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If a website’s WebGL canvas appears blank in a screenshot, the drawing buffer may have been cleared after the browser displayed it. If you control the page, request preserveDrawingBuffer: true when creating its WebGL context, then check performance. If you control the rendering code, capturing synchronously during rendering is another option. For an OffscreenCanvas using transferToImageBitmap(), that option does not prevent the transfer from leaving the canvas bitmap blank.
Why is my WebGL canvas screenshot blank?
WebGL’s drawing buffer is not preserved by default. After the browser composites the canvas, it may clear the buffer to its default values. A later screenshot or canvas readback can then see no rendered pixels, even if the 3D content was visible on screen moments earlier.
The WebGL specification says that using a context as an image source after rendering returns can have undefined behavior when the drawing buffer is not preserved. The affected operations it names include readPixels, toDataURL, drawImage, and ImageBitmap creation. See the Khronos WebGL Specification.
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Before changing rendering code, make sure the blank area is actually the canvas you expect to capture. Another layer, an overlay, or an iframe may cover or contain the content. This is a diagnostic check, not a guarantee about how a particular browser automation tool captures pages.
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- Inspect the page and identify the canvas displaying the missing content.
- Check whether the visible content is inside an iframe or whether another page element sits over it.
- Confirm that the canvas has completed the desired draw before capturing.
If you do not control the website, you cannot change its WebGL context options. You can try capturing after the desired draw has completed, using the screenshot tool’s supported browser capture path. Exact timing and automation settings vary by tool and version; there is no universal automation flag established here.
If you control the page, preserve the drawing buffer
Find the first call that creates the WebGL context and request preservation there. For a WebGL 1 context, the initialization pattern is:
const canvas = document.querySelector("canvas");
const gl = canvas.getContext("webgl", {
preserveDrawingBuffer: true
});
if (!gl) {
throw new Error("WebGL context could not be created");
}For WebGL 2, use the corresponding context name:
const canvas = document.querySelector("canvas");
const gl = canvas.getContext("webgl2", {
preserveDrawingBuffer: true
});
if (!gl) {
throw new Error("WebGL 2 context could not be created");
}The option is a context-creation parameter. Adding it after an existing context has already been created does not change that context; update the original initialization call and recreate the context as needed.
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Capture the page after the change and verify that the canvas pixels appear. Then test rendering on the devices and platforms you support. Khronos warns that preserving the drawing buffer can cause significant performance loss on some platforms; the specification does not give a universal numerical penalty. See the Khronos WebGL 1.0 API Quick Reference Card and the WebGL Specification.
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If you control rendering, capture during the draw
Instead of preserving the buffer for later use, you can arrange to read or export the canvas synchronously while rendering is taking place. The WebGL specification identifies synchronous readPixels or toDataURL calls in the rendering function as an alternative technique. The WebGL Fundamentals guide to WebGL tips demonstrates canvas export with toBlob.
For example, with a 2D canvas, an export callback can be requested with:
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canvas.toBlob((blob) => {
if (!blob) {
throw new Error("Canvas export returned no image");
}
// Use or save the image blob here.
}, "image/png");For WebGL, make the readback or export part of the rendering operation rather than relying on a later read after the browser has composited and possibly cleared the buffer. An application-owned framebuffer and readback workflow is another option when your renderer needs tighter control over when pixels are read.
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If the page renders through an OffscreenCanvas, check whether it calls transferToImageBitmap(). That method transfers the current bitmap and replaces it with a new blank bitmap. The WHATWG HTML Standard specifies that preserveDrawingBuffer has no effect for this transfer when the rendering context is WebGL. In that case, changing the preservation option does not prevent the blank replacement; account for the transfer in the capture workflow.
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Frequently Asked Questions
Does preserveDrawingBuffer default to true?
No. The WebGL drawing buffer is not preserved by default.
Can I add preserveDrawingBuffer after creating the WebGL context?
No. It is a context-creation parameter; change the initial context-creation call and recreate the context if necessary.
Will preserveDrawingBuffer fix a blank OffscreenCanvas transfer?
No. The HTML Standard says it has no effect for the WebGL OffscreenCanvas transfer made by transferToImageBitmap().
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