(no subject)

[Ralph Giles]

On Tue 13 Jun 2000 K<bob> wrote:

> I never heard of MNG until just now. (So color me ignorant.) 

Sorry, I should have included some urls.

To briefly summarize, mng is an animated image file format from the people
who designed png, and is essentially a multiframe extension of the same.

png (pronounced 'ping') is a format developed by a group of volunteers as
a replacement for GIF for lossless image compression and transfer over the
internet. Its primary features are: free and unencumbered, lossless
compression comparable to GIF, support for paletted and truecolor images
up to 16 bits per channel, alpha transparency, gamma correction, a
progressive display option, and clever file integrity checks.

Unfortunately, although the final specification was released in 1997 (as
RFC 2083) mainstream adoption has been slow, mostly because of poor
support in Netscape and IE. My guess is this was a combination of lack of
commitment from the client developers, and graphics in general "not being
ready for alpha blending" at the time. Fortunately, the growing awareness
of patent issues among open source developers is making for widespread
adoption in that community, as I'm sure many here can appreciate.

mng (pronounced 'ming') is a multi-image extension of png. It can consist
of a series of png images, or a series of 'jng' images, or a mixture of
the two. jng is a sub-format of mng that wraps the most-commonly-
implemented subset of jpeg compression in a png-like file format, with the
additional option of a png-compressed alpha channel. So it can losslessly
transcode motion-jpeg. There's also a png-encoded 'image delta' option. 

Finally, it allows arbitrary composition of the the encoded images
with translation, simple clipping, alpha transparency and loops to create
frames for display, which is what makes it so powerful for animation.

> I read this on the front page of the MNG web site. "while [MNG] has 
> fairly extensive animation and image-manipulation capabilities, there 
> is no serious expectation that it will ever integrate audio or video." 

That's right. In terms of popular distribution of video, mng would
do a really poor job. And the designers have been steadfast in limiting
its scope to a well-defined feature set. While the chunk structure of the
files could be made to include other data types, I think mng-over-ogg
makes more sense than ogg-over-mng.

> So, is it a video format, or not? What I really mean is, does MNG 
> have a real timebase, or are MNG animations free-running? A real 
> timebase is necessary to synch up with audio. 

Hmm. Having thought about this more carefully, there may be some issues
here. I haven't read the entire spec, so I may be missing some ideas, but
this is my current understanding:

First of all, mng can be used for non-temporal image series like
slices in a 3D dataset or catalogs of separate but somehow related images.
I think there can also be multiple animations. So we probably want to
define some meta-specifications for inclusion in ogg.

But, for image-series-as-animation, there is a frame counter, a (variable)
inter-frame delay in arbitrary units, and a scale for converting that to
real time. Additionally, there are seek chucks that can mark an absolute
position in the stream, both in terms of frame number and timecode. (These
can be either 4 or 8 byte integers.) Would that be sufficient?

> Also, is there a MNG mailing list? I couldn't find one on the 
> web site. 

While it's still marked a 'draft specification' mng has been frozen since
May 1999, so you'd have to be pretty compelling to effect any design
modifications at this point. We could probably add an ancilliary chuck
type if it were necessary.

As for lists, I think the original png development took place on the
comp.graphics newsgroups. I don't know about mng. However, the libmng
implementer has a list <libmng-public at lists.sourceforge.net> you might
try. libmng itself is very much under development, so I'm sure input would
be welcome there.

URLs:

http://www.libpng.org/pub/png/	(the png website)
http://www.libpng.org/pub/mng/	(the mng website)
http://www.libmng.com/		(libmng implementation site)
http://sourceforge.net/project/?group_id=5635 (libmng on sf.net)

Hope that helps,
 -ralph


--
giles at ashlu.bc.ca

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