320 lines
16 KiB
HTML
320 lines
16 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
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"http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd">
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<html>
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<head>
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<title>Commentary on Meta Information Formats</title>
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<link rel=stylesheet type='text/css' href='style.css' title='Style'>
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<style type="text/css">
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<!--
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a.ref { text-decoration: none; font-size: x-small;
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font-weight: normal; vertical-align: super; }
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.norm { font-weight: normal }
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-->
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</style>
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</head>
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<body>
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<div class='index'>
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<b>What sucks:</b>
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<br><a href="#EXIF">EXIF / TIFF</a>
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<br><a href="#Maker">EXIF Maker notes</a>
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<br><a href="#JPEG">JPEG</a>
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<br><a href="#IPTC">IPTC IIM</a>
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<br><a href="#XMP">XMP</a>
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<br><a href="#PDF">PDF</a>
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<br><a href="#PNG">PNG</a>
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<br><a href="#PICT">PICT</a>
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<br><a href="#FlashPix">FlashPix</a>
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<br><a href="#DICOM">DICOM</a>
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<br><a href="#AVCHD">AVCHD</a>
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<br><a href="#ISO">ISO</a>
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<br><b>What doesn't:</b>
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<br><a href="#CRW">CRW (CIFF)</a>
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<br><a href="#QuickTime">QuickTime</a>
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<br><a href="#MIE">MIE</a>
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</div>
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<h2 class='up'>Commentary on Meta Information Formats<br>
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<span class='sm'>(or <i>"Why this or that Format Sucks"</i>)</span></h2>
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<a name='EXIF'></a>
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<h3>EXIF / TIFF</h3>
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<p>EXIF uses the TIFF format to store information. The biggest problem with
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this format is that all pointers are absolute (ie. relative to the start of
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the file). This means that the pointers need fixing up if the position of
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any information changes. This is significant because pointers are often
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hidden inside proprietary structures which should remain opaque (such as
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maker notes). The result is that some EXIF information is easily corrupted
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when editing a file. This problem is ignored by many software packages.
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Much effort was required in the development of ExifTool to avoid the loss of
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information such as this.</p>
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<p>A significant problem is that an IFD (image file directory) pointer has
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the same format code as a long integer, so information in any non-recognized
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IFD is lost when the image is rewritten. There is a seldom-used format
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code of 13 that has been proposed to indicate an IFD, but this is not part
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of the EXIF/TIFF specification and is not in common usage. <i class=lt>[Hey!
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Olympus has just started using this code for the IFD's in the maker notes of
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their new Camera models! - 2007/03/08]</i></p>
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<p>Also, it was wrong to specify a count instead of a size for each
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directory entry because this makes it impossible to add new format types
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without requiring that older readers simply discard data of unknown type
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(since the data size isn't known, the old readers don't know how to copy the
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data).</p>
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<p>All these shortcomings make TIFF a very non-extensible format which by
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design will result in the loss of non-standard information.</p>
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<p>And we don't learn from our mistakes (OK, maybe some of us can, but
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apparently not big corporations like Adobe or Microsoft), because the new
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DNG and WDP formats are TIFF-based too.</p>
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<p>The EXIF specification also has some significant shortcomings to add to the
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TIFF problems, like minimal special character abilities, no language support,
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and no way to specify time zone.</p>
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<a name='Maker'></a>
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<h3>EXIF Maker Notes</h3>
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<p>Maker notes are a can of worms. The EXIF specification unfortunately
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made no mention about the information format of the maker notes. If
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properly designed, the specification would have provided a maker note IFD
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tag to allow IFD-style maker notes to be used. Many manufacturers
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implemented it this way, but each with a different technique and many with
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<a href="idiosyncracies.html">odd quirks</a>. This deficiency combined with
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problems in the TIFF (EXIF) format mean that most software will quite
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unknowingly scramble your maker notes when an image is rewritten. To
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prevent this, specific knowledge about files written by individual
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manufacturers is necessary, and of course the manufacturers are not prone to
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divulging this information to the public so it must all be reverse
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engineered.</p>
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<p>The official TIFF recommendation is to delete unknown information when
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rewriting an image<a class=ref href="#ref1">[1]</a>, and this irresponsible
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strategy has been adopted by some applications such as Adobe Photoshop,
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resulting in a total loss of maker note information. This is very
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unfortunate because even corrupted maker notes contain useful information,
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and smart utilities (like ExifTool) are often able to recover the
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information from damaged maker notes.</p>
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<a name='JPEG'></a>
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<h3>JPEG</h3>
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<p>The JPEG format acts only as a container for meta information, and
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defines no meta information format itself. But it sucks just a as a
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container format because it limits the maximum contiguous block size to
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64kB. This causes problems because many cameras store more than 64kB of
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additional information in their JPEG images. Usually, this is due to the
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medium-sized JPEG image that many cameras embed for preview purposes. It
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would have been nice if there was a standard way to embed this preview in
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JPEG images (without the 64kB limitation). <i class=lt>[Update: In February
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2009 CIPA released the MPF format specification which allows storage of
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preview images larger than 64kB, but unfortunately this specification
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<a href="standards.html#MPF">has problems of its own</a>.]</i></p>
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<p>Aside: The EXIF specification could have easily allowed the information
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to span multiple JPEG segments, which would have been smart, but still
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wouldn't have solved the problem entirely since it could result in a
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non-contiguous preview image (this would be a problem for cameras since they
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typically want to use this image for quick review of pictures, and
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re-mapping the image into a contiguous memory space would be costly).</p>
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<a name='IPTC'></a>
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<h3>IPTC IIM</h3>
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<p>IPTC IIM earns a few points because it is the easiest to process of the
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common meta information formats. However, the format is overly restrictive
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and not very extensible. Format information and human-readable tag names
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are not part of the specification, so it is not possible to meaningfully
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interpret unknown values. Also, there are many interdependencies between
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tags that make it very annoying for writing, and the special character
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support is ill-conceived.</p>
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<a name='XMP'></a>
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<h3>XMP</h3>
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<p>The XMP format is a good example of a designed-by-committee
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specification. It is based on XML, which is much more complicated than
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necessary and tries to make everyone happy by providing a multitude of
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format styles and features (shorthand format, etc, etc). This makes it very
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difficult to properly implement the complete specification. Not even the
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Adobe software supports all of the format options of their own XMP
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specification. And the format is exceedingly verbose, wasting disk space,
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bandwidth and time.</p>
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<p>Also, XMP only supports textual information. Binary information must be
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text-encoded if stored in XMP. This makes it very bulky, slow and
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completely unsuitable for storage of binary information.</p>
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<p>Mainly because of the complexity of the specification, and despite
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Adobe's best efforts, XMP still isn't very well supported by meta
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information editing tools. The best hope for small application developers
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is to link against an off-the-shelf XMP or XML library. Apparently Adobe
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realizes this and is now providing an XMP development kit, but this solution
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is only useful for those platforms and computer languages that they choose
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to support.</p>
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<a name='PDF'></a>
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<h3>PDF</h3>
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<p>...and I thought XMP was too complicated. PDF is just insane. Because
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of this, it is unlikely that ExifTool will ever support this format for
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writing. <i class=lt>[Dec. 18, 2007 Update: I have just added PDF write support
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using the incremental update feature of PDF. It was still a bit of work, but
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somewhat simpler (and MUCH faster!) than rewriting the entire PDF. But there
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are disadvantages: The resulting PDF is not "linearized", and there are privacy
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and security issues with leaving old metadata in the file.]</i></p>
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<a name='PNG'></a>
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<h3>PNG</h3>
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<p>PNG sucks because meta information can be stored only in text format.
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This was a huge mistake, and as a work-around software (eg. ImageMagick)
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must encode binary information in ASCII-hex, then compress it to offset the
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increase in size, and store it as a compressed text block. This adds
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unnecessary complexity, greatly slows down processing, and makes it much
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harder to develop software which supports PNG meta information.</p>
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<a name='PICT'></a>
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<h3>PICT</h3>
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<p>The PICT format just plain sucks. It is one of the worst-designed image
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formats in existence, second only to FlashPix. I don't believe that it was
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originally designed as a file format. Instead it appears to have emerged
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from the internal Apple QuickDraw structures. The files are not designed to
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be processed by any software other than the Mac OS. Intimate knowledge of
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the QuickDraw structures is required just to parse the PICT image, even if
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the reader doesn't care about the contained information. Because of this,
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PICT images are very fragile and a simple data error or programming bug will
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invalidate an entire image. Possibly for these reasons the PICT format
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never gained popularity outside the Apple world. Also, the concept of meta
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information is all but ignored in the PICT design.</p>
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<a name='FlashPix'></a>
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<h3>FlashPix</h3>
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<p>Oh. My. God. Don't get me started on this one. Let's just say that basing
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an image file format on the File Allocation Table (FAT) structure of a floppy
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disk with fixed 512-byte sectors is monumentally idiotic. Microsoft Word,
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Excel and PowerPoint documents also use this insane format.</p>
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<a name='DICOM'></a>
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<h3>DICOM Medical Images</h3>
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<p>The DICOM format is reasonably well designed. The UID's are cumbersome,
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but achieve their purpose. The biggest bungle in the design is the implicit
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VR syntax, which presumably exists for historical reasons to provide
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backward compatibility with pre-existing ACR-NEMA images. The DICOM
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specification document is horrible and obviously written by committee, and
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was possibly the single largest impediment to implementing support for this
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format.</p>
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<p>Technical: The files would be easier to parse if the transfer syntax
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became effective immediately after the transfer syntax data element instead
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of at the end of the meta information group. Also, the deflated data should
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have contained the zlib header, as in PNG and PDF images. This would make
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it easier to read and allow CRC validation of the datastream. -- Can you
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tell I wrote this just after adding DICOM support to ExifTool?</p>
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<a name='AVCHD'></a>
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<h3>AVCHD (.M2TS) <span class='sm norm'><i class=lt>(added 2009-10-29)</i></span></h3>
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<p>This format is used in .M2TS video files (and Blu-Ray HD DVD's). It is
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painfully obvious that this was never designed as a storage format. It uses
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MPEG-2 transport stream (M2TS) container, which is a communications protocol
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and never should have been used for storage. The M2TS format is based on a
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188-byte packet size which makes no sense for modern filesystems. And to
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make things worse, metadata in AVCHD files is stored in the H.264 video
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stream (!!) which uses insane and convoluted data structures such as the
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variable-bit-length exponential-Golomb which are painful and inefficient to
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parse in software, and as far as I can tell the format for the metadata
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stored in these streams is proprietary and undocumented.</p>
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<a name='ISO'></a>
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<h3>ISO Considered Harmless</h3>
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<p>This is not a meta information format, but since many format
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specifications are imprisoned by the ISO it deserves mentioning...</p>
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<p>In my experience, the International Standards Organization is extremely
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counter-productive. Their goal is to promote standardization, but by
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charging too much money for copies of the standards documents they defeat
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their own purpose. The effect is that small companies and individuals
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developing software (including open source software such as ExifTool) do not
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have access to the official standard. Instead, software is often based on
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old, obsolete documents or drafts which are sometimes publicly available, or
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on 3rd party descriptions which are often incomplete. To sum things up: ISO
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sucks.</p>
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<hr>
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<h2>OK. So all those formats suck. What doesn't?</h2>
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<a name='CRW'></a>
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<h3>Canon CRW Images (CIFF format)</h3>
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<p>This format uses a directory structure similar to TIFF, but all offsets
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are relative and unknown information can handled properly when rewriting.
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Because of this, it is much more extensible than TIFF, but the format is
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still limited by 16-bit integer tag ID's among other TIFF weaknesses. It
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isn't great, but at least it doesn't suck.</p>
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<a name='QuickTime'></a>
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<h3>QuickTime Videos</h3>
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<p>The Apple QuickTime MOV format is very nice format. (Which is a breath
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of fresh air after the Apple PICT travesty.) It was a smart move to use
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this format for the MP4 specification. I would only change 2 things about
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this format: 1) Use longer tag names instead of the current 4 characters (I
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know that 4 characters is convenient because it can be used as an integer in
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lookup tables, but it would be nice if the tag names were a bit more
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meaningful). 2) Add a format code to the Atom definition so simple unknown
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information could be decoded properly. These changes would allow some
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unknown information to be interpreted, which would make it possible for
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information added by 3rd parties to be useful.</p>
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<p>I only have 2 problems with the QuickTime specification:</p>
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<p>1) The metadata organization is a mess because tags may be stored in more
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than one location (eg. UserData vs. InfoList), and conflicting metadata
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directories may exist, even in the same location (as written by iTunes).</p>
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<p>2) The length of 'udta' text strings with language codes is written
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incorrectly by many (most?) utilties, including Apple iTunes. The language
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strings are supposed to use a "small integer atom format" where the first
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two bytes give the size of the string including the length word and language
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codes. But often the size doesn't include these 4 bytes, which sort of
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defeats the purpose of the multiple languages because it is difficult to
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parse past the first entry if you don't know the size.</p>
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<a name='MIE'></a>
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<h3>My Format (Meta Information Encapsulation, or "MIE")</h3>
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<p>Now it is time to put my money where my mouth is: I have designed a
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format which doesn't suck. It is called "MIE", which stands for Meta
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Information Encapsulation.</p>
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<p>The MIE format is hierarchical like QuickTime, but it uses meaningful tag
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names and specifies a data format for all values. As well, this format
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offers a number of other features:</p>
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<ul>
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<li>Can be used as a wrapper around any type of file, as a trailer appended
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to other formats, or as a stand-alone meta information format
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<li>Extensible
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<li>Compact
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<li>Meaningful tag ID's
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<li>Streamable (and single-pass writing possible)
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<li>Relocatable data elements (ie. no fixed offsets)
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<li>Relatively simple to implement reader/writer
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<li>Supports large data lengths (up to 4GB*4GB)
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<li>Localized text feature
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<li>Built-in support for numerical units of measure
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<li>Multiple documents in a single file
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</ul>
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<p>The specification for this format can be found
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<a href="MIE1.1-20070121.pdf">here</a>, and is implemented in
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<a href="http://search.cpan.org/dist/Image-ExifTool/lib/Image/ExifTool/MIE.pm">Image::ExifTool::MIE.pm</a></p>
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<p>OK. Flame away. I can take it.</p>
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<a name="refs"></a>
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<h3>References</h3>
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<ol>
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<li><a name="ref1" href="http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf">http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf</a>, page 26</li>
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</ol>
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<hr>
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<i>Created Nov. 15, 2005</i><br>
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<i>Last revised July 29, 2010</i>
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<p class='lf'><a href="index.html"><-- Back to ExifTool home page</a></p>
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