JNG was created as an adjunct to the MNG animation format, but may be used as a stand-alone format. JNG files embed an 8-bit or 12-bit JPEG datastream in order to store color data, and may embed another datastream (1, 2, 4, 8, 16-bit PNG, or 8-bit JPEG grayscale image) for transparency information. However, a JNG may contain two separate JPEG datastreams for color information (one 8-bit and one 12-bit) to permit decoders that are unable to (or do not wish to) handle 12-bit datastreams to display the 8-bit datastream instead, if one is present.
Version 1.0 of the JNG specification was released on January 31, 2001 (initially as part of the MNG specification). Usually, all the applications supporting the MNG file format can handle JNG files too. For example, Konqueror has native MNG/JNG support, and MNG/JNG plugins are available for Opera, Internet Explorer, and Mozilla Firefox. The Mozilla Application Suite (and hence Netscape) originally supported MNG/JNG, but native support was removed in Mozilla 1.5a by developers, and Mozilla has not supported the format since, despite requests from its users. Safari does not support MNG/JNG.
JNG enhances the capabilities of the JFIF format (the usual JPEG file format) by supporting transparency, two consecutive color streams (one 8-bit and the other 12-bit), and other useful PNG features like color correction, gamma correction, embedded color profiles, PNG-style metadata, checksums, etc. The transparency information inside a JNG file (as an alpha channel) can be saved either in lossless PNG format or in lossy JPEG format. This way, users can benefit from the power of JPEG compression while preserving lossless (PNG-compressed) transparency information.
The chunk-based structure of JNG files is essentially the same as that of PNG files, differing only in the slightly different signature and the use of different chunks.