card; paid plans start at $5 for 3,000 shots. Those capabilities serve website capture; they do not replace configuring Firebase access when your own browser code must read a private image and build a PDF.
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Does setting crossOrigin = "anonymous" make a Firebase image public?
No. It requests the image without credentials in CORS mode; Firebase Storage Rules still determine whether the object can be read.
Can a Firebase download URL work in an <img> but fail in jsPDF?
Yes. Displaying an image does not establish that its pixels are readable by canvas, which is needed before jsPDF can use the canvas output.
Can html2canvas’s proxy option help if I cannot change the bucket policy?
It can route image loading through a proxy, but that proxy must be controlled and secured; it does not remove the need to authorize access to the image.
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