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What is the JNG format?

JPEG Network Graphics

The JNG (JPEG Network Graphics) format is an image file format that was designed as a sub-format of the more widely known MNG (Multiple-image Network Graphics) format. It was primarily developed to provide a solution for lossy and lossless compression within a single image format, which was not possible with other common formats such as JPEG or PNG at the time of its creation. JNG files are typically used for images that require both a high-quality, photographic-style representation and an optional alpha channel for transparency, which is not supported by standard JPEG images.

JNG is not a standalone format but is part of the MNG file format suite, which was designed to be the animated version of PNG. The MNG suite includes both MNG and JNG formats, with MNG supporting animations and JNG being a single-image format. The JNG format was created by the same team that developed the PNG format, and it was intended to complement PNG by adding JPEG-compressed color data while maintaining the possibility of a separate alpha channel, which is a feature that PNG supports but JPEG does not.

The structure of a JNG file is similar to that of a MNG file, but it is simpler since it is intended for single images only. A JNG file consists of a series of chunks, each of which contains a specific type of data. The most important chunks in a JNG file are the JHDR chunk, which contains the header information; the JDAT chunk, which contains the JPEG-compressed image data; the JSEP chunk, which may be present to indicate the end of the JPEG data stream; and the alpha channel chunks, which are optional and can be either IDAT chunks (containing PNG-compressed alpha data) or JDAA chunks (containing JPEG-compressed alpha data).

The JHDR chunk is the first chunk in a JNG file and is critical as it defines the properties of the image. It includes information such as the image's width and height, color depth, whether an alpha channel is present, the color space used, and the compression method for the alpha channel. This chunk allows decoders to understand how to process the subsequent data within the file.

The JDAT chunk contains the actual image data, which is compressed using the JPEG standard compression techniques. This compression allows for efficient storage of photographic images, which often contain complex color gradients and subtle variations in tone. The JPEG compression within JNG is identical to that used in standalone JPEG files, making it possible for standard JPEG decoders to read the image data from a JNG file without needing to understand the entire JNG format.

If an alpha channel is present in the JNG image, it is stored in either IDAT or JDAA chunks. The IDAT chunks are the same as those used in PNG files and contain PNG-compressed alpha data. This allows for lossless compression of the alpha channel, ensuring that transparency information is preserved without any quality loss. The JDAA chunks, on the other hand, contain JPEG-compressed alpha data, which allows for smaller file sizes at the cost of potential lossy compression artifacts in the alpha channel.

The JSEP chunk is an optional chunk that signals the end of the JPEG data stream. It is useful in cases where the JNG file is being streamed over a network, and the decoder needs to know when to stop reading JPEG data and start looking for alpha channel data. This chunk is not required if the file is being read from a local storage medium where the end of the JPEG data can be determined from the file structure itself.

JNG also supports color correction by including an ICCP chunk, which contains an embedded ICC color profile. This profile allows for accurate color representation across different devices and is particularly important for images that will be viewed on a variety of screens or printed. The inclusion of color management capabilities is a significant advantage of the JNG format over standalone JPEG files, which do not inherently support embedded color profiles.

Despite its capabilities, the JNG format has not seen widespread adoption. This is partly due to the dominance of the JPEG format for photographic images and the PNG format for images requiring transparency. Additionally, the rise of formats like WebP and HEIF, which also support both lossy and lossless compression as well as transparency, has further reduced the need for a separate format like JNG. However, JNG remains a viable option for specific use cases where its unique combination of features is required.

One of the reasons for the lack of widespread adoption of JNG is the complexity of the MNG file format suite. While JNG itself is relatively simple, it is part of a larger and more complex set of specifications that were not widely implemented. Many software developers chose to support the simpler and more popular JPEG and PNG formats instead, which met most users' needs without the additional complexity of MNG and JNG.

Another factor that has limited the adoption of JNG is the lack of support in popular image editing and viewing software. While some specialized software may support JNG, many of the most commonly used programs do not. This lack of support means that users and developers are less likely to encounter or use JNG files, further diminishing its presence in the marketplace.

Despite these challenges, JNG does have its proponents, particularly among those who appreciate its technical capabilities. For instance, JNG can be useful in applications where a single file needs to contain both a high-quality photographic image and a separate alpha channel for transparency. This can be important in graphic design, game development, and other fields where images need to be composited against various backgrounds.

The technical design of JNG also allows for potential optimizations in file size and quality. For example, by separating the color and alpha data, it is possible to apply different levels of compression to each, optimizing for the best balance between file size and image quality. This can result in smaller files than if a single compression method were applied to the entire image, as is the case with formats like PNG.

In conclusion, the JNG image format is a specialized file format that offers a unique combination of features, including support for both lossy and lossless compression, an optional alpha channel for transparency, and color management capabilities. While it has not achieved widespread adoption, it remains a technically capable format that may be suitable for specific applications. Its future relevance will likely depend on whether there is a renewed interest in its capabilities and whether software support for the format expands. For now, JNG stands as a testament to the ongoing evolution of image formats and the search for the perfect balance of compression, quality, and functionality.

Supported formats

AAI.aai

AAI Dune image

AI.ai

Adobe Illustrator CS2

AVIF.avif

AV1 Image File Format

AVS.avs

AVS X image

BAYER.bayer

Raw Bayer Image

BMP.bmp

Microsoft Windows bitmap image

CIN.cin

Cineon Image File

CLIP.clip

Image Clip Mask

CMYK.cmyk

Raw cyan, magenta, yellow, and black samples

CMYKA.cmyka

Raw cyan, magenta, yellow, black, and alpha samples

CUR.cur

Microsoft icon

DCX.dcx

ZSoft IBM PC multi-page Paintbrush

DDS.dds

Microsoft DirectDraw Surface

DPX.dpx

SMTPE 268M-2003 (DPX 2.0) image

DXT1.dxt1

Microsoft DirectDraw Surface

EPDF.epdf

Encapsulated Portable Document Format

EPI.epi

Adobe Encapsulated PostScript Interchange format

EPS.eps

Adobe Encapsulated PostScript

EPSF.epsf

Adobe Encapsulated PostScript

EPSI.epsi

Adobe Encapsulated PostScript Interchange format

EPT.ept

Encapsulated PostScript with TIFF preview

EPT2.ept2

Encapsulated PostScript Level II with TIFF preview

EXR.exr

High dynamic-range (HDR) image

FARBFELD.ff

Farbfeld

FF.ff

Farbfeld

FITS.fits

Flexible Image Transport System

GIF.gif

CompuServe graphics interchange format

GIF87.gif87

CompuServe graphics interchange format (version 87a)

GROUP4.group4

Raw CCITT Group4

HDR.hdr

High Dynamic Range image

HRZ.hrz

Slow Scan TeleVision

ICO.ico

Microsoft icon

ICON.icon

Microsoft icon

IPL.ipl

IP2 Location Image

J2C.j2c

JPEG-2000 codestream

J2K.j2k

JPEG-2000 codestream

JNG.jng

JPEG Network Graphics

JP2.jp2

JPEG-2000 File Format Syntax

JPC.jpc

JPEG-2000 codestream

JPE.jpe

Joint Photographic Experts Group JFIF format

JPEG.jpeg

Joint Photographic Experts Group JFIF format

JPG.jpg

Joint Photographic Experts Group JFIF format

JPM.jpm

JPEG-2000 File Format Syntax

JPS.jps

Joint Photographic Experts Group JPS format

JPT.jpt

JPEG-2000 File Format Syntax

JXL.jxl

JPEG XL image

MAP.map

Multi-resolution Seamless Image Database (MrSID)

MAT.mat

MATLAB level 5 image format

PAL.pal

Palm pixmap

PALM.palm

Palm pixmap

PAM.pam

Common 2-dimensional bitmap format

PBM.pbm

Portable bitmap format (black and white)

PCD.pcd

Photo CD

PCDS.pcds

Photo CD

PCT.pct

Apple Macintosh QuickDraw/PICT

PCX.pcx

ZSoft IBM PC Paintbrush

PDB.pdb

Palm Database ImageViewer Format

PDF.pdf

Portable Document Format

PDFA.pdfa

Portable Document Archive Format

PFM.pfm

Portable float format

PGM.pgm

Portable graymap format (gray scale)

PGX.pgx

JPEG 2000 uncompressed format

PICON.picon

Personal Icon

PICT.pict

Apple Macintosh QuickDraw/PICT

PJPEG.pjpeg

Joint Photographic Experts Group JFIF format

PNG.png

Portable Network Graphics

PNG00.png00

PNG inheriting bit-depth, color-type from original image

PNG24.png24

Opaque or binary transparent 24-bit RGB (zlib 1.2.11)

PNG32.png32

Opaque or binary transparent 32-bit RGBA

PNG48.png48

Opaque or binary transparent 48-bit RGB

PNG64.png64

Opaque or binary transparent 64-bit RGBA

PNG8.png8

Opaque or binary transparent 8-bit indexed

PNM.pnm

Portable anymap

PPM.ppm

Portable pixmap format (color)

PS.ps

Adobe PostScript file

PSB.psb

Adobe Large Document Format

PSD.psd

Adobe Photoshop bitmap

RGB.rgb

Raw red, green, and blue samples

RGBA.rgba

Raw red, green, blue, and alpha samples

RGBO.rgbo

Raw red, green, blue, and opacity samples

SIX.six

DEC SIXEL Graphics Format

SUN.sun

Sun Rasterfile

SVG.svg

Scalable Vector Graphics

SVGZ.svgz

Compressed Scalable Vector Graphics

TIFF.tiff

Tagged Image File Format

VDA.vda

Truevision Targa image

VIPS.vips

VIPS image

WBMP.wbmp

Wireless Bitmap (level 0) image

WEBP.webp

WebP Image Format

YUV.yuv

CCIR 601 4:1:1 or 4:2:2

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