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+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
+* 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
+* 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.
+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.
+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:
+ DAC (not to appear unless arithmetic coding is used)
+ 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.
+ Tag = 347 (15B.H)
+ 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
+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:
+ DAC (not to appear unless arithmetic coding is used)
+ 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
+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.
+[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.
+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
+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!