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canvas.toBlob(..., 'image/webp', 1.0) can produce a pixel-identical WebP in a particular browser, but the Canvas API does not guarantee that quality 1.0 always selects lossless encoding. In one reported Chromium 149 test, the 1.0 file was about 3.5 times the size of the same screenshot encoded at 0.999. Treat that as an observation for one image and browser, not a universal rule.
What the quality value does—and does not—promise
The Canvas API accepts an optional image type and quality value. MDN describes quality as a number from 0 to 1 for formats that support lossy compression, including WebP. It does not specify that 1.0 means lossless in every browser. MDN’s toBlob() documentation also says that if the requested image type is unsupported, the browser falls back to PNG.
WebP itself supports both lossy and lossless encoding, as described in the Google WebP Container Specification and IETF RFC 9649. That format capability is separate from how a browser maps a particular quality number to its encoder. A high quality setting alone is not proof that the result is lossless.
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What the 1.0 versus 0.999 test found
NullPointerZen reported a test of a 1440×2400 help-page screenshot on an Apple M4 laptop with 16 GB of memory, using a Chromium 149 build. In that test, quality 1.0 produced a 771,126-byte WebP; quality 0.999 produced a 222,292-byte file. The author reported that the 1.0 result decoded to the same pixel values as the source, with all 13,824,000 channel values matching. The 1.0 file was about 3.5 times larger. These are results from one reported test, not a general browser benchmark. Read the author’s test and stated limits.
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The author said only Chromium 149 on that laptop was tested. The result therefore does not establish that other Chromium versions, browsers, images, or operating systems will behave the same way. Nor does it show that every 0.999 result is lossy or every 1.0 result is lossless.
How to verify the output in your application
If output format, file size, or pixel fidelity matters, test the actual content and browser versions your application supports. Check the returned Blob rather than assuming the requested type was honored:
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canvas.toBlob((blob) => {
if (!blob) {
throw new Error('Canvas encoding failed');
}
console.log(blob.type, blob.size);
}, 'image/webp', 1.0);For the test to be meaningful, keep the source canvas and image content fixed while comparing quality values. Record the browser and version, inspect blob.type to confirm the output format, and compare the encoded Blob’s byte size. If losslessness is required, decode the output and compare its pixels with the original canvas; file type and quality value do not establish pixel identity.
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If your application searches quality values to fit an upload or storage limit, measure the resulting size for representative images in each target browser. The cited test suggests that the top endpoint can be unexpectedly expensive in at least one environment, but it does not establish a universally best cap or quality value. NullPointerZen said their own tool capped its search at 0.99; that is an implementation choice, not a cross-browser recommendation.
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Choose based on the requirements you can verify: whether the output must be pixel-identical, the permitted byte size for real images, and the browser versions you support. Where those constraints are strict, make the encoder’s observed behavior part of your application’s compatibility testing rather than relying on a quality number as a mode switch.
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