[Link to the wiki page] | [Link to the help page]

This comes up regularly so I figured I'd make a help page so we have something we can link to people, which will hopefully help people figure out why we sometimes keep smaller PNG files, delete bigger jpg files, or delete upscales, etc. etc.

Please let me know what you think of this page. Is it concise enough? Is it easy enough to understand for someone not tech-savy? Is the tutorial clear enough (even for beginners)? Are there any parts missing?

A common confusion that users may have is that because their art is of high quality - in the sense of the factors listed on this page - that it should be accepted.

I foresee that this page will be used by confused and angry users who have had their images deleted for not meeting quality standards, as a sort of "look, I did everything on the page, why was my art deleted!?!?"

I suggest that these pages include a distinction between technical quality, as is described on that page, and artistic quality, such that users can see that these are not the only standards used when judging whether or not a particular piece will stay (rather than this, which only applies to comparisons between two versions).

Otherwise I think it is fairly well done.

Great wiki page! Here are some changes I would make. Feel free to copy them (warning: LONG!)

What are your thoughts? I hope what I wrote was good! :D

Q1

I don't know the below cases' standard:

I downsample the high-res image to match the low-res one (Is it a right way to do?)

and followed the "Tutorial on how to compare images" in help page

Below link contains the original images and differenced images

files(#NSFW)

Q2

I know two same images with different sizes , their image qualities may be the same. (ex. my deleted post )

So after comparing the files sizes, I should followed the "Tutorial on how to compare images" in help page

But sometimes it is hard to judge , due to the screen, eye limit, or eye strain

Here's the case:

93.2 KB (95,480 bytes) v.s. 93.2 KB (95,526 bytes)

First, I followed the tutorial and get the mostly black image

and I use program to check that image, it is not pure black ( my deleted post black image is pure black )

Then I used SSIM to compare them , the result is 0.9999561 , not equal to 1

I checked my deleted post , SSIM is 1

So if the image size is bigger than the post on e621 and their SSIM is not equal to 1, (and same image type, ex:jpg, and same resolution)

Can I reupload the post?

Or recommend other algorithms or approaches?

darkazma said:

Here's the case:

93.2 KB (95,480 bytes) v.s. 93.2 KB (95,526 bytes)

Then I used SSIM to compare them , the result is 0.9999561 , not equal to 1

I checked my deleted post , SSIM is 1

So if the image size is bigger than the post on e621 and their SSIM is not equal to 1, (and same image type, ex:jpg)

Can I reupload the post?

Or recommend other algorithms or approaches?

For a few bytes? No.

AoBird said:
For a few bytes? No.

Filesize. Is. Irrelevant.

darkazma said:
bunch of text

is JPG resaved as PNG, you should see JPG compression clearly. 1496542.jpg is preferred as it's higher quality, higher resolution and no funky stuff with filetype.
764152.jpg is only ever so slightly higher resolution, but 764152-2.png is lossless, so added resolution doesn't justify details lost with compression, so PNG is preferred.

When the resolution between two images is differend, then you shouldn't compare differenciation as downscaling will effect image quality and it can change depending on integrer and what downscaling method you used (nearest neighbor, bilinear, lanczos, etc.). With two differend sized images you should simply compare manually with eye. If you have problems, simply use zoom, image editors really commonly have nearest neighbor zoom which makes it even easier while browsers usually apply filtering to zoom.

With post #843349, there's difference, but we are nearing the point where it's first of all impossible to determine which version is the superior one (effected areas are on white so they look visually identical) and when the difference amount is 0.0001% it becomes nitpicking so usually that's in the area where it's ruled identical, even if on paper it's not 1:1 identical. Keep common sense in the game, even if sometimes the difference on replaced posts are low.

So below are the solutions to different cases

Please tell me which parts are wrong or fine.

Below links' images are NSFW.

Same type, Same resolution, Different size

case 1

if the image has not been uploaded, choose the biggest size

else, check their differenced image,

> if you can see the slightly colored outlines in it, then you can reupload it.

> if you cannot see the slightly colored outlines, then don't reupload it.

Different type, Same resolution, Different size

case 2

(?)Same as above

Same type, Different resolution, Different size

case 3

(?)compare two images by eyes ( zoom in )

Different type, Different resolution, Different size

case 4

case 5

for case 4 and case 5, both of them need to compare two images by eyes ( zoom in )

but in case 4, I thought B mostly wins.

Edited:above images are all from artist's websites, and two images' SSIM > 0.9 but not equal to 1

darkazma said:

Same type, Same resolution, Different size

case 1

name	resolution	size
A.jpg	1000x1000	100KB
B.jpg	1000x1000	200KB

if the image has not been uploaded, choose the biggest size

You seem to have missed this part in Mairo's explanation so let me just reiterate this:

File size doesn't matter. Period.

Always check the files manually, and if in doubt go with the best looking original version directly from one of the artist's galleries.

File size is literally never a good indicator of quality because it's possible to inflate it with bogus-data.

Saving PNGs with different compression levels (eg 2 vs 9) can also result in files of a notably different filesize (without any bogus data), that are still pixel-for-pixel identical.