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!
