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-<pre>
-DRAFT TIFF Technical Note #2				17-Mar-95
-============================
-
-This Technical Note describes serious problems that have been found in
-TIFF 6.0's design for embedding JPEG-compressed data in TIFF (Section 22
-of the TIFF 6.0 spec of 3 June 1992).  A replacement TIFF/JPEG
-specification is given.  Some corrections to Section 21 are also given.
-
-To permit TIFF implementations to continue to read existing files, the 6.0
-JPEG fields and tag values will remain reserved indefinitely.  However,
-TIFF writers are strongly discouraged from using the 6.0 JPEG design.  It
-is expected that the next full release of the TIFF specification will not
-describe the old design at all, except to note that certain tag numbers
-are reserved.  The existing Section 22 will be replaced by the
-specification text given in the second part of this Tech Note.
-
-
-Problems in TIFF 6.0 JPEG
-=========================
-
-Abandoning a published spec is not a step to be taken lightly.  This
-section summarizes the reasons that have forced this decision.
-TIFF 6.0's JPEG design suffers from design errors and limitations,
-ambiguities, and unnecessary complexity.
-
-
-Design errors and limitations
------------------------------
-
-The fundamental design error in the existing Section 22 is that JPEG's
-various tables and parameters are broken out as separate fields which the
-TIFF control logic must manage.  This is bad software engineering: that
-information should be treated as private to the JPEG codec
-(compressor/decompressor).  Worse, the fields themselves are specified
-without sufficient thought for future extension and without regard to
-well-established TIFF conventions.  Here are some of the significant
-problems:
-
-* The JPEGxxTable fields do not store the table data directly in the
-IFD/field structure; rather, the fields hold pointers to information
-elsewhere in the file.  This requires special-purpose code to be added to
-*every* TIFF-manipulating application, whether it needs to decode JPEG
-image data or not.  Even a trivial TIFF editor, for example a program to
-add an ImageDescription field to a TIFF file, must be explicitly aware of
-the internal structure of the JPEG-related tables, or else it will probably
-break the file.  Every other auxiliary field in the TIFF spec contains
-data, not pointers, and can be copied or relocated by standard code that
-doesn't know anything about the particular field.  This is a crucial
-property of the TIFF format that must not be given up.
-
-* To manipulate these fields, the TIFF control logic is required to know a
-great deal about JPEG details, for example such arcana as how to compute
-the length of a Huffman code table --- the length is not supplied in the
-field structure and can only be found by inspecting the table contents.
-This is again a violation of good software practice.  Moreover, it will
-prevent easy adoption of future JPEG extensions that might change these
-low-level details.
-
-* The design neglects the fact that baseline JPEG codecs support only two
-sets of Huffman tables: it specifies a separate table for each color
-component.  This implies that encoders must waste space (by storing
-duplicate Huffman tables) or else violate the well-founded TIFF convention
-that prohibits duplicate pointers.  Furthermore, baseline decoders must
-test to find out which tables are identical, a waste of time and code
-space.
-
-* The JPEGInterchangeFormat field also violates TIFF's proscription against
-duplicate pointers: the normal strip/tile pointers are expected to point
-into the larger data area pointed to by JPEGInterchangeFormat.  All TIFF
-editing applications must be specifically aware of this relationship, since
-they must maintain it or else delete the JPEGInterchangeFormat field.  The
-JPEGxxTables fields are also likely to point into the JPEGInterchangeFormat
-area, creating additional pointer relationships that must be maintained.
-
-* The JPEGQTables field is fixed at a byte per table entry; there is no
-way to support 16-bit quantization values.  This is a serious impediment
-to extending TIFF to use 12-bit JPEG.
-
-* The 6.0 design cannot support using different quantization tables in
-different strips/tiles of an image (so as to encode some areas at higher
-quality than others).  Furthermore, since quantization tables are tied
-one-for-one to color components, the design cannot support table switching
-options that are likely to be added in future JPEG revisions.
-
-
-Ambiguities
------------
-
-Several incompatible interpretations are possible for 6.0's treatment of
-JPEG restart markers:
-
-  * It is unclear whether restart markers must be omitted at TIFF segment
-    (strip/tile) boundaries, or whether they are optional.
-
-  * It is unclear whether the segment size is required to be chosen as
-    a multiple of the specified restart interval (if any); perhaps the
-    JPEG codec is supposed to be reset at each segment boundary as if
-    there were a restart marker there, even if the boundary does not fall
-    at a multiple of the nominal restart interval.
-
-  * The spec fails to address the question of restart marker numbering:
-    do the numbers begin again within each segment, or not?
-
-That last point is particularly nasty.  If we make numbering begin again
-within each segment, we give up the ability to impose a TIFF strip/tile
-structure on an existing JPEG datastream with restarts (which was clearly a
-goal of Section 22's authors).  But the other choice interferes with random
-access to the image segments: a reader must compute the first restart
-number to be expected within a segment, and must have a way to reset its
-JPEG decoder to expect a nonzero restart number first.  This may not even
-be possible with some JPEG chips.
-
-The tile height restriction found on page 104 contradicts Section 15's
-general description of tiles.  For an image that is not vertically
-downsampled, page 104 specifies a tile height of one MCU or 8 pixels; but
-Section 15 requires tiles to be a multiple of 16 pixels high.
-
-This Tech Note does not attempt to resolve these ambiguities, so
-implementations that follow the 6.0 design should be aware that
-inter-application compatibility problems are likely to arise.
-
-
-Unnecessary complexity
-----------------------
-
-The 6.0 design creates problems for implementations that need to keep the
-JPEG codec separate from the TIFF control logic --- for example, consider
-using a JPEG chip that was not designed specifically for TIFF.  JPEG codecs
-generally want to produce or consume a standard ISO JPEG datastream, not
-just raw compressed data.  (If they were to handle raw data, a separate
-out-of-band mechanism would be needed to load tables into the codec.)
-With such a codec, the TIFF control logic must parse JPEG markers emitted
-by the codec to create the TIFF table fields (when writing) or synthesize
-JPEG markers from the TIFF fields to feed the codec (when reading).  This
-means that the control logic must know a great deal more about JPEG details
-than we would like.  The parsing and reconstruction of the markers also
-represents a fair amount of unnecessary work.
-
-Quite a few implementors have proposed writing "TIFF/JPEG" files in which
-a standard JPEG datastream is simply dumped into the file and pointed to
-by JPEGInterchangeFormat.  To avoid parsing the JPEG datastream, they
-suggest not writing the JPEG auxiliary fields (JPEGxxTables etc) nor even
-the basic TIFF strip/tile data pointers.  This approach is incompatible
-with implementations that handle the full TIFF 6.0 JPEG design, since they
-will expect to find strip/tile pointers and auxiliary fields.  Indeed this
-is arguably not TIFF at all, since *all* TIFF-reading applications expect
-to find strip or tile pointers.  A subset implementation that is not
-upward-compatible with the full spec is clearly unacceptable.  However,
-the frequency with which this idea has come up makes it clear that
-implementors find the existing Section 22 too complex.
-
-
-Overview of the solution
-========================
-
-To solve these problems, we adopt a new design for embedding
-JPEG-compressed data in TIFF files.  The new design uses only complete,
-uninterpreted ISO JPEG datastreams, so it should be much more forgiving of
-extensions to the ISO standard.  It should also be far easier to implement
-using unmodified JPEG codecs.
-
-To reduce overhead in multi-segment TIFF files, we allow JPEG overhead
-tables to be stored just once in a JPEGTables auxiliary field.  This
-feature does not violate the integrity of the JPEG datastreams, because it
-uses the notions of "tables-only datastreams" and "abbreviated image
-datastreams" as defined by the ISO standard.
-
-To prevent confusion with the old design, the new design is given a new
-Compression tag value, Compression=7.  Readers that need to handle
-existing 6.0 JPEG files may read both old and new files, using whatever
-interpretation of the 6.0 spec they did before.  Compression tag value 6
-and the field tag numbers defined by 6.0 section 22 will remain reserved
-indefinitely, even though detailed descriptions of them will be dropped
-from future editions of the TIFF specification.
-
-
-Replacement TIFF/JPEG specification
-===================================
-
-[This section of the Tech Note is expected to replace Section 22 in the
-next release of the TIFF specification.]
-
-This section describes TIFF compression scheme 7, a high-performance
-compression method for continuous-tone images.
-
-Introduction
-------------
-
-This TIFF compression method uses the international standard for image
-compression ISO/IEC 10918-1, usually known as "JPEG" (after the original
-name of the standards committee, Joint Photographic Experts Group).  JPEG
-is a joint ISO/CCITT standard for compression of continuous-tone images.
-
-The JPEG committee decided that because of the broad scope of the standard,
-no one algorithmic procedure was able to satisfy the requirements of all
-applications.  Instead, the JPEG standard became a "toolkit" of multiple
-algorithms and optional capabilities.  Individual applications may select
-a subset of the JPEG standard that meets their requirements.
-
-The most important distinction among the JPEG processes is between lossy
-and lossless compression.  Lossy compression methods provide high
-compression but allow only approximate reconstruction of the original
-image.  JPEG's lossy processes allow the encoder to trade off compressed
-file size against reconstruction fidelity over a wide range.  Typically,
-10:1 or more compression of full-color data can be obtained while keeping
-the reconstructed image visually indistinguishable from the original.  Much
-higher compression ratios are possible if a low-quality reconstructed image
-is acceptable.  Lossless compression provides exact reconstruction of the
-source data, but the achievable compression ratio is much lower than for
-the lossy processes; JPEG's rather simple lossless process typically
-achieves around 2:1 compression of full-color data.
-
-The most widely implemented JPEG subset is the "baseline" JPEG process.
-This provides lossy compression of 8-bit-per-channel data.  Optional
-extensions include 12-bit-per-channel data, arithmetic entropy coding for
-better compression, and progressive/hierarchical representations.  The
-lossless process is an independent algorithm that has little in
-common with the lossy processes.
-
-It should be noted that the optional arithmetic-coding extension is subject
-to several US and Japanese patents.  To avoid patent problems, use of
-arithmetic coding processes in TIFF files intended for inter-application
-interchange is discouraged.
-
-All of the JPEG processes are useful only for "continuous tone" data,
-in which the difference between adjacent pixel values is usually small.
-Low-bit-depth source data is not appropriate for JPEG compression, nor
-are palette-color images good candidates.  The JPEG processes work well
-on grayscale and full-color data.
-
-Describing the JPEG compression algorithms in sufficient detail to permit
-implementation would require more space than we have here.  Instead, we
-refer the reader to the References section.
-
-
-What data is being compressed?
-------------------------------
-
-In lossy JPEG compression, it is customary to convert color source data
-to YCbCr and then downsample it before JPEG compression.  This gives
-2:1 data compression with hardly any visible image degradation, and it
-permits additional space savings within the JPEG compression step proper.
-However, these steps are not considered part of the ISO JPEG standard.
-The ISO standard is "color blind": it accepts data in any color space.
-
-For TIFF purposes, the JPEG compression tag is considered to represent the
-ISO JPEG compression standard only.  The ISO standard is applied to the
-same data that would be stored in the TIFF file if no compression were
-used.  Therefore, if color conversion or downsampling are used, they must
-be reflected in the regular TIFF fields; these steps are not considered to
-be implicit in the JPEG compression tag value.  PhotometricInterpretation
-and related fields shall describe the color space actually stored in the
-file.  With the TIFF 6.0 field definitions, downsampling is permissible
-only for YCbCr data, and it must correspond to the YCbCrSubSampling field.
-(Note that the default value for this field is not 1,1; so the default for
-YCbCr is to apply downsampling!)  It is likely that future versions of TIFF
-will provide additional PhotometricInterpretation values and a more general
-way of defining subsampling, so as to allow more flexibility in
-JPEG-compressed files.  But that issue is not addressed in this Tech Note.
-
-Implementors should note that many popular JPEG codecs
-(compressor/decompressors) provide automatic color conversion and
-downsampling, so that the application may supply full-size RGB data which
-is nonetheless converted to downsampled YCbCr.  This is an implementation
-convenience which does not excuse the TIFF control layer from its
-responsibility to know what is really going on.  The
-PhotometricInterpretation and subsampling fields written to the file must
-describe what is actually in the file.
-
-A JPEG-compressed TIFF file will typically have PhotometricInterpretation =
-YCbCr and YCbCrSubSampling = [2,1] or [2,2], unless the source data was
-grayscale or CMYK.
-
-
-Basic representation of JPEG-compressed images
-----------------------------------------------
-
-JPEG compression works in either strip-based or tile-based TIFF files.
-Rather than repeating "strip or tile" constantly, we will use the term
-"segment" to mean either a strip or a tile.
-
-When the Compression field has the value 7, each image segment contains
-a complete JPEG datastream which is valid according to the ISO JPEG
-standard (ISO/IEC 10918-1).  Any sequential JPEG process can be used,
-including lossless JPEG, but progressive and hierarchical processes are not
-supported.  Since JPEG is useful only for continuous-tone images, the
-PhotometricInterpretation of the image shall not be 3 (palette color) nor
-4 (transparency mask).  The bit depth of the data is also restricted as
-specified below.
-
-Each image segment in a JPEG-compressed TIFF file shall contain a valid
-JPEG datastream according to the ISO JPEG standard's rules for
-interchange-format or abbreviated-image-format data.  The datastream shall
-contain a single JPEG frame storing that segment of the image.  The
-required JPEG markers within a segment are:
-	SOI	(must appear at very beginning of segment)
-	SOFn
-	SOS	(one for each scan, if there is more than one scan)
-	EOI	(must appear at very end of segment)
-The actual compressed data follows SOS; it may contain RSTn markers if DRI
-is used.
-
-Additional JPEG "tables and miscellaneous" markers may appear between SOI
-and SOFn, between SOFn and SOS, and before each subsequent SOS if there is
-more than one scan.  These markers include:
-	DQT
-	DHT
-	DAC	(not to appear unless arithmetic coding is used)
-	DRI
-	APPn	(shall be ignored by TIFF readers)
-	COM	(shall be ignored by TIFF readers)
-DNL markers shall not be used in TIFF files.  Readers should abort if any
-other marker type is found, especially the JPEG reserved markers;
-occurrence of such a marker is likely to indicate a JPEG extension.
-
-The tables/miscellaneous markers may appear in any order.  Readers are
-cautioned that although the SOFn marker refers to DQT tables, JPEG does not
-require those tables to precede the SOFn, only the SOS.  Missing-table
-checks should be made when SOS is reached.
-
-If no JPEGTables field is used, then each image segment shall be a complete
-JPEG interchange datastream.  Each segment must define all the tables it
-references.  To allow readers to decode segments in any order, no segment
-may rely on tables being carried over from a previous segment.
-
-When a JPEGTables field is used, image segments may omit tables that have
-been specified in the JPEGTables field.  Further details appear below.
-
-The SOFn marker shall be of type SOF0 for strict baseline JPEG data, of
-type SOF1 for non-baseline lossy JPEG data, or of type SOF3 for lossless
-JPEG data.  (SOF9 or SOF11 would be used for arithmetic coding.)  All
-segments of a JPEG-compressed TIFF image shall use the same JPEG
-compression process, in particular the same SOFn type.
-
-The data precision field of the SOFn marker shall agree with the TIFF
-BitsPerSample field.  (Note that when PlanarConfiguration=1, this implies
-that all components must have the same BitsPerSample value; when
-PlanarConfiguration=2, different components could have different bit
-depths.)  For SOF0 only precision 8 is permitted; for SOF1, precision 8 or
-12 is permitted; for SOF3, precisions 2 to 16 are permitted.
-
-The image dimensions given in the SOFn marker shall agree with the logical
-dimensions of that particular strip or tile.  For strip images, the SOFn
-image width shall equal ImageWidth and the height shall equal RowsPerStrip,
-except in the last strip; its SOFn height shall equal the number of rows
-remaining in the ImageLength.  (In other words, no padding data is counted
-in the SOFn dimensions.)  For tile images, each SOFn shall have width
-TileWidth and height TileHeight; adding and removing any padding needed in
-the edge tiles is the concern of some higher level of the TIFF software.
-(The dimensional rules are slightly different when PlanarConfiguration=2,
-as described below.)
-
-The ISO JPEG standard only permits images up to 65535 pixels in width or
-height, due to 2-byte fields in the SOFn markers.  In TIFF, this limits
-the size of an individual JPEG-compressed strip or tile, but the total
-image size can be greater.
-
-The number of components in the JPEG datastream shall equal SamplesPerPixel
-for PlanarConfiguration=1, and shall be 1 for PlanarConfiguration=2.  The
-components shall be stored in the same order as they are described at the
-TIFF field level.  (This applies both to their order in the SOFn marker,
-and to the order in which they are scanned if multiple JPEG scans are
-used.)  The component ID bytes are arbitrary so long as each component
-within an image segment is given a distinct ID.  To avoid any possible
-confusion, we require that all segments of a TIFF image use the same ID
-code for a given component.
-
-In PlanarConfiguration 1, the sampling factors given in SOFn markers shall
-agree with the sampling factors defined by the related TIFF fields (or with
-the default values that are specified in the absence of those fields).
-
-When DCT-based JPEG is used in a strip TIFF file, RowsPerStrip is required
-to be a multiple of 8 times the largest vertical sampling factor, i.e., a
-multiple of the height of an interleaved MCU.  (For simplicity of
-specification, we require this even if the data is not actually
-interleaved.)  For example, if YCbCrSubSampling = [2,2] then RowsPerStrip
-must be a multiple of 16.  An exception to this rule is made for
-single-strip images (RowsPerStrip >= ImageLength): the exact value of
-RowsPerStrip is unimportant in that case.  This rule ensures that no data
-padding is needed at the bottom of a strip, except perhaps the last strip.
-Any padding required at the right edge of the image, or at the bottom of
-the last strip, is expected to occur internally to the JPEG codec.
-
-When DCT-based JPEG is used in a tiled TIFF file, TileLength is required
-to be a multiple of 8 times the largest vertical sampling factor, i.e.,
-a multiple of the height of an interleaved MCU; and TileWidth is required
-to be a multiple of 8 times the largest horizontal sampling factor, i.e.,
-a multiple of the width of an interleaved MCU.  (For simplicity of
-specification, we require this even if the data is not actually
-interleaved.)  All edge padding required will therefore occur in the course
-of normal TIFF tile padding; it is not special to JPEG.
-
-Lossless JPEG does not impose these constraints on strip and tile sizes,
-since it is not DCT-based.
-
-Note that within JPEG datastreams, multibyte values appear in the MSB-first
-order specified by the JPEG standard, regardless of the byte ordering of
-the surrounding TIFF file.
-
-
-JPEGTables field
-----------------
-
-The only auxiliary TIFF field added for Compression=7 is the optional
-JPEGTables field.  The purpose of JPEGTables is to predefine JPEG
-quantization and/or Huffman tables for subsequent use by JPEG image
-segments.  When this is done, these rather bulky tables need not be
-duplicated in each segment, thus saving space and processing time.
-JPEGTables may be used even in a single-segment file, although there is no
-space savings in that case.
-
-JPEGTables:
-	Tag = 347 (15B.H)
-	Type = UNDEFINED
-	N = number of bytes in tables datastream, typically a few hundred
-JPEGTables provides default JPEG quantization and/or Huffman tables which
-are used whenever a segment datastream does not contain its own tables, as
-specified below.
-
-Notice that the JPEGTables field is required to have type code UNDEFINED,
-not type code BYTE.  This is to cue readers that expanding individual bytes
-to short or long integers is not appropriate.  A TIFF reader will generally
-need to store the field value as an uninterpreted byte sequence until it is
-fed to the JPEG decoder.
-
-Multibyte quantities within the tables follow the ISO JPEG convention of
-MSB-first storage, regardless of the byte ordering of the surrounding TIFF
-file.
-
-When the JPEGTables field is present, it shall contain a valid JPEG
-"abbreviated table specification" datastream.  This datastream shall begin
-with SOI and end with EOI.  It may contain zero or more JPEG "tables and
-miscellaneous" markers, namely:
-	DQT
-	DHT
-	DAC	(not to appear unless arithmetic coding is used)
-	DRI
-	APPn	(shall be ignored by TIFF readers)
-	COM	(shall be ignored by TIFF readers)
-Since JPEG defines the SOI marker to reset the DAC and DRI state, these two
-markers' values cannot be carried over into any image datastream, and thus
-they are effectively no-ops in the JPEGTables field.  To avoid confusion,
-it is recommended that writers not place DAC or DRI markers in JPEGTables.
-However readers must properly skip over them if they appear.
-
-When JPEGTables is present, readers shall load the table specifications
-contained in JPEGTables before processing image segment datastreams.
-Image segments may simply refer to these preloaded tables without defining
-them.  An image segment can still define and use its own tables, subject to
-the restrictions below.
-
-An image segment may not redefine any table defined in JPEGTables.  (This
-restriction is imposed to allow readers to process image segments in random
-order without having to reload JPEGTables between segments.)  Therefore, use
-of JPEGTables divides the available table slots into two groups: "global"
-slots are defined in JPEGTables and may be used but not redefined by
-segments; "local" slots are available for local definition and use in each
-segment.  To permit random access, a segment may not reference any local
-tables that it does not itself define.
-
-
-Special considerations for PlanarConfiguration 2
-------------------------------------------------
-
-In PlanarConfiguration 2, each image segment contains data for only one
-color component.  To avoid confusing the JPEG codec, we wish the segments
-to look like valid single-channel (i.e., grayscale) JPEG datastreams.  This
-means that different rules must be used for the SOFn parameters.
-
-In PlanarConfiguration 2, the dimensions given in the SOFn of a subsampled
-component shall be scaled down by the sampling factors compared to the SOFn
-dimensions that would be used in PlanarConfiguration 1.  This is necessary
-to match the actual number of samples stored in that segment, so that the
-JPEG codec doesn't complain about too much or too little data.  In strip
-TIFF files the computed dimensions may need to be rounded up to the next
-integer; in tiled files, the restrictions on tile size make this case
-impossible.
-
-Furthermore, all SOFn sampling factors shall be given as 1.  (This is
-merely to avoid confusion, since the sampling factors in a single-channel
-JPEG datastream have no real effect.)
-
-Any downsampling will need to happen externally to the JPEG codec, since
-JPEG sampling factors are defined with reference to the full-precision
-component.  In PlanarConfiguration 2, the JPEG codec will be working on
-only one component at a time and thus will have no reference component to
-downsample against.
-
-
-Minimum requirements for TIFF/JPEG
-----------------------------------
-
-ISO JPEG is a large and complex standard; most implementations support only
-a subset of it.  Here we define a "core" subset of TIFF/JPEG which readers
-must support to claim TIFF/JPEG compatibility.  For maximum
-cross-application compatibility, we recommend that writers confine
-themselves to this subset unless there is very good reason to do otherwise.
-
-Use the ISO baseline JPEG process: 8-bit data precision, Huffman coding,
-with no more than 2 DC and 2 AC Huffman tables.  Note that this implies
-BitsPerSample = 8 for each component.  We recommend deviating from baseline
-JPEG only if 12-bit data precision or lossless coding is required.
-
-Use no subsampling (all JPEG sampling factors = 1) for color spaces other
-than YCbCr.  (This is, in fact, required with the TIFF 6.0 field
-definitions, but may not be so in future revisions.)  For YCbCr, use one of
-the following choices:
-	YCbCrSubSampling field		JPEG sampling factors
-	1,1				1h1v, 1h1v, 1h1v
-	2,1				2h1v, 1h1v, 1h1v
-	2,2  (default value)		2h2v, 1h1v, 1h1v
-We recommend that RGB source data be converted to YCbCr for best compression
-results.  Other source data colorspaces should probably be left alone.
-Minimal readers need not support JPEG images with colorspaces other than
-YCbCr and grayscale (PhotometricInterpretation = 6 or 1).
-
-A minimal reader also need not support JPEG YCbCr images with nondefault
-values of YCbCrCoefficients or YCbCrPositioning, nor with values of
-ReferenceBlackWhite other than [0,255,128,255,128,255].  (These values
-correspond to the RGB<=>YCbCr conversion specified by JFIF, which is widely
-implemented in JPEG codecs.)
-
-Writers are reminded that a ReferenceBlackWhite field *must* be included
-when PhotometricInterpretation is YCbCr, because the default
-ReferenceBlackWhite values are inappropriate for YCbCr.
-
-If any subsampling is used, PlanarConfiguration=1 is preferred to avoid the
-possibly-confusing requirements of PlanarConfiguration=2.  In any case,
-readers are not required to support PlanarConfiguration=2.
-
-If possible, use a single interleaved scan in each image segment.  This is
-not legal JPEG if there are more than 4 SamplesPerPixel or if the sampling
-factors are such that more than 10 blocks would be needed per MCU; in that
-case, use a separate scan for each component.  (The recommended color
-spaces and sampling factors will not run into that restriction, so a
-minimal reader need not support more than one scan per segment.)
-
-To claim TIFF/JPEG compatibility, readers shall support multiple-strip TIFF
-files and the optional JPEGTables field; it is not acceptable to read only
-single-datastream files.  Support for tiled TIFF files is strongly
-recommended but not required.
-
-
-Other recommendations for implementors
---------------------------------------
-
-The TIFF tag Compression=7 guarantees only that the compressed data is
-represented as ISO JPEG datastreams.  Since JPEG is a large and evolving
-standard, readers should apply careful error checking to the JPEG markers
-to ensure that the compression process is within their capabilities.  In
-particular, to avoid being confused by future extensions to the JPEG
-standard, it is important to abort if unknown marker codes are seen.
-
-The point of requiring that all image segments use the same JPEG process is
-to ensure that a reader need check only one segment to determine whether it
-can handle the image.  For example, consider a TIFF reader that has access
-to fast but restricted JPEG hardware, as well as a slower, more general
-software implementation.  It is desirable to check only one image segment
-to find out whether the fast hardware can be used.  Thus, writers should
-try to ensure that all segments of an image look as much "alike" as
-possible: there should be no variation in scan layout, use of options such
-as DRI, etc.  Ideally, segments will be processed identically except
-perhaps for using different local quantization or entropy-coding tables.
-
-Writers should avoid including "noise" JPEG markers (COM and APPn markers).
-Standard TIFF fields provide a better way to transport any non-image data.
-Some JPEG codecs may change behavior if they see an APPn marker they
-think they understand; since the TIFF spec requires these markers to be
-ignored, this behavior is undesirable.
-
-It is possible to convert an interchange-JPEG file (e.g., a JFIF file) to
-TIFF simply by dropping the interchange datastream into a single strip.
-(However, designers are reminded that the TIFF spec discourages huge
-strips; splitting the image is somewhat more work but may give better
-results.)  Conversion from TIFF to interchange JPEG is more complex.  A
-strip-based TIFF/JPEG file can be converted fairly easily if all strips use
-identical JPEG tables and no RSTn markers: just delete the overhead markers
-and insert RSTn markers between strips.  Converting tiled images is harder,
-since the data will usually not be in the right order (unless the tiles are
-only one MCU high).  This can still be done losslessly, but it will require
-undoing and redoing the entropy coding so that the DC coefficient
-differences can be updated.
-
-There is no default value for JPEGTables: standard TIFF files must define all
-tables that they reference.  For some closed systems in which many files will
-have identical tables, it might make sense to define a default JPEGTables
-value to avoid actually storing the tables.  Or even better, invent a
-private field selecting one of N default JPEGTables settings, so as to allow
-for future expansion.  Either of these must be regarded as a private
-extension that will render the files unreadable by other applications.
-
-
-References
-----------
-
-[1] Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
-Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44.
-
-This is the best short technical introduction to the JPEG algorithms.
-It is a good overview but does not provide sufficiently detailed
-information to write an implementation.
-
-[2] Pennebaker, William B. and Mitchell, Joan L.  "JPEG Still Image Data
-Compression Standard", Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1.
-638pp.
-
-This textbook is by far the most complete exposition of JPEG in existence.
-It includes the full text of the ISO JPEG standards (DIS 10918-1 and draft
-DIS 10918-2).  No would-be JPEG implementor should be without it.
-
-[3] ISO/IEC IS 10918-1, "Digital Compression and Coding of Continuous-tone
-Still Images, Part 1: Requirements and guidelines", February 1994.
-ISO/IEC DIS 10918-2, "Digital Compression and Coding of Continuous-tone
-Still Images, Part 2: Compliance testing", final approval expected 1994.
-
-These are the official standards documents.  Note that the Pennebaker and
-Mitchell textbook is likely to be cheaper and more useful than the official
-standards.
-
-
-Changes to Section 21: YCbCr Images
-===================================
-
-[This section of the Tech Note clarifies section 21 to make clear the
-interpretation of image dimensions in a subsampled image.  Furthermore,
-the section is changed to allow the original image dimensions not to be
-multiples of the sampling factors.  This change is necessary to support use
-of JPEG compression on odd-size images.]
-
-Add the following paragraphs to the Section 21 introduction (p. 89),
-just after the paragraph beginning "When a Class Y image is subsampled":
-
-	In a subsampled image, it is understood that all TIFF image
-	dimensions are measured in terms of the highest-resolution
-	(luminance) component.  In particular, ImageWidth, ImageLength,
-	RowsPerStrip, TileWidth, TileLength, XResolution, and YResolution
-	are measured in luminance samples.
-
-	RowsPerStrip, TileWidth, and TileLength are constrained so that
-	there are an integral number of samples of each component in a
-	complete strip or tile.  However, ImageWidth/ImageLength are not
-	constrained.  If an odd-size image is to be converted to subsampled
-	format, the writer should pad the source data to a multiple of the
-	sampling factors by replication of the last column and/or row, then
-	downsample.  The number of luminance samples actually stored in the
-	file will be a multiple of the sampling factors.  Conversely,
-	readers must ignore any extra data (outside the specified image
-	dimensions) after upsampling.
-
-	When PlanarConfiguration=2, each strip or tile covers the same
-	image area despite subsampling; that is, the total number of strips
-	or tiles in the image is the same for each component.  Therefore
-	strips or tiles of the subsampled components contain fewer samples
-	than strips or tiles of the luminance component.
-
-	If there are extra samples per pixel (see field ExtraSamples),
-	these data channels have the same number of samples as the
-	luminance component.
-
-Rewrite the YCbCrSubSampling field description (pp 91-92) as follows
-(largely to eliminate possibly-misleading references to
-ImageWidth/ImageLength of the subsampled components):
-
-	(first paragraph unchanged)
-
-	The two elements of this field are defined as follows:
-
-	Short 0: ChromaSubsampleHoriz:
-
-	1 = there are equal numbers of luma and chroma samples horizontally.
-
-	2 = there are twice as many luma samples as chroma samples
-	horizontally.
-
-	4 = there are four times as many luma samples as chroma samples
-	horizontally.
-
-	Short 1: ChromaSubsampleVert:
-
-	1 = there are equal numbers of luma and chroma samples vertically.
-
-	2 = there are twice as many luma samples as chroma samples
-	vertically.
-
-	4 = there are four times as many luma samples as chroma samples
-	vertically.
-
-	ChromaSubsampleVert shall always be less than or equal to
-	ChromaSubsampleHoriz.  Note that Cb and Cr have the same sampling
-	ratios.
-
-	In a strip TIFF file, RowsPerStrip is required to be an integer
-	multiple of ChromaSubSampleVert (unless RowsPerStrip >=
-	ImageLength, in which case its exact value is unimportant).
-	If ImageWidth and ImageLength are not multiples of
-	ChromaSubsampleHoriz and ChromaSubsampleVert respectively, then the
-	source data shall be padded to the next integer multiple of these
-	values before downsampling.
-
-	In a tiled TIFF file, TileWidth must be an integer multiple of
-	ChromaSubsampleHoriz and TileLength must be an integer multiple of
-	ChromaSubsampleVert.  Padding will occur to tile boundaries.
-
-	The default values of this field are [ 2,2 ].  Thus, YCbCr data is
-	downsampled by default!
-</pre>