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!