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I tried uploading a full-page screenshot of a deleted question to share with low-rep users, and I noticed that the result was barely legible.

For example let's take a full-page screenshot of the XY problem FAQ page. The end result is a pixel-perfect PNG which occupies 1845833 bytes (1.85 kB or 1.76 kiB). Both are under the 2 MB upload limit of the UI.

But when I upload the image, this happens. What we get is an overcompressed mess of a JPEG where the text is barely legible. I didn't ask for this. At least the image is 742 kB in size; that's great, right?

If I convert the PNG to a JPEG myself using gimp I can end up with a sub-2MB image if I use 77% quality and chroma quartered subsampling. The result is a bit more blurry and dithered than the PNG but still easily legible, and its size is 1916719 bytes.

So let's upload the JPEG with the hopes of preventing automatic conversion (which is there for whatever reason). And we end up with this. Oh, it got compressed to death again! 735 kB size this time.

What I believe I know:

  • I have uploaded walls of meta screenshots before, and the quality was always fine
  • If I repeat the uploading process with something smaller (I tried with some images I found in some of my answers, all below 0.5 MB) the PNG to JPEG conversion doesn't even happen, so the problem is not universal.

Has something changed about how uploaded images are treated or is this something I just haven't noticed? And is there a size limit (either in bytes or in number of pixels) that triggers an aggressively compressed conversion to JPEG? And can we do anything about the latter? I would think that the 2 MB limit on uploaded files should in itself be enough of a safeguard, and such automatic conversion shouldn't happen at all.

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+50

Digging deeper based on this imgur forum post led me to imgur's help section on "What files can I upload? What is the size limit?". It's spelled out clearly there (bold mine):

Maximum File Size

The maximum file size for non-animated images (think JPG, PNG, etc) is 20MB. PNG files over 5MB will be converted to JPEGs.

[...]

Non-animated images over 1MB for anonymous uploads and 5MB for account holders will be lossily compressed.

Users of the SE image upload facility would be anonymous and hence our images would be compressed when over 1 MB.

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    Thanks for the find! The only thing I find weird is that I'm sure this used to work (or at least I could upload walls of meta posts without them being compressed so badly), but the userecho thread you found 8 years old (I've been here for 5). Either 1. stack imgur used to count as non-anonymous and now it doesn't, or 2. the lossy compression has been made more aggressive in recent years, or 3. I merely misremember or earlier examples happened to somehow fall below the 1MB limit. – Andras Deak Jul 1 at 8:55

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