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Section - [14] Why all the argument about file formats?
Strictly speaking, JPEG refers only to a family of compression algorithms; it does *not* refer to a specific image
format. The JPEG committee was prevented from defining a file format by turf wars within the international
standards organizations.
Since we can't actually exchange images with anyone else unless we agree on a common file format, this leaves us
with a problem. In the absence of official standards, a number of JPEG program writers have just gone off to "do
their own thing", and as a result their programs aren't compatible with anyone else's.
The closest thing we have to a standard JPEG format is some work that's been coordinated by people at C-Cube
Microsystems. They have defined two JPEG-based file formats:
* JFIF (JPEG File Interchange Format), a "low-end" format that transports pixels and not much else.
* TIFF/JPEG, aka TIFF 6.0, an extension of the Aldus TIFF format. TIFF is a "high-end" format
that will let you record just about everything you ever wanted to know about an image, and a lot more besides :-).
JFIF has emerged as the de-facto standard on Internet, and is what is most commonly meant by "a JPEG file".
Most JFIF readers are also capable of handling some not-quite-JFIF-legal variant formats.
The TIFF 6.0 spec for incorporating JPEG is not widely implemented, partly because it has some serious design
flaws. A revised TIFF/JPEG design is now described by TIFF Technical Note #2; this design will be the one
used in TIFF 7.0. New implementations of TIFF should use the Tech Note's design for embedding JPEG, not
the TIFF 6.0 design. (As far as I know, NeXTStep systems are the only ones making any significant use of TIFF
6.0 style TIFF/JPEG.) Even when TIFF/JPEG is stable, it will never be as widely used as JFIF. TIFF is far more
complex than JFIF, and is generally less transportable because different vendors often implement slightly different,
nonoverlapping subsets of TIFF. Adding JPEG to the mix hasn't helped any.
Apple's Macintosh QuickTime software uses a JFIF-compatible datastream wrapped inside the Mac-specific PICT
format.Conversion between JFIF and PICT/JPEG is pretty straightforward, and several Mac programs are available
to do it (see part 2, item 8). If you have an editor that handles binary files, you can even strip a PICT/JPEG file down
to JFIF by hand; see the next section for details.
News flash: the ISO JPEG committee seems to have won their turf wars. They have defined a complete file format
spec called SPIFF in the new " art 3" extensions to the JPEG standard. It's pretty late in the game though, so whether
this will have much impact on real-world files remains to be seen. SPIFF is upward compatible with JFIF, so if it does
get widely adopted, most users probably won't even notice. |
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发表于 2015-5-6 17:45:41
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