MNG and PNJ: Status Report 19980804
MNG: A Multiple-Image Format in the PNG Family
The Portable Network Graphics (PNG)
 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
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
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.
PNJ: Portable Network JPEG Format
Because the images making up a MNG are in PNG and PNJ format, MNG shares
the good features of PNG:
- It is unencumbered by patents.
- It is streamable.
- It has excellent, lossless compression.
- It stores up to 16 bits per channel.
- It provides transparency and an alpha channel.
- It provides platform-independent rendition of colors by
inclusion of gamma and chromaticity information.
- It provides early detection of common file transmission
errors and robust detection of file corruption.
- Single-image GIF ("Graphical Interchange Format") files can be
losslessly converted to PNG.
- It provides animation with loops and variable interframe delays.
- It allows composition of frames containing multiple images.
- It facilitates the use of images as "sprites."
- It capitalizes on frame-to-frame similarities to reduce the
amount of data that must be included in the datastream.
- It provides "restart" points at which an animation can be
resumed in case of data loss or corruption.
- A "frame priority" chunk allows authors to indicate which
frame should be displayed by single-image viewers, and a subset
of the frames that should be displayed by slow viewers.
- Multiple-image GIF files can be losslessly converted to MNG.
- It includes a lossy sub-format that provides JPEG with alpha.
- It is complementary to MPEG and does not attempt to replace MPEG
for lossy storage of video.
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
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
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-
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:
- Each image is a separate frame (as in the two examples shown
- A group of images makes up a frame.
- restoring background between frames
- not restoring background
- A group of images makes up a series of frames, in which the
images become visible one at a time, like cards being dealt.
- restoring background before displaying the series
- not restoring background
Status of MNG
The MNG proposal is being designed by the PNG developers and is being
discussed in the "firstname.lastname@example.org" mailing list.
Interested persons can subscribe by sending a message to
that contains the line "subscribe mpng-list" (and nothing else) in
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
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
- "ARG: Animated Raster Graphics"
- "DOH: Dancing Optical Hallucinations" (this is a temporary name; if
this proposal is accepted in lieu of MNG, it will probably be called MNG).
1. PNG (Portable Network Graphics) Specification, Version 1.0,
Boutell, T., et. al., PNG (Portable Network Graphics Format Version 1.0),
3. MNG (Multiple-image Network Graphics) Proposal,
- GIF is a service mark of CompuServe Incorporated.
- SGI and IRIX are trademarks of Silicon Graphics, Inc.
- Windows-95 is a trademark of MicroSoft, Inc.
Request for Comments
Comments on the proposed MNG format are welcome and should be addressed to