WIP

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Even though e621 aims to host all works as high quality as possible and archive them, it is also a directly browsable website, meaning that all content uploaded should be accessible and viewable with modern up-to-date browsers. Because of this, only specific file formats are accepted and there are additional limitations on aspects like file size and resolution, meaning other formats or huge files need to be transcoded into an acceptable format before they can be uploaded.

Transcoding means the conversion from digital format to another digital format.

If the material in question is already in format that e621 supports, any kind of file alteration, modification, transcoding, etc. is generally highly discouraged!
This is because the file's MD5 hash will be altered. This breaks one way to reverse search the artwork (e.g. MD5:<MD5 Hash> status:any search), in addition to the currently-in-use duplicate detection system, resulting in accidental duplicate uploads and making it harder to verify the integrity of a post compared to its source. Transcoding also isn't always perfect and can be altering the file in some ways that are not immediately visible, resulting in us hosting an inferior quality copy of the work.

Image files

Image files are—generally speaking—the easiest ones to handle as most mainstream formats also work on browsers directly, but additionally lossless formats are extremely common because file sizes are a non-issue with modern computers and internet speeds.

The three image file formats that e621 accepts are:

• .jpeg/.jpg (an initialism/acronym of its developer, the Joint Photographic Experts Group; pronounced "JAY-peg"), which is a long-standing (since 1992) image format using lossy compression made to store highly detailed digital images such as photographs at a much smaller file size without losing too much image quality. Saving any image in this format will result in a loss of visual data, hence "lossy", leading to compression artifacts.
• .gif (Graphics Interchange Format; officially pronounced "JIF" [soft 'g'] like the peanut butter brand but it's much more commonly pronounced "GHIF" [hard 'g'] as in "gift" or "graphic"), which is a bitmap format developed in the 1980s, and is used for simple graphics. It supports transparency and remains the standard raster image format for animations, despite its age and several limitations, the most notable of which is that it cannot display more than 256 colors per frame, although there are ways to get around that.
• .png (Portable Network Graphics; officially pronounced "ping" but commonly pronounced by letter as "PEE-en-JEE"), which is an image format that's younger than the other two above (since 1996). It uses lossless compression, which means that images will not degrade upon saving, and can store higher-quality graphics than GIF does. It was made with the internet in mind. It supports 8-bit alpha transparency, unlike GIF (where transparency has to be allocated to a space in a color palette) or JPEG (which does not support transparency at all). However, it does not support animations, although an unofficial extension of the format known as APNG (Animated Portable Network Graphics) supports such and has gained support in all five major desktop browsers.

Source files

Source files mean file formats which are not meant for viewing, but are files containing all of the data used when creating the image. Common ones are .psd (Photoshop Document), .xcf (GIMP), .kra (Krita), .sai (PaintTool SAI) and .clip (Clip Studio Paint).
These are extremely uncommon files to bump into and are generally only accessible trough first party channels or through paid channels, so make sure these aren't paid material or possibly bases as these can be against e621 rules and guidelines.

As these files are almost always software specific, the best way to handle these are to open these files with their native application and then exporting them as a PNG file. Many graphics editing software have ability to open source files created in other software (e.g. GIMP can open .psd). However, depending on content and what it utilizes, the importing process can alter the information or even straight up discard it, resulting in an altered file from author's original vision, so this is not recommended!

Vector files

When talking of raster image files, they generally have a specified resolution and all of the pixels on the image are determined to be specific ones. However, there are also vector files, which instead of saving individual pixels saves instructions how to draw the image in real time. What this means is that the image scales indefinitely, where regular images would need to be interpolated, resulting scaling artifacts and decreasing the quality of the image immensely.

Most universal format for vector graphics is .svg (Scalable Vector Graphics), with .ai being the Adobe Illustrator Artwork format. Additionally some file formats can have vector elements inside them including .pdf (Portable Document Format).

Unlike source files, vector files should look exactly the same regardless of what software is used to open and modify them. However, as e621 does not support vectorized files, these would need to be rendered into lossless bitmap format PNG. This can be problematic as there is no hard rules as to what resolution to render the file at and in most instances, maximum dimension of 15000px can be excessive. However, many files should contain information of their intended resolution, some files can contain information of DPI which should tell the printing dimensions for guidance. With PDF files, sometimes there can be other bitmap elements on the page which can be used to determine the desired render resolution, utilities to decompile content from PDF files can be utilized to determine these.

Other image files

There are multitude of other file formats for images, usually these are extremely old, extremely new or formats meant for specific purpose. Some of the newer filetypes are compatible with web browsers, e.g. AVIF so these could possibly be replaced with native files in future, however for current time being, transcoding them into accepted format is the way to go.
Generally speaking, converting these file formats into PNG is usually the way to go, as metadata is considered as irrelevant data (and can be put into post description if necessary) and using lossy formats like JPEG can lead into generation loss where two different lossy compressions stack decreasing the quality further.

• .bmp (bitmap), which is an extremely common format to save images pixel perfectly. Bitmap files can get massive because they don't use any file compression.
• .tiff (Tag Image File Format), which is similar to JPEG in that it can include additional metadata, but it is lossless quality.
• .webp (WebP) is a file format that can be lossless (but chroma subsampled), lossy, static or animated, and supports alpha transparency.
• .heif (High Efficiency Image Format) is the newest standard for lossy image files and provides far higher image quality and lower file sizes compared to currently widespread standard JPEG. The file format can utilize different formats including HEVC (.heic), AVC (.avci) and AV1 (.avif).

Video and animation files

Audio files

Generally speaking, e621 does not consider audio only content relevant and as such audio files are not acceptable to be uploaded and are generally deleted under uploading guidelines.
However, the general gist of previous sections still apply; there are lossy files (AAC, Opus, MP3, etc.), lossless files (WAV, FLAC, ALAC, etc.) and "vector" formats, a.k.a. formats which are instructions to play the music (MIDI, MOD, XM, etc.).

See also

• e621:image_quality
• e621:supported_filetypes

• Transcoding - Wikipedia

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