MNG and PNJ: Status Report 19980804

MNG: A Multiple-Image Format in the PNG Family
PNJ: Portable Network JPEG Format

The Portable Network Graphics (PNG) [1] format for bit-mapped images was approved in October 1996 by the World Wide Web Consortium (W3C) as a W3C Recommendation and has also been published as RFC-2083 [2]. MNG ("Multiple-image Network Graphics") and PNJ ("Portable Network JPEG") are proposed additions to the PNG family, that are now being designed by the PNG developers [3]. MNG is for storing and transmitting multiple-image animations and composite frames, and PNJ is a MNG sub-format for encapsulating JPEG images, with alpha-channel transparency, in a format usable by MNG.

Because the images making up a MNG are in PNG and PNJ format, MNG shares the good features of PNG:

In addition,

A Simple MNG Datastream

The MNG format uses the same chunk structure (length, name, data, check value) as that used by PNG. The simplest form of MNG is an 8-byte MNG file signature, a MNG header chunk (MHDR), plus a series of one or more PNG or PNJ datastreams (less their 8-byte signatures), followed by a MEND chunk. MNG is more powerful than that, however. It is frequently true that the images will be similar, and data from the first image can be reused in constructing the second to conserve on the amount of data that must be stored or transmitted. In this example, the palette and the gamma and chromaticity data from the first image are reused in the second image:
    138 M N G CR LF 26 LF       # MNG 8-byte signature
    MHDR maxwidth maxheight ... # MNG Header chunk
    DEFI 1 visible concrete     # MNG Define image chunk
    IHDR width height ...       # PNG Header chunk
    gAMA 50000                  # PNG Gamma chunk
    cHRM ...                    # PNG Chromaticity chunk
    PLTE ...                    # PNG Palette
    IDAT ...                    # PNG Pixel data
    IEND                        # End of first PNG datastream
    DHDR 1 png replace width ht # Delta-PNG header
    IDAT ...                    # Delta-PNG pixels
    DEND                        # End of Delta-PNG
    MEND                        # End of MNG datastream
When only a smaller rectangle within the second image has pixels that are different from those in the first image, the DHDR chunk can specify that only a smaller rectangle of pixels (sometimes called a "change box") will be transmitted. Whether the pixels for the full image or for a smaller rectangle are changed, the data can be presented as new values that replace the old ones or as deltas (differences) from the corresponding pixels in the previous image. Usually the data in delta form is much more compressible. Several movies of finite-element calculational results by the U. S. Army Research Laboratory required only about a quarter of the file space when converted from a simple series of PNGs to delta-encoded PNGs.

It is possible to change just the alpha samples in the image, or in selected parts of it, to fade an image in or out against a background image.

Further dramatic savings in the size of the datastream can be achieved when an image or a portion of one is merely relocated. MNG provides a MOVE chunk in which the new coordinates of the image are transmitted instead of having to retransmit the entire image. A CLIP chunk is also available, to make it possible to show only a portion of a previously transmitted image. The MOVE and CLIP chunks can be used for scrolling or panning across an image that is larger than the display area.


MNG has a simple loop structure that can be used for repeating images. In this example, five images are defined and displayed in order 1-2-3-4-5 and then played ten times in order 4-3-2-1- 2-3-4-5:
  138 M N G CR LF 26 LF               # MNG 8-byte signature
  MHDR maxwidth maxheight ...         # MNG Header Chunk
  IHDR width height ...               # PNG Header Chunk
  DHDR 1 ... PLTE ...   IDAT ... IEND # Define Image 1
  CLON 1 2   DHDR 2 1 1 IDAT ... IEND # Define Image 2
  CLON 2 3   DHDR 3 1 1 IDAT ... IEND # Define Image 3
  CLON 3 4   DHDR 4 1 1 IDAT ... IEND # Define Image 4
  CLON 4 5   DHDR 5 1 1 IDAT ... IEND # Define Image 5
  LOOP 0 0 10                         # Begin Loop
  SHOW 4 2 SHOW 1 5                   # Show images 4-2, 1-5
  ENDL 0                              # End Loop
  MEND                                # End MNG

Composite Frames and Sprites

In addition to the simple single-image frames described thus far, MNG can also describe composite images that are built up of two or more PNG images. For example, one image could be a full-screen background while others could be small sprites that are moved around by means of the MOVE chunk. Examples that demonstrate these capabilities and others (including scrolling, tiling, storing 3-D tomographic data, and converting GIF animations to MNG format) are given as appendices in the MNG proposal.

MNG provides five framing modes that can be used with composite images. The framing modes include:

Status of MNG

The MNG proposal is being designed by the PNG developers and is being discussed in the "" mailing list. Interested persons can subscribe by sending a message to
that contains the line "subscribe mpng-list" (and nothing else) in the body.

The MNG format specification has not yet been frozen, but it has reached a state where test implementations are possible. There has been discussion of MNG since the completion of the PNG design in March 1995, and the first informal MNG drafts appeared on June 25, 1996. As of August 4, 1998, 45 drafts had been produced for review by the PNG group. The only major changes since Draft 33 (issued in late January, 1997) have been the recent addition of the PNJ format and improvement of palette animation via a new PPLT chunk. The examples shown above are consistent with Draft 33.

Several prototype MNG datastreams have been written, and two viewers have been written that are able to process a subset of MNG datastreams (simple movies and composite frames), including delta-encoded images. One has already been used by the U. S. Army for real presentation work as early as September 1996. The prototype MNG files and a viewer ("viewpng", written at the U.S. Army Research Laboratory for use on SGI workstations running IRIX 5.3) and the latest version of the MNG proposal from the PNG Development Group are available at the MNG ftp site,

A more complete viewer for Windows-95, by Gerard Juhn, was released on August 1, 1998. It is available at <URL:>

Competing proposals

Two competing proposals have also been written, both around January 1997. The documentation for these proposals can be found at the MNG ftp site mentioned above. They are


1. PNG (Portable Network Graphics) Specification, Version 1.0, <URL:>.

2. Boutell, T., et. al., PNG (Portable Network Graphics Format Version 1.0), RFC 2083, <URL:>

3. MNG (Multiple-image Network Graphics) Proposal, <URL:>.


Request for Comments

Comments on the proposed MNG format are welcome and should be addressed to