Editing JPEG Network Graphics (section)

{{Short description|File format for image files}}
{{More footnotes|date=March 2014}}

'''JPEG Network Graphics''' ('''JNG''', {{IPAc-en|ˈ|dʒ|ɪ|ŋ}}) is a [[JPEG]]-based [[graphics file format]] which is closely related to [[Portable Network Graphics|PNG]]: it uses the PNG file structure (with a different signature) as a [[Container format (digital)|container format]] to wrap JPEG-encoded image data.

JNG was created as an adjunct to the [[Multiple-image Network Graphics|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 (web browser)|Opera]], [[Internet Explorer]], and [[Mozilla Firefox]]. The [[Mozilla Application Suite]] (and hence [[Netscape (web browser)|Netscape]]) originally supported MNG/JNG, but native support was removed in Mozilla 1.5a by developers, and Mozilla has not supported the format since. [[Safari (web browser)|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.

{| class="wikitable"
|+ Network Graphics signatures
!rowspan="2"| Name !!colspan="2"| Signature
|-
! [[hexadecimal]] !! [[ASCII]] + [[C0 and C1 control codes|C0, C1]]
|-
! PNG
| <code>89 50 4E 47 0D 0A 1A 0A</code> || <code title="HTJ">0x80<nowiki>|</nowiki>[[Horizontal Tab|␉]]</code>PNG[[Carriage Return|␍]][[Line Feed|␊]][[Substitute character|␚]] ␊
|-
! MNG
| <code>8A 4D 4E 47 0D 0A 1A 0A</code> || <code title="VTS">0x80<nowiki>|</nowiki>␊</code>MNG␍␊␚␊
|-
! JNG
| <code>8B 4A 4E 47 0D 0A 1A 0A</code> || <code title="PLD">0x80<nowiki>|</nowiki>[[Vertical Tab|␋]]</code>JNG␍␊␚␊
|}

JNG does not have a registered [[Internet media type]], but <code>image/x-jng</code> can be used.<ref>[http://www.libpng.org/pub/mng/spec/jng.html Official JNG specification], section 1</ref>