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A different free JPEG implementation, written by the PVRG
07
group at Stanford, is available from ftp://
havefun.stanford.edu/pub/jpeg/. This program is designed
for research and experimentation rather than production
use; it is slower, harder to use, and less portable than the IJG
code, but it is easier to read and modify. Also, the PVRG code
supports lossless JPEG, which we do not. (On the other
hand, it doesn't do progressive JPEG.)
FILE FORMAT WARS
Some JPEG programs produce files that are not compatible
with our library. The root of the problem is that the ISO JPEG
committee failed to specify a concrete file format. Some
vendors "filled in the blanks" on their own, creating
proprietary formats that no one else could read. (For
example, none of the early commercial JPEG
implementations for the Macintosh were able to exchange
compressed files.)
The file format we have adopted is called JFIF (see
REFERENCES). This format has been agreed to by a number
of major commercial JPEG vendors, and it has become the
de facto standard. JFIF is a minimal or "low end"
representation. We recommend the use of TIFF/JPEG (TIFF
revision 6.0 as modified by TIFF Technical Note #2) for "high
end" applications that need to record a lot of additional data
about an image. TIFF/JPEG is fairly new and not yet widely
supported, unfortunately.
The upcoming JPEG Part 3 standard defines a file format
called SPIFF. SPIFF is interoperable with JFIF, in the sense
that most JFIF decoders should be able to read the most
common variant of SPIFF. SPIFF has some technical
advantages over JFIF, but its major claim to fame is simply
that it is an official standard rather than an informal one. At
this point it is unclear whether SPIFF will supersede JFIF or
whether JFIF will remain the de-facto standard. IJG intends
to support SPIFF once the standard is frozen, but we have
not decided whether it should become our default output
format or not. (In any case, our decoder will remain capable
of reading JFIF indefinitely.)
Various proprietary file formats incorporating JPEG
compression also exist. We have little or no sympathy for the
existence of these formats. Indeed, one of the original
reasons for developing this free software was to help force
convergence on common, open format standards for JPEG
files. Don't use a proprietary file format!
TO DO
The major thrust for v7 will probably be improvement of
visual quality. The current method for scaling the
quantization tables is known not to be very good at low Q
values. We also intend to investigate block boundary
smoothing, "poor man's variable quantization", and other
means of improving quality-vs-file-size performance without
sacrificing compatibility.
In future versions, we are considering supporting some of
the upcoming JPEG Part 3 extensions --- principally, variable
quantization and the SPIFF file format.
As always, speeding things up is of great interest.
Please send bug reports, offers of help, etc. to jpeg-
info@uunet.uu.net.
libupnp
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