Recently my post #4134253 has been replaced by lower quality JPG, original I upload with PNG version, and replaced by JPG version, are e621 can't allow for upload PNG version from Twitter? Even in same resolution?

The replacement reason are "Artificial PNG conversion", even though I did not convert the image

Your "PNG version" is a pixel-for-pixel match to the JPG version. Twitter did not serve you the original PNG.

You might not have converted it personally, but Twitter did.

The source you uploaded from:
https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=png&name=large
This is a PNG sample generated by Twitter, of an originally JPG file. It's evident that this is converted because of the amount of compression artifacts that would not be present on an original PNG.

The replacement:
https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=jpg&name=orig
With the orig param this is the closest to the "original" you're going to get on Twitter. Changing the params in the URL you'll find that https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=png&name=orig doesn't exist, so there's no original PNG available.

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"Original" PNGs do exist on Twitter, though it's rare you'll see them. For example: https://pbs.twimg.com/media/Fw47hC8acAE3Q-W?format=png&name=orig

If you try to request an original JPG from that URL you'll be met with a 404: https://pbs.twimg.com/media/Fw47hC8acAE3Q-W?format=jpg&name=orig

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If you upload by URL the uploader will actually attempt to prevent you making this mistake.

faucet said:
You might not have converted it personally, but Twitter did.

The source you uploaded from:
https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=png&name=large
This is a PNG sample generated by Twitter, of an originally JPG file. It's evident that this is converted because of the amount of compression artifacts that would not be present on an original PNG.

The replacement:
https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=jpg&name=orig
With the orig param this is the closest to the "original" you're going to get on Twitter. Changing the params in the URL you'll find that https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=png&name=orig doesn't exist, so there's no original PNG available.

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If you upload by URL the uploader will actually attempt to prevent you making this mistake.

If "large" are not original, then why https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=jpg&name=orig and https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=jpg&name=large still the same image and data size?

Btw, I download the PNG version first, and then upload with file instead of URL

jonathanwolf said:
If "large" are not original, then why https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=jpg&name=orig and https://pbs.twimg.com/media/Fzjt0UEaAAAsEBK?format=jpg&name=large still the same image and data size?

It's all dependent on the resolution and size of the image. If it's a file that's not too large with a resolution of 2048 pixels or lower on its biggest dimension, the large is likely the same as the orig, but uploading the orig is still preferred because it's the only way to know for certain that it's the best available image that Twitter will supply.

There's also a lesser known 4096x4096 option that's seen even more rarely, which from my experience has been the same as the orig at least 99% of the time, but there's no reason to upload that version when you can upload the orig and be certain about it.

faucet said:
It's all dependent on the resolution and size of the image. If it's a file that's not too large with a resolution of 2048 pixels or lower on its biggest dimension, the large is likely the same as the orig, but uploading the orig is still preferred because it's the only way to know for certain that it's the best available image that Twitter will supply.

There's also a lesser known 4096x4096 option that's seen even more rarely, which from my experience has been the same as the orig at least 99% of the time, but there's no reason to upload that version when you can upload the orig and be certain about it.

So technically it's still the same image if large and original image had same resolution, than e621 should allow for PNG version if the resolution same in both large and original

jonathanwolf said:
So technically it's still the same image if large and original image had same resolution, than e621 should allow for PNG version if the resolution same in both large and original

The PNG version is still just the JPG version re-saved as a PNG, though. All that does is inflate the file size without adding any quality, because you’re retaining all of the compression artifacts from the JPG. So it’s better to just have the JPG in this case.

jonathanwolf said:
So technically it's still the same image if large and original image had same resolution, than e621 should allow for PNG version if the resolution same in both large and original

The PNG version is a pixel-for-pixel replica of the JPG, so it's just the exact same thing with an unnecessarily higher filesize. I personally wouldn't have gone out of my way to replace it, but it's correct that it never should've been uploaded that way in the first place.

To put another way, a picture url that ends with &name=orig is guaranteed to be the original, most desirable, best version there is. We want this one.

A picture url that ends with &name=small, &name=medium, &name=large, or &name=[insert number here]x[insert same number here] has been replaced by pod people had things done to it. We don't want any of these.

A .png that's a resaved .jpg is a pod person actually a poor copy of the original .jpg. The main problem with .jpgs is that their compression causes artifacting, damaged pixels between color changes. That's why we normally prefer .pngs when saved straight from the original work file. However, .pngs are not always good. Resave a .jpg as a .png, and you do not get rid of those damaged pixels. There is no improvement in quality. Worse, the file size gets bigger. This is one of those times when the .jpg is actually the superior file to have.

clawstripe said:
To put another way, a picture url that ends with &name=orig is guaranteed to be the original, most desirable, best version there is. We want this one.

A picture url that ends with &name=small, &name=medium, &name=large, or &name=[insert number here]x[insert same number here] has been replaced by pod people had things done to it. We don't want any of these.

A .png that's a resaved .jpg is a pod person actually a poor copy of the original .jpg. The main problem with .jpgs is that their compression causes artifacting, damaged pixels between color changes. That's why we normally prefer .pngs when saved straight from the original work file. However, .pngs are not always good. Resave a .jpg as a .png, and you do not get rid of those damaged pixels. There is no improvement in quality. Worse, the file size gets bigger. This is one of those times when the .jpg is actually the superior file to have.

There's also JPEG files 'saved' as PNG as in, like IrfanView throws a fit and asks if you want to rename it to *.JPG. Not even recompressed - just the exact same bits. XD