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I've noticed that a lot of objective measures of image quality prefer blur to ringing, but many subjective measurements tilt the other way.

by ryandamm3 hours ago|

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That's why you should prefer LPIPS to PSNR. LPIPS is a learned metric that uses subjective human input.

That said, it's limited due to the training set, iirc. So we still use PSNR regularly — it's an easily-understood metric with widely-understood limitations, which is in practice not too bad. Devil you know, etc.

by m-schuetz5 minutes ago|

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Ironically when we did compression quality evaluations, me and my colleague found that PSNR way better matches our perception of quality, than LPIPS or FLIP. Our assumption is that LPIPS is biased towards quality artifacts that is not representative of compression quality artifacts.

by gutechh3 hours ago|

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I do prefer the firefox look too! I'd be curious to hear your take on what could be the root cause of this. I'm far from a image rendering expert. But I remember when inspecting the edges and curves of the images in a DCT visualizer they were all in the AC coefficients which is what led me to the conclusion of the post.

by debazel2 hours ago|

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I'm far from an image rendering expert myself, but I don't think there is a single root cause here. I don't doubt the partial decoding would affect the final result just as the scaling algorithm itself, so in the end it's going to be a combination of it all.

The reason I believe the scaling algorithm is contributing more in your particular example, is because it looks almost exactly like a classic B/W comparison between a sharp ringing heavy algorithm and a more blurry one.

For reference here is a quick comparison between Lanczos/Bilinear I threw together: https://twinlens.app/compare?share=40973d84a174

It would be interesting to see how your example would look if you used colors instead, where the ringing artifacts become a lot more obvious. Like this one: https://twinlens.app/compare?share=8fb890a5820e

by gutechh41 minutes ago|

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Thanks for these! Indeed I understand your point now. I'll add a little note on the post later today.

Another interesting bit is that It wasn't so easy to get an image showing the issue a clear as we had it. My hunch is that moiré patterns might have a role in this, as most images where the the issue showed, had some kind of grid/repetition. I suppose some images might be more affected by the IDCT and other more affected by the scaling algorithm, making it more or less visible. It's a mix like you said!

by ack_complete2 hours ago|

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I wonder if this is a gamma correction related. JPEG encodes directly in a non linear color encoding (full-range YCbCr), so the partial decoding may be effectively scaling without gamma correction.