To capture a WebGL canvas you control, export its pixels with toBlob() or toDataURL() while the intended frame is still available. The default WebGL drawing buffer may be cleared after the browser composites it, so a later export can be blank or stale. If you need the capture later, you can request preserveDrawingBuffer: true when creating the context, but that option can reduce performance. If you cannot change the app, use a browser-level screenshot workflow instead.
Choose the capture method that matches your goal
| What you need | Use this method | Important limitation |
|---|---|---|
| Save only the WebGL render, with access to the app code | Export the canvas with toBlob() or toDataURL() while the frame is readable. |
Timing matters because the default drawing buffer is not guaranteed to retain its contents after compositing. Khronos WebGL Specification 1.0 |
| Export after the rendering function has returned | Consider preserveDrawingBuffer: true at context creation, or redesign capture around a synchronous read or framebuffer. |
Preservation may hurt performance, and context attributes cannot be changed after the first getContext() call. Chrome for Developers |
| Capture the visible browser tab, including browser-rendered content | Use a browser screenshot feature or an extension capture API supported by the browser. | This captures the visible view, not a direct export of the canvas pixels. The html2canvas FAQ names chrome.tabs.captureVisibleTab() for Chrome, Edge, and Opera. html2canvas FAQ |
| Recreate a page area from DOM and styles | Use a DOM-reconstruction library such as html2canvas, then export its output. | It builds an image from DOM information rather than taking a native browser screenshot, so the result may differ from what the browser displayed. html2canvas documentation |
Export the WebGL canvas from your application
A WebGL canvas is still an HTML canvas, so canvas image-export methods can save its rendered content when the pixels are available. Call the export at a point in the render flow when the intended frame is present; do not assume it will remain readable after the browser has composited the frame.
Export a PNG with toBlob()
Call this after drawing the frame you want to save. The callback receives a PNG blob that can be downloaded or passed to other code.
function saveCanvasPng(canvas) {
canvas.toBlob((blob) => {
if (!blob) {
console.error('Canvas export returned no image.');
return;
}
const link = document.createElement('a');
link.href = URL.createObjectURL(blob);
link.download = 'webgl-capture.png';
link.click();
URL.revokeObjectURL(link.href);
}, 'image/png');
}
// Call after rendering the frame you intend to capture:
renderer.render(scene, camera);
saveCanvasPng(renderer.domElement);Replace renderer.domElement with the actual canvas element used by your app. If your rendering loop schedules another frame or clears the drawing buffer before the export reads it, move the capture call into the appropriate render step.
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Export as a data URL
toDataURL() returns an encoded image string. It is convenient for small captures or when an API expects a data URL; for larger images, toBlob() avoids holding the complete base64 string in memory.
const canvas = document.querySelector('canvas');
const pngDataUrl = canvas.toDataURL('image/png');
const link = document.createElement('a');
link.href = pngDataUrl;
link.download = 'webgl-capture.png';
link.click();When to use preserveDrawingBuffer
WebGL does not guarantee that the default drawing buffer keeps its contents after compositing. With preservation disabled, later operations that use the canvas as a source can have undefined behavior. If capture must happen after the render function returns, you can request preservation when creating the context:
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const canvas = document.querySelector('canvas');
const gl = canvas.getContext('webgl', {
preserveDrawingBuffer: true
});
// Initialize WebGL resources and render as usual.
// Later, while the desired frame is present:
canvas.toBlob((blob) => {
if (!blob) return;
// Save or process the blob.
}, 'image/png');This must be set on the initial context creation call. If some library or framework has already called getContext() on that canvas, you cannot change the context attribute afterward; create the context with the option from the start or use a different capture strategy. The Khronos specification warns that buffer preservation can cause significant performance loss on some platforms, so use it only when the capture timing requires it. Khronos WebGL Specification 1.0 and Chrome for Developers
Capture the browser view when you cannot change the app
If you do not control the page or cannot change how it creates its WebGL context, capture the browser-rendered view rather than relying on a later canvas export. An extension can use a supported visible-tab capture API; the html2canvas FAQ lists chrome.tabs.captureVisibleTab() for Chrome, Edge, and Opera. A real-browser automation workflow with Puppeteer or Playwright is another option for server-side screenshots. Check the current API and permissions for your chosen browser and tool before implementing it; capture behavior and available APIs are not identical across browsers. html2canvas FAQ
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A browser-level screenshot represents the visible view at capture time. It is a different result from directly exporting canvas pixels: the browser screenshot can include surrounding page content, while the canvas export gives you the canvas image itself.
Use html2canvas only when DOM reconstruction is suitable
html2canvas can render a DOM section into a canvas and export it, with crop and scale options. Its documentation explains that it reconstructs the image using page DOM and style information rather than taking an actual browser screenshot; it may not exactly match the real browser representation. Do not rely on it for pixel-accurate WebGL output. Cross-origin content and differences in browser rendering can also limit what appears in the generated image. See the html2canvas documentation, examples, and FAQ.
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Account for alpha and transparency
If the canvas output has unexpected color around transparent areas, check the WebGL context’s premultipliedAlpha behavior and the image format you export. The WebGL specification notes that toDataURL() may need to de-multiply color values when the output format does not specify premultiplied alpha, and that operation is lossy. Decide whether you need a transparent background or an opaque composite, and validate the exported image in the format and viewer that will consume it. Khronos WebGL Specification 1.0
Troubleshoot blank, stale, or inaccurate captures
- The export is blank or shows an old frame: move the export closer to the draw call for the desired frame. Check whether it happens after the browser has composited or cleared the drawing buffer. If later capture is required, consider preservation at context creation or a synchronous drawing-buffer/framebuffer approach.
preserveDrawingBufferappears to have no effect: verify it was included in the firstgetContext()call. Context attributes cannot be changed on an already-created context.toBlob()returnsnull: treat this as a failed export, then inspect frame timing and whether the canvas still contains the intended pixels. Try exporting at the point of rendering rather than later.- Colors or transparency look different: account for the WebGL context’s alpha handling and the export format’s alpha expectations; de-multiplication can be lossy.
- html2canvas omits or misrenders WebGL or external content: it reconstructs from DOM and style information, not from the browser’s final pixels. Switch to direct canvas export when you control the canvas, or a browser screenshot when you need the rendered tab.
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For a rendered website screenshot, ScreenshotNeo offers a one-request API; it is not a substitute for exporting pixels from a WebGL app you control when you specifically need the canvas buffer. Its screenshot workflow accepts cookie banners and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture. Bot checks and CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and responses report the page verdict and billing status in headers. ScreenshotNeo also provides an MCP server with screenshot tools for AI agents.
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