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A WebGL canvas can look correct on the page yet export as a blank image because the browser may clear its drawing buffer after presenting the frame. If you control the renderer, draw immediately before capturing or capture from the render loop. Use a framebuffer when pixels must persist across operations; request preserveDrawingBuffer: true only when its simplicity is worth a possible performance cost.
Why a WebGL screenshot can be blank
WebGL renders into a drawing buffer. After the browser presents that buffer to the page compositor, the default context behavior allows its contents to be cleared when preserveDrawingBuffer is false. A later call to readPixels, canvas.toDataURL(), drawImage(), or ImageBitmap creation may therefore no longer see the pixels that appeared on screen. The WebGL specification describes the later result as undefined in this situation, so a blank capture is not necessarily evidence that the original draw failed. Khronos WebGL Specification
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This timing explanation applies directly to an in-page read or export of the WebGL canvas. A full-page screenshot taken by a browser or desktop tool is a different capture path and can involve behavior not established by the specification’s canvas readback rules.
Choose the fix for your capture path
Redraw immediately before export
If you control the app’s renderer, make the draw call immediately before requesting the image. The capture then reads pixels from a freshly rendered frame, rather than relying on a buffer that may already have been presented and cleared. Keep state updates separate from the draw operation if redrawing for a capture should not advance the animation. WebGL Fundamentals: WebGL Tips
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Capture after drawing in the render loop
When rendering is continuous or scheduled, set a capture-request flag when the user asks for an image. In the render pass, draw the requested frame first and perform the read or export directly afterward. This keeps the capture aligned with the draw that produced the pixels.
Render to a framebuffer object for multi-step work
If the rendered result must remain available across multiple operations, render into a framebuffer object (FBO) and read or process that attachment as needed. The specification identifies an FBO as an option when rendering to the same drawing buffer over a series of calls. It also suggests synchronous access while rendering as an alternative where appropriate. Khronos WebGL Specification
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Request buffer preservation at context creation
For a simple export path, ask for a preserved drawing buffer when the context is first created:
const gl = canvas.getContext('webgl', { preserveDrawingBuffer: true });This is a context-creation attribute, not a setting that can be switched on later for an existing context. If a rendering library created the context, inspect its renderer or context-creation configuration rather than calling getContext again with different attributes. The WebGL specification warns that preservation can cause significant performance loss on some platforms, and WebGL Fundamentals notes that it can prevent certain optimizations. Prefer capture timing or an FBO when they fit the application better. Khronos WebGL Specification · WebGL Fundamentals
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Diagnose a blank capture step by step
- Identify how the image is captured. Determine whether the code uses
toBlob,toDataURL,drawImage, orreadPixels, or whether a browser or desktop tool captures the page. The drawing-buffer explanation directly covers in-page canvas reads and image-source use; do not assume it explains every full-page capture failure. - Check whether the canvas itself renders. If the visible canvas is also blank, investigate rendering and context setup first. A late capture cannot explain pixels that were never drawn; there is no single universal fix for shader, asset-loading, or context-creation failures.
- Move capture next to the draw. Test a capture immediately after drawing. If that works, put the capture after the draw call in the render loop or redraw immediately before export.
- Compare requested and actual buffer dimensions. Inspect
gl.drawingBufferWidthandgl.drawingBufferHeight. The implementation may allocate a smaller buffer than the requested canvas size if it cannot meet the request. Khronos WebGL Specification - Verify resize and viewport handling. Changing the canvas dimensions does not automatically update the WebGL viewport. After resizing, update it to the actual drawing-buffer dimensions where appropriate:
gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight); - Isolate a browser- or device-specific failure. Compare browser and operating-system versions and test a capture made during rendering. A historical WebKit report documents blank
glReadPixelsbehavior associated with iOS 14.2-era Safari; it is not evidence of a universal current Safari issue or a reason to toggle unrelated context settings. WebKit Bug 219023
Or skip the browser setup
For a page-level screenshot rather than an app’s own WebGL readback, ScreenshotNeo offers a screenshot API and an MCP server. A single request can return a PNG, JPEG, WebP, or PDF. For example, this cURL request saves a WebP screenshot of a page; replace the sample URL with the page you want to capture:
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 API documentation for request options. It removes cookie and consent banners, newsletter popups, and chat widgets before capture; those cleanup steps can be turned off. Bot checks and CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and the response headers identify the page verdict and billing status. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools for Claude, Cursor, and other MCP clients. The free plan includes 1,000 screenshots per month with no card; paid plans start at $5 for 3,000 screenshots. ScreenshotNeo is a separate page-capture route, not a replacement for correcting an app’s WebGL readback timing. ScreenshotNeo
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How to choose between the fixes
- You control the renderer and need one export: redraw and capture immediately, or capture after drawing in the render loop.
- You need pixels for several later operations: use an FBO-backed render target.
- You need the simplest context-level change: request
preserveDrawingBuffer: truewhen creating the context, then check performance on the platforms that matter to your app. - You only need a screenshot of the page: use a browser or screenshot service, while treating its behavior separately from canvas readback.
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