Ugoira is a custom animation format used by Pixiv. A Ugoira is a Zip file of images, played back using a custom Javascript player.
To upload a Ugoira to Danbooru, paste the Pixiv URL into the source field on the upload page, or upload using the bookmarklet.
To download a Ugoira from Pixiv, you can use Pixiv Toolkit or gallery-dl. Ugoiras downloaded this way can also be uploaded to Danbooru.
Technically, a Ugoira consists of Zip file containing each frame in JPEG or PNG format, and a separate JSON file containing the delay between each frame. Since each frame has a separate delay, it's possible for a Ugoira to have a variable frame rate. A custom Javascript player is used to play animations back. Since the JSON file containing the frame delays is separate from the Zip file containing the images, saving raw Ugoiras and playing them outside of a browser is difficult.
Ugoira is a custom format exclusive to Pixiv. Animations from other sites cannot be Ugoira.
Ugoira is often considered inferior to animations found on other sites due to Pixiv's limitations on uploading GIFs or MP4 videos. As a result, artists may convert their animations to GIFs before uploading, introducing dithering and/or color banding, and Pixiv's recompression process can introduce additional JPEG artifacts, affecting the overall quality of Ugoira animations.
