Tag Image File Format Rev 4.0

April 31, 1987

Tag Image File Format  Rev 4.0
April 31, 1987

This memorandum has been prepared jointly by Aldus and Microsoft in conjunction
with leading scanner and printer manufacturers.  This document does not
represent a commitment on the part of either Microsoft or Aldus to provide
support for this file format in any application.  When responding to specific
issues raised in this memo, or when requesting additional tag or field
assignments, please address your correspondence to either:

          Tim Davenport                       Manny Vellon
          Aldus Corporation                   Windows Marketing Group
          411 First Ave. South                Microsoft Corporation
          Suite 200                           16011 NE 36th Way
          Seattle, WA  98104                  Box 97017
                                              Redmond, WA  98073-9717

Revision Notes

This release of the TIFF specification has been given a Revision number.  It is
really the fourth major revision, so the Revision number was set to 4.0.

Abstract

This document describes TIFF, a tag based file format that is designed to
promote the interchange of digital image data.

The fields were defined primarily with desktop publishing and related
applications in mind, although it is conceivable that other sorts of imaging
applications may find TIFF to be useful.

The general scenario for which TIFF was invented assumes that applications
software for scanning or painting creates a TIFF file, which can then be read
and incorporated into a document or publication by an application such as a
desktop publishing package.

The intent of TIFF is to organize and codify existing practice with respect to
the definition and usage of desktop digital data, not to blaze new paths or
promote unproven techniques.  Yet a very high priority has been given to
structuring the data in such a way as to minimize the pain of future additions.
TIFF was designed to be a very extensible interchange format.

TIFF is not a printer language or page description language, nor is it intended
to be a general document interchange standard.  It may be useful as is for some
image editing applications, but is probably inappropriate for and would thus
need to be translated into some intermediate data structures by other image
editing applications.  The primary design goal was to provide a rich
environment within which the exchange of image data between application
programs can be accomplished.  This richness is required in order to take
advantage of the varying capabilities of scanners and similar devices.  TIFF is
therefore designed to be a superset of existing image file formats for desktop
scanners (and paint programs and anything else that produces images with pixels
in them) and will be enhanced on a continuing basis as new capabilities arise.

Although TIFF is claimed to be in some sense a rich format, it can easily be
used for simple scanners and applications as well, since the application
developer need only be concerned with the capabilities that he requires.  The
mechanisms for accomplishing this goal are discussed in the next section.

TIFF is intended to be independent of specific operating systems, filing
systems, compilers, and processors.  The only significant assumption is that
the storage medium supports something like a file, defined as a sequence of
8-bit bytes, where the bytes are numbered from 0 to N.  The largest possible
TIFF file is 2**32 bytes.  Since pointers (byte offsets) are used liberally, a
TIFF file is most easily read from a random access device, although it is
possible to read and write TIFF files on sequential media such as magnetic
tape.

The recommended MS-DOS file extension for TIFF files is .TIF.  The recommended
Macintosh filetype is TIFF.  Conventions in other computing environments have
not yet been established.

1) Conformance

Many of the application programs that read the contents of TIFF image files
will not support all of the features described in this document.  In some
cases, little more than the default options will be supported.  It is up to
each organization to determine the costs and benefits associated with different
levels of conformity.  Therefore, claims of conformity to this specification
should be interpreted with a certain amount of caution.

It follows that the usage of this specification does not preclude the need for
coordination between image file writers and image file readers.  It is up to
the application designer that initially writes a file in this format to verify
that the desired file options are supported by the applications that will read
the file.

2) Structure

In TIFF, individual fields are identified with a unique tag.  This allows
particular fields to be present or absent from the file as required by the
application.

Some TIFF files will have only a few fields in them; others will have many.
Software that creates TIFF files should write out as many fields as it believes
will be meaningful and useful (and no more).  Software that reads TIFF files
should do the best it can with the fields that it finds there.

See Appendix A: Tag Structure Rationale.

There are many ways in which a tag-oriented file format scheme can be
implemented.  TIFF uses the following approach:

There are three main parts to a TIFF file.  First is a short image file header.
Next is a directory of all the fields that are to be found in this file.
Finally, we have the data for the fields.

3) Header and Directory

A TIFF file begins with a small amount of positionally defined data, containing
the following information:

Bytes 0-1:

The first word of the file serves to specify the byte order used within the
file.  The currently defined values are:

     II        (hex 4949)
     MM        (hex 4D4D)

In the II format, byte order is always from least significant to most
significant, for both 16-bit and 32-bit integers.

In the MM format, byte order is always from most significant to least
significant, for both 16-bit and 32-bit integers.

In both formats, character strings are stored into sequential byte locations.

It is certainly not required that all applications software be able to handle
both formats.  It should be apparent which is the native format for a
particular machine.

Bytes 2-3:

The second word of the file is the TIFF version number.  This number shouldnt
change.  This document describes Version 42, so 42 (2A in hex) should be stored
in this word.

Bytes 4-7:

This long word contains the offset (in bytes) of the first Image File
Directory.  The directory may be at any location in the file after the header
but must begin on a word boundary.

(The term byte offset is always used in this document to refer to a location
with respect to the beginning of the file.  The first byte of the file has an
offset of 0.)

An IFD consists of a 2-byte count of the number of entries (i.e., the number of
fields), followed by a sequence of 12-byte field entries, followed by a 4-byte
offset of the next Image File Directory (or 0 if none).  Each 12-byte field
entry has the following format:

Bytes 0-1 contain the Tag for the field.  Bytes 2-3 contain the field Type.
Bytes 4-7 contain the Length (Count might have been a better term) of the
field.  Bytes 8-11 contain the file offset (in bytes) of the Value for the
field.  The Value is expected to begin on a word boundary; the corresponding
file offset will thus be an even number.

The entries in an IFD must be sorted in ascending order by Tag.  Note that this
is not the order in which the fields are described in this document.  The
Values to which directory entries point need not be in any particular order in
the file.

If the Value fits within 4 bytes, the Offset is interpreted to contain the
Value instead of pointing to the Value, to save a little time and space.  If
the Value is less than 4 bytes, it is left-justified.  Whether or not it fits
within 4 bytes can be determined by looking at the Type and Length of the
field.

The Length part is specified in terms of the data type.  A single 16-bit word
(SHORT) has a Length of 1, not 2, for example.  The data types and their
lengths are described below:

     1 = BYTE.      8-bit unsigned integer.
     2 = ASCII.     8-bit bytes that store ASCII codes; the last byte must
                     be null.
     3 = SHORT.     A 16-bit (2-byte) unsigned integer.
     4 = LONG.      A 32-bit (4-byte) unsigned integer.
     5 = RATIONAL.  Two LONGs: the first represents the numerator of a
                     fraction, the second the denominator.

The value of the Length part of an ASCII field entry includes the null.  If
padding is necessary, the Length does not include the pad byte.

The reader should check the type to ensure that it is what he expects.  TIFF
currently allows more than 1 valid type for a given field.  For example,
ImageWidth and ImageLength were specified as having type SHORT.  Very large
images with more than 64k rows or columns are possible with some devices even
now.  Rather than add parallel LONG tags for these fields, it is cleaner to
allow both SHORT and LONG for ImageWidth and similar fields.  Writers of TIFF
files are, however, encouraged to use the default type values as indicated in
this document to insure compatbility with existing TIFF reader applications.

Note that there may be more than one IFD.  Each IFD is said to define a
subfile.  One potential use of subsequent subfiles is to describe a sub-image
that is somehow related to the main image, such as a reduced resolution or
screen resolution image.  Another use is to represent multiple page images -
for example, a facsimile document requiring more than one page.  Subsequent
IFDs will in general contain many of the same fields as the first IFD but will
usually point to or contain different Values for those fields.

4) Definitions

The TIFF structure itself is not specific to imaging applications in any way.
It is only the definitions of the fields themselves that jointly describe an
image.  Before we begin describing the fields, a few image related definitions
may be useful.

An image is defined to be a rectangular array of pixels, each of which consists
of one or more samples.  With monochromatic data, we have one sample per pixel,
and sample and pixel can be used interchangeably.  Color data usually contains
three samples per pixel, as in, for example, an RGB scheme.

5) The Fields

The following fields are defined in this version of TIFF.  More will be added
in future versions, if possible in such a way so as not to break old software
that encounters a newer TIFF file.  An attempt has been made to group related
fields, although the grouping is necessarily somewhat arbitrary.

The documentation for each field contains the name of the field (quite
arbitrary, but convenient), the Tag value, the field Type, the Number of Values
(N) expected (per IFD, in the case of multiple subfiles), comments describing
the field, and the default, if any.  The default value is used if the field
does not exist.

A fairly large number of fields has already been defined, and the number will
grow.  Please keep in mind that many common images can be described using only
a handful of these fields (see the Examples section).

General Description

SubfileType
Tag  = 255  (FF)
Type = SHORT
N    = 1

A general indication of the kind of data that is contained in this subfile.
Currently defined values are:
1 = full resolution image data  ImageWidth, ImageLength, and StripOffsets are
required fields.
2 = reduced resolution image data ImageWidth, ImageLength, and StripOffsets
are required fields.  It is further assumed that a reduced resolution image is
a reduced version of the entire extent of the corresponding full resolution
data.
3 = Single page of a multi-page image (see the PageNumber tag description).

If your kind of image data doesnt fit nicely into either description, contact
either Aldus or Microsoft to define an additional value.  Note that both image
types can be found in a single TIFF file, with each subfile described by its
own IFD.
No default.

Data Architecture

ImageWidth
Tag  = 256  (100)
Type = SHORT
N    = 1

The images width, in pixels (X: horizontal).  The number of columns in the
image.
No default.

ImageLength
Tag  = 257  (101)
Type = SHORT
N    = 1

The images length (height) in pixels (Y: vertical).  The number of rows
(sometimes described as scan lines) in the image.  ImageLength and ImageWidth
refer only to how the pixels are stored in the file and do not imply anything
about where the visual top or left side of the image may be.  See Orientation
for this information.
No default.

RowsPerStrip
Tag  = 278  (116)
Type = SHORT or LONG
N    = 1

The number of rows per strip.  The image data is organized into strips for fast
access to individual rows when the data is compressed (though this field is
valid even if the data is not compressed).

Note that either SHORT or LONG values can be used to specify RowsPerStrip.
SHORT values may be used for small TIFF files.  It should be noted, however,
that earlier TIFF specifications required LONG values and that some software
may not expect SHORT values.

Default is 2**32 - 1, which is effectively infinity.  That is, the entire image
is one strip.

[StripsPerImage]
N    = 1

The number of strips per image.  This value is not a field, since it can be
computed from two other fields, but it is convenient to give it a name in order
to clarify the use of other fields.  The equation to use is StripsPerImage =
(ImageLength + RowsPerStrip - 1) / RowsPerStrip, assuming integer arithmetic.

StripOffsets
Tag  = 273  (111)
Type = SHORT or LONG
N    = StripsPerImage for PlanarConfiguration equal to 1.
     = SamplesPerPixel * StripsPerImage for PlanarConfiguration equal to 2

For each strip, the byte offset of that strip.  The offset is specified with
respect to the beginning of the TIFF file.  Note that this implies that each
strip has a location independent of the locations of other strips.  This
feature may be useful for certain editing applications.  This field is the only
way for a reader to find the image data, and hence must exist.

Note that either SHORT or LONG values can be used to specify the strip offsets.
SHORT values may be used for small TIFF files.  It should be noted, however,
that earlier TIFF specifications required LONG strip offsets and that some
software may not expect SHORT values.
No default.

StripByteCounts
Tag  = 279  (117)
Type = LONG
N    = StripsPerImage for PlanarConfiguration equal to 1.
     = SamplesPerPixel * StripsPerImage for PlanarConfiguration equal to 2

For each strip, the number of bytes in that strip.
No default.

SamplesPerPixel
Tag  = 277  (115)
Type = SHORT
N    = 1

The number of samples per pixel.  Usually 1 for monochromatic data and 3 for
color data (i.e. one sample for each of the color planes.)
Default = 1.

BitsPerSample
Tag  = 258  (102)
Type = SHORT
N    = SamplesPerPixel

Number of bits per sample.  Note that this tag allows a different number of
bits per sample for each sample corresponding to a pixel.  For example, RGB
color data could use a different number of bits per sample for each of the
three color planes.
Default = 1.

PlanarConfiguration
Tag  = 284  (11C)
Type = SHORT
N    = 1

1 = the sample values for each pixel are stored contiguously, so that there is
a single image plane.  See PhotometricInterpretation to determine the order of
the samples within the pixel data.
2 = the samples are stored in separate sample planes.  The values in
StripOffsets and StripByteCounts are then arranged as a 2-dimensional array,
with SamplesPerPixel rows and StripsPerImage columns.  (All of the columns for
row 0 are stored first, followed by the columns of row 1, and so on.)
PhotometricInterpretation describes the type of data that is stored in each
sample plane.

If SamplesPerPixel is 1, a PlanarConfiguration value of 1 is equivalent to a
value of 2.
No default.

Compression
Tag  = 259  (103)
Type = SHORT
N    = SamplesPerPixel for PlanarConfiguration equal to 1 or 2.

Note that a value is provided for each sample, allowing different compression
schemes to be applied to different planes of data.

1 = No compression, but pack data into bytes as tightly as possible, with no
unused bits except at the end of a row.  See also FillOrder.  The bytes are
stored as an array of type BYTE, for BitsPerSample <= 8, SHORT if BitsPerSample
> 8 and <= 16, and LONG if BitsPerSample > 16 and <= 32.  The byte ordering of
data >8 bits must be consistent with that specified in the TIFF file header
(bytes 0 and 1).  Intel format files will have the least significant bytes
preceeding the most significant bytes while Motorola format files will have the
opposite.

If the number of bits per sample is not a power of 2, and you are willing to
give up some space for better performance, you may wish to use the next higher
power of 2.  For example, if your data can be represented in 6 bits, you may
wish to specify that it is 8 bits deep.  If you take this approach, you should
be sure that MinSampleValue and MaxSampleValue are given correct values
(probably 0 and 63 for intrinsically 6-bit data.)  TIFF file readers should use
MinSampleValue and MaxSampleValue to determine the range of values in the data
rather than BitsPerSample.

Rows are required to begin on byte boundaries.

2 = CCITT Group 3 1-Dimensional Modified Huffman run length encoding.  See
Appendix B: Data Compression Scheme 2.  BitsPerSample must be 1, since this
type of compression is defined only for binary images.

3 = Facsimile-compatible CCITT Group 3, exactly as specified in Standardization
of Group 3 facsimile apparatus for document transmission, Recommendation T.4,
Volume VII, Fascicle VII.3, Terminal Equipment and Protocols for Telematic
Services, The International Telegraph and Telephone Consultative Committee
(CCITT), Geneva, 1985, pages 16 through 31.  Each strip must begin on a byte
boundary.  (But recall that an image can be a single strip.)  Rows that are not
the first row of a strip are not required to begin on a byte boundary.  The
data is stored as bytes, not words byte-reversal is not allowed.  Note that
the FillOrder field still applies.  See the Group3Options field for Group 3
options such as 1D vs 2D coding.

4 = Facsimile-compatible CCITT Group 4, exactly as specified in Facsimile
Coding Schemes and Coding Control Functions for Group 4 Facsimile Apparatus,
Recommendation T.6, Volume VII, Fascicle VII.3, Terminal Equipment and
Protocols for Telematic Services, The International Telegraph and Telephone
Consultative Committee (CCITT), Geneva, 1985, pages 40 through 48.  Each strip
must begin on a byte boundary.  Rows that are not the first row of a strip are
not required to begin on a byte boundary.  The data is stored as bytes, not
words.  Note that the FillOrder field still applies.  See the Group4Options
field for Group 4 options.

32771 = the same thing as Compression type 1 (no compression), except that each
row begins on the next available word boundary, instead of byte boundary.

32773 = PackBits compression, a relatively simple byte-oriented run-length
scheme.  See Appendix C.

Data compression only applies to pixel data, as pointed to by StripOffsets.
All other TIFF information is unaffected.

To be determined are additional compression schemes for gray and colored
images.  We encourage your suggestions, especially if accompanied by full
specifications and performance information.  It is of course desirable to
minimize the number of compression schemes that are being used, but this is
clearly an area in which extremely significant time and space tradeoffs exist.
Default = 1.

Group3Options
Tag  = 292  (124)
Type = LONG
N    = 1

This field is made up of a set of 32 flag bits.  Unused bits are expected to be
0.  Bit 0 is the low-order bit.  It is probably not safe to try to read the
file if any bit of this field is set that you dont know the meaning of.

Bit 0 is 1 for 2-dimensional coding (else 1-dimensional is assumed).  For 2-D
coding, if more than one strip is specified, each strip must begin with a
1-dimensionally coded line.  That is, RowsPerStrip should be a multiple of
Parameter K as documented in the CCITT specification.

Bit 1 is 1 if uncompressed mode is used.

Bit 2 is 1 if fill bits have been added as necessary before EOL codes such that
EOL always ends on a byte boundary, thus ensuring an eol-sequence of a 1 byte
preceded by a zero nibble: xxxx-0000 0000-0001.

Default is 0, for basic 1-dimensional coding.

Group4Options
Tag  =  293  (125)
Type = LONG
N    = 1

This field is made up of a set of 32 flag bits.  Unused bits are expected to be
0.  Bit 0 is the low-order bit.  It is probably not safe to try to read the
file if any bit of this field is set that you dont know the meaning of.  Gray
scale and color coding schemes are under study, and will be added when
finalized.

For 2-D coding, each strip is encoded as if it were a separate image.  In
particular, each strip begins on a byte boundary; and the coding for the first
row of a strip is encoded independently of the previous row, using horizontal
codes, as if the previous row is entirely white.  Each strip ends with the
24-bit end-of-facsimile block (EOFB).

Bit 0 is unused.

Bit 1 is 1 if uncompressed mode is used.

Default is 0, for basic 2-dimensional binary compression.

FillOrder
Tag  = 266  (10A)
Type = SHORT
N    = 1

The order of data values within a byte.
1 = most significant bits of the byte are filled first.  That is, data values
(or code words) are ordered from high order bit to low order bit within a byte.

2 = least significant bits are filled first.
Default is FillOrder = 1.

Threshholding
Tag  = 263  (107)
Type = SHORT
N    = 1

1 = a bilevel line art scan.  BitsPerSample must be 1.
2 = a halftone or dithered scan, usually of continuous tone data such as
photographs.  BitsPerSample must be 1.
3 = Error Diffused.
Default is Threshholding = 1.

CellWidth
Tag  = 264  (108)
Type = SHORT
N    = 1

The width, in 1-bit samples, of the dithering/halftoning matrix.  Assumes that
Threshholding = 2.  That is, this field is only relevant if Threshholding = 2.

No default.

CellLength
Tag  = 265  (109)
Type = SHORT
N    = 1

The length, in 1-bit samples, of the dithering/halftoning matrix.  Assumes that
Threshholding = 2.  This field and the previous field may be useful for
converting from halftoned to true gray level data.
No default.

Photometrics

These fields are useful in determining the visual meaning of the sample data.

MinSampleValue
Tag  = 280  (118)
Type = SHORT
N    = SamplesPerPixel

The minimum valid sample value.
Default is 0.

MaxSampleValue
Tag  = 281  (119)
Type = SHORT
N    = SamplesPerPixel

The maximum valid sample value.
Default is 2**(BitsPerSample) - 1.

PhotometricInterpretation
Tag  = 262  (106)
Type = SHORT
N    = 1

0 = MinSampleValue should be imaged as white.  MaxSampleValue should be imaged
as black.  If the bit-map represents gray scale, then the values between the
minimum and maximum sample values should be interpreted according to either the
gray scale response curve information (if included) or according to the result
of some more arbitrary rule.  See GrayResponseCurve.

1 = MinSampleValue should be imaged as black.  MaxSampleValue should be imaged
as white.  If the bit-map represents gray scale, then the values between the
minimum and maximum sample values should be interpreted according to either the
gray scale response curve information (if included) or according to the result
of some more arbitrary rule.

2 = RGB.  In the RGB model, a color is described as a combination of the three
primary colors of light (red, green, and blue) in particular concentrations.
For each of the three samples, MinSampleValue represents minimum intensity, and
MaxSampleValue represents maximum intensity.  For PlanarConfiguration = 1, the
samples are stored in the indicated order within a pixel: first Red, then
Green, then Blue.  For PlanarConfiguration = 2, the sample planes are stored in
the indicated order: first the Red sample plane, then the Green plane, then
the Blue plane.

The Red, Green and Blue intensity values are defined according to the NTSC
specifications for primary color chromaticity.  These specifications assume the
illumination to be CIE D6500.  See the Red, Green and Blue color response curve
tags.

Note: some compression schemes, such as the CCITT schemes, imply a particular
PhotometricInterpretation.  Therefore, when reading such data, TIFF readers
should ignore PhotometricInterpretation.  And, ideally, TIFF writers should not
write out the field when using one of these schemes.

No default.

GrayResponseUnit
Tag  = 290 (122)
Type = SHORT
N    = 1

1 = number represents tenths of a unit.
2 = number represents hundredths of a unit.
3 = number represents thousandths of a unit.
4 = number represents ten-thousandths of a unit.
5 = number represents hundred-thousandths of a unit.
Default is 2.

GrayResponseCurve
Tag  = 291 (123)
Type = SHORT
N    = 2**BitsPerSample

The purpose of the gray response curve and the gray units is to further provide
photometric interpretation information for gray scale image data.  The gray
response curve specifies for given levels of gray between the minimum and
maximum sample values the actual photometric gray level of the value.  It
represents this gray level in terms of optical density.

The GrayScaleResponseUnits specifies the accuracy of the information contained
in the curve.  Since optical density is specified in terms of fractional
numbers, this tag is necessary to know how to interpret the stored integer
information.  For example, if GrayScaleResponseUnits is set to 4
(ten-thousandths of a unit), and a GrayScaleResponseCurve number for gray level
4 is 3455, then the resulting actual value is 0.3455.

If the gray scale response curve is known for the data in the TIFF file, and if
the gray scale response of the output device is known, then an intelligent
conversion can be made between the input data and the output device.  For
example, the output can be made to look just like the input.  In addition, if
the input image lacks contrast (as can be seen from the response curve), then
appropriate contrast enhancements can be made.

The purpose of the grey scale response curve is to act as a lookup table
mapping values from 0 to 2**BitsPerSample-1 into specific intensity values.
Refer to the PhotometricInterpretation tag to determine how the mapping should
be done.

ColorResponseUnit
Tag  = 300 (12C)
Type = SHORT
N    = 1

1 = number represents tenths of a unit.
2 = number represents hundredths of a unit.
3 = number represents thousandths of a unit.
4 = number represents ten-thousandths of a unit.
5 = number represents hundred-thousandths of a unit.
Default is 2.

ColorResponseCurves
Tag  = 301 (12D)
Type = SHORT
N    = 2**BitsPerSample (for Red samples) +
                2**BitsPerSample (for Green samples) +
                2**BitsPerSample (for Blue samples)

This tag defines three color response curves (one each for Red, Green and Blue
color information).  The curves are stored sequentially (in red-green-blue
order).  The size of each table is 2**BitsPerSample, using the BitsPerSample
value corresponding to the respective color.  The ColorResponseUnit further
specifies how each entry in the table is to be interpreted.

The purpose of the color response curves is to act as a lookup table mapping
values from 0 to 2**BitsPerSample-1 into specific intensity values.  The
intensity values are as specified by the NTSC color strandard assuming
illumination to be CIE D6500.

Correspondence to the Physical World

XResolution
Tag  = 282  (11A)
Type = RATIONAL
N    = 1

The number of pixels per ResolutionUnit (see below) in the X direction, i.e.,
in the ImageWidth direction.  It is, of course, not mandatory that the image be
actually printed at the size implied by this parameter.  It is up to the
application to use this information as it wishes.

As is the case for many of these fields, XResolution may be invalid and
irrelevant for some images (e.g., images made with a hand-held digitizing
camera, which has a three-dimensional nature) and should therefore be absent
from the image file.
No default.

YResolution
Tag  = 283  (11B)
Type = RATIONAL
N    = 1

The number of pixels per ResolutionUnit in the Y direction, i.e., in the
ImageLength direction.
No default.

ResolutionUnit
Tag  = 296 (128)
Type = SHORT
N    = 1

To be used with XResolution and YResolution.
1 = no absolute unit of measurement.  Used for images that may have a
non-square aspect ratio, but no meaningful absolute dimensions.
2 = inch
3 = centimeter
Default is 2

Orientation
Tag  = 274 (112)
Type = SHORT
N    = 1

1 =  The 0th row represents the visual top of the image, and the 0th column
represents the visual left hand side.
2 =  The 0th row represents the visual top of the image, and the 0th column
represents the visual right hand side.
3 =  The 0th row represents the visual bottom of the image, and the 0th column
represents the visual right hand side.
4 =  The 0th row represents the visual bottom of the image, and the 0th column
represents the visual left hand side.
5 =  The 0th row represents the visual left hand side of the image, and the 0th
column represents the visual top.
6 =  The 0th row represents the visual right hand side of the image, and the
0th column represents the visual top.
7 =  The 0th row represents the visual right hand side of the image, and the
0th column represents the visual bottom.
8 =  The 0th row represents the visual left hand side of the image, and the 0th
column represents the visual bottom.
Default is 1.

Document Context

DocumentName
Tag  = 269  (10D)
Type = ASCII

The name of the document from which this image was scanned.
No default.

PageName
Tag  = 285  (11D)
Type = ASCII

The name of the page from which this image was scanned.
No default.

XPosition
Tag  = 286  (11E)
Type = RATIONAL

The X offset of the left side of the image, with respect to the left side of
the page, in inches.
No default.

YPosition
Tag  = 287  (11F)
Type = RATIONAL

The Y offset of the top of the image, with respect to the top of the page, in
inches.  In the TIFF coordinate scheme, the positive Y direction is down, so
that YPosition is always positive.
No default.

PageNumber
Tag  = 297  (129)
Type = SHORT
N    = 2

This tag is used to specify page numbers of a multiple page (e.g. facsimile)
document.  Two SHORT values are specified.  The first value is the page number;
the second value is the total number of pages in the document.

Note that pages need not appear in numerical order.

Miscellaneous Strings

ImageDescription
Tag  = 270 (10E)
Type = ASCII

Useful or interesting information about the image.
No default.

Make
Tag  = 271  (10F)
Type = ASCII

The name of the scanner manufacturer.
No default.

Model
Tag  = 272  (110)
Type = ASCII

The model name/number of the scanner.
No default.

Storage Management

These fields may be useful in certain dynamic editing situations.  Software
that merely reads TIFF files will probably not need to care about these fields.
And, of course, software that creates TIFF files is by no means required to
write these fields.

FreeOffsets
Tag  = 288  (120)
Type = LONG

For each free block in the file, its byte offset.
No default.

FreeByteCounts
Tag  = 289  (121)
Type = LONG

For each free block in the file, the number of bytes in the block.

6) Examples

A binary image from a paint program might contain only SubfileType, ImageWidth,
ImageLength, StripOffsets, and PhotometricInterpretation fields.

A typical line art scan might require that XResolution and YResolution be added
to the above list.

7) Private Fields

An organization may wish to store with the image file information that is
meaningful only to that organization.  Tags numbered 32768 or higher are
reserved for that purpose.  Upon request, the administrator will allocate and
register a block of private tags for an organization, to avoid possible
conflicts with other organizations.

Private enumerated values can be accommodated in a similar fashion.
Enumeration constants numbered 32768 or higher are reserved for private usage.
Upon request, the administrator will allocate and register a block of
enumerated values for a particular field, to avoid possible conflicts.

Tags and values which are allocated in the private number range are not
prohibited from being included in a future revision of this specification.
Several such instances can be found in this revision.

8) A List of Possible Future Enhancements

In the future TIFF will very likely be expanded to support more compression
schemes, more photometric schemes, color lookup tables, and non-rectangular
images.  Please refer all questions regarding enhancements to TIFF to the
contacts listed at the beginning of the document.  Written submissions should
be in Microsoft Windows Write format, to ensure timely and error-free
incorporation into the specification.

Tag Image File Format  Rev 4.0
April 31, 1987

Appendix A: Tag Structure Rationale

A file format is defined by both form (structure) and content.  The content
of TIFF consists of definitions of individual fields.  It is therefore the
content that we are ultimately interested in.  The structure merely tells us
how to find the fields.

Yet the structure deserves serious consideration for a number of reasons that
are not at all obvious at first glance.  Since the structure described herein
departs significantly from several other approaches, it may be useful to
discuss the rationale behind it.

The simplest, most straightforward structure for something like an image file
is a positional format.  In a positional scheme, the location of the data
defines what the data means.  For example, the field for number of rows might
begin at byte offset 30 in the image file.

This approach is simple and easy to implement and is perfect for static
environments.  But if a significant amount of ongoing change must be
accommodated, some subtle problems start showing up.  For example, suppose
that a field must be superseded by a new, more general field.  You could bump
a version number to flag the change.  Then new software has no problem doing
something sensible with old data, and all old software will reject the new
data, even software that didnt care about the old field.  This may seem like
no more than a minor annoyance at first glance, but causing old software to
break more often than it would really need to can be very costly and,
inevitably, causes much gnashing of teeth among customers.

Furthermore, it can be avoided.  One approach is to store a valid flag bit
for each field.  Now you dont have to bump the version number, as long as you
can put the new field somewhere that doesnt disturb any of the old fields.
Old software that didnt care about that old field anyway can continue to
function.  (Old software that did care will of course have to give up, but
this is an unavoidable price to be paid for the sake of progress, barring
total omniscience.)

Another problem that crops up frequently is that certain fields are likely to
make sense only if other fields have certain values.  This is not such a
serious problem in practice; it just makes things more confusing.
Nevertheless, we note that the valid flag bits described in the previous
paragraph can help to clarify the situation.

Field-dumping programs can be very helpful for diagnostic purposes.  A
desirable characteristic of such a program is that it doesnt have to know
much about what it is dumping.  In particular, it would be nice if the
program could dump ASCII data in ASCII format, integer data in integer
format, and so on, without having to teach the program about new fields all
the time.  So maybe we should add a data type component to our fields, plus
information on how long the field is, so that our dump program can walk
through the fields without knowing what the fields mean.

But note that if we add one more component to each field, namely a tag that
tells what the field means, we can dispense with the valid flag bits, and we
can also avoid wasting space on the non-valid fields in the file.  Simple
image creation applications can write out several fields and be done.

We have now derived the essentials of a tag based image file format.

Finally, a caveat.  A tag based scheme cannot guarantee painless growth.  But
is does provide a useful tool to assist in the process.

                      Table 1/T.4

                     Make-up codes

                   Terminating codes

White run        Code word       Black run       Code word
 length                                           length

     0           00110101           0            0000110111
     1           000111             1            010
     2           0111               2            11
     3           1000               3            10
     4           1011               4            011
     5           1100               5            0011
     6           1110               6            0010
     7           1111               7            00011
     8           10011              8            000101
     9           10100              9            000100
    10           00111             10            0000100
    11           01000             11            0000101
    12           001000            12            0000111
    13           000011            13            00000100
    14           110100            14            00000111
    15           110101            15            000011000
    16           101010            16            0000010111
    17           101011            17            0000011000
    18           0100111           18            0000001000
    19           0001100           19            00001100111
    20           0001000           20            00001101000
    21           0010111           21            00001101100
    22           0000011           22            00000110111
    23           0000100           23            00000101000
    24           0101000           24            00000010111
    25           0101011           25            00000011000
    26           0010011           26            000011001010
    27           0100100           27            000011001011
    28           0011000           28            000011001100
    29           00000010          29            000011001101
    30           00000011          30            000001101000
    31           00011010          31            000001101001
    32           00011011          32            000001101010
    33           00010010          33            000001101011
    34           00010011          34            000011010010
    35           00010100          35            000011010011
    36           00010101          36            000011010100
    37           00010110          37            000011010101
    38           00010111          38            000011010110
    39           00101000          39            000011010111
    40           00101001          40            000001101100
    41           00101010          41            000001101101
    42           00101011          42            000011011010
    43           00101100          43            000011011011
    44           00101101          44            000001010100
    45           00000100          45            000001010101
    46           00000101          46            000001010110
    47           00001010          47            000001010111
    48           00001011          48            000001100100
    49           01010010          49            000001100101
    50           01010011          50            000001010010
    51           01010100          51            000001010011
    52           01010101          52            000000100100
    53           00100100          53            000000110111
    54           00100101          54            000000111000
    55           01011000          55            000000100111
    56           01011001          56            000000101000
    57           01011010          57            000001011000
    58           01011011          58            000001011001
    59           01001010          59            000000101011
    60           01001011          60            000000101100
    61           00110010          61            000001011010
    62           00110011          62            000001100110
    63           00110100          63            000001100111

                      Table 2/T.4

                     Make-up codes

White run        Code word       Black run       Code word
 length                                           length

    64           11011             64            0000001111
   128           10010            128            000011001000
   192           010111           192            000011001001
   256           0110111          256            000001011011
   320           00110110         320            000000110011
   384           00110111         384            000000110100
   448           01100100         448            000000110101
   512           01100101         512            0000001101100
   576           01101000         576            0000001101101
   640           01100111         640            0000001001010
   704           011001100        704            0000001001011
   768           011001101        768            0000001001100
   832           011010010        832            0000001001101
   896           011010011        896            0000001110010
   960           011010100        960            0000001110011
  1024           011010101        1024           0000001110100
  1088           011010110        1088           0000001110101
  1152           011010111        1152           0000001110110
  1216           011011000        1216           0000001110111
  1280           011011001        1280           0000001010010
  1344           011011010        1344           0000001010011
  1408           011011011        1408           0000001010100
  1472           010011000        1472           0000001010101
  1536           010011001        1536           0000001011010
  1600           010011010        1600           0000001011011
  1664           011000           1664           0000001100100
  1728           010011011        1728           0000001100101
  EOL            000000000001     EOL            000000000001

Note  it is recognized that machines exist which accommodate larger paper
widths whilst maintaining the standard horizontal resolution.  This option
has been provided for by the addition of the Make-up code set defined as
follows:

                     Run length              Make-up codes
                  (black and white)

                        1792                  00000001000
                        1856                  00000001100
                        1920                  00000001101
                        1984                  000000010010
                        2048                  000000010011
                        2112                  000000010100
                        2176                  000000010101
                        2240                  000000010110
                        2304                  000000010111
                        2368                  000000011100
                        2432                  000000011101
                        2496                  000000011110
                        2560                  000000011111


Appendix B: Data Compression Scheme 2

Abstract

This document describes a method for compressing bilevel data that is based
on the CCITT Group 3 1D facsimile compression scheme.  It is intended that it
be used in conjunction with the Tag Image File Format.

References

1.  Standardization of Group 3 facsimile apparatus for document transmission,
Recommendation T.4, Volume VII, Fascicle VII.3, Terminal Equipment and
Protocols for Telematic Services, The International Telegraph and Telephone
Consultative Committee (CCITT), Geneva, 1985, pages 16 through 31.

2.  Facsimile Coding Schemes and Coding Control Functions for Group 4
Facsimile Apparatus, Recommendation T.6, Volume VII, Fascicle VII.3, Terminal
Equipment and Protocols for Telematic Services, The International Telegraph
and Telephone Consultative Committee (CCITT), Geneva, 1985, pages 40 through
48.

Relationship to the CCITT Specifications

The CCITT Group 3 and Group 4 specifications describe communications
protocols for a particular class of devices.  They are not by themselves
sufficient to describe a disk data format.  Fortunately, however, the CCITT
coding schemes can be readily adapted to this different environment.  The
following is one such adaptation.

Coding Scheme

A line (row) of data is composed of a series of variable length code words.
Each code word represents a run length of either all white or all black.
(Actually, more than one code word may be required to code a given run, in a
manner described below.)  White runs and black runs alternate.

In order to ensure that the receiver (decompressor) maintains color
synchronization, all data lines will begin with a white run length code word
set.  If the actual scan line begins with a black run, a white run length of
zero will be sent (written).  Black or white run lengths are defined by the
code words in Tables 1 and 2.  The code words are of two types: Terminating
code words and Make-up code words.  Each run length is represented by zero or
more Make-up code words followed by exactly one Terminating code word.

Run lengths in the range of 0 to 63 pels (pixels) are encoded with their
appropriate Terminating code word.  Note that there is a different list of
code words for black and white run lengths.

Run lengths in the range of 64 to 2623 (2560+63) pels are encoded first by
the Make-up code word representing the run length that is nearest to, not
longer than, that required.  This is then followed by the Terminating code
word representing the difference between the required run length and the run
length represented by the Make-up code.

Run lengths in the range of lengths longer than or equal to 2624 pels are
coded first by the Make-up code of 2560.  If the remaining part of the run
(after the first Make-up code of 2560) is 2560 pels or greater, additional
Make-up code(s) of 2560 are issued until the remaining part of the run
becomes less than 2560 pels.  Then the remaining part of the run is encoded
by Terminating code or by Make-up code plus Terminating code, according to
the range mentioned above.

It is considered an unrecoverable error if the sum of the run lengths for a
line do not equal the value of the ImageWidth field.

New rows always begin on the next available byte boundary.

No EOL code words are used.  No fill bits are used, except for the ignored
bits at the end of the last byte of a row.  RTC is not used.


Appendix C: Data Compression Scheme 32773 PackBits

Abstract

This document describes a compression scheme for paint type files.  It is
intended for use in conjunction with the Tag Image File Format.

1) Motivation

The current TIFF specification allows for two compression schemes.
Compression type 1 is really no compression, other than basic pixel packing.
Compression type 2, based on CCITT 1D compression, is powerful, but not
trivial to implement and is designed for scanned data more than data
generated by paint programs.  Simple byte-oriented run length schemes tend to
work well with paint data, since paint data often has large areas of white
space and areas filled with 8-bit patterns.

2) Description

Since several good schemes already exist, we may as well use one of them.  We
somewhat arbitrarily pick the Macintosh PackBits scheme.  It is byte
oriented, so there is no problem with word alignment.  And it has a good
worst case behavior (at most 1 extra byte for every 128 input bytes).  For
Macintosh users, there are toolbox utilities PackBits and UnPackBits that
will do the work for you, but it is easy to implement your own.

A pseudo code fragment to unpack it might look like this:

Loop until you get the number of unpacked bytes you are expecting:
    Read the next source byte into n.
    If n is between 0 and 127 inclusive, copy the next n+1 bytes literally.
    Else if n is between -127 and -1 inclusive, copy the next byte -n+1 times.
    Else if n is 128, noop.
Endloop

In the inverse routine, its best to encode a 2 byte repeat run as a replicate
run except when preceded and followed by a literal run, in which case its
best to merge the three into one literal run.  Always encode 3 byte repeats
as replicate runs.

So thats the algorithm.  Other rules:

Each row must be packed separately.  Do not compress across row boundaries.

The number of uncompressed bytes per row is defined to be (ImageWidth + 7) /
8.  If the uncompressed bitmap is required to have an even number of bytes
per row, decompress into word-aligned buffers.

If a run is larger than 128 bytes, simply encode the remainder of the run as
one or more additional replicate runs.

When PackBits data is uncompressed, the result should be interpreted as per
compression type 1 (no compression); i.e. the SamplesPerPixel, BitsPerSample
and PlanarConfiguration tags should be consulted to interpret the image.


Appendix D: Using the Microsoft Windows Clipboard

The Microsoft Windows Clipboard provides a mechanism that allows applications
to pass information to each other.  Pictures created in Microsoft Paint, for
example, may be passed as bitmaps to Microsoft Write.

In general, the Clipboard is not recommended as a way of passing TIFF
information between applications.  The amount of data associate with image
data can be very large.  Currently, data passed through the Microsoft Windows
Clipboard is limited to 64K bytes.  It is suggested that applications employ
file-based mechanisms to exchange TIFF data.  Aldus PageMaker, for example,
implements a File Place command to allow TIFF files to be imported.

For images requiring less than 64K bytes of TIFF data, a new Clipboard format
has been defined:

           CF_TIFF             Microsoft Tag Image File Format

(this symbol will be defined in the windows.h file distributed with the
Microsoft Windows Software Development Kit.)

The data associated with this format is a handle to a block of global memory
containing the same data as would be stored in a disk TIFF file.  When
interpreting this memory, TIFF readers should interpret file offsets as
offsets from the beginning of the memory block.

Applications that are capable of passing TIFF information through the
Microsoft Windows Clipboard should preferably not render the TIFF information
until requested to do so.  In addition to passingTIFF data, these
applications should also place bitmaps (Clipboard format CF_BITMAP) on the
Clipboard corresponding to the TIFF data.  Applications should judge whether
to render these bitmaps formatted for the display or for the currently
selected output device.  Placing a bitmap on the Clipboard will allow the
Clipboard viewer application (CLIPBRD.EXE) to display a likeness of the image
and will allow non-TIFF applications to import, at least, an approximate
bitmap.  These and other Clipboard techniques are described in the Microsoft
Windows Programming Guide, a document in the Microsoft Windows Software
Development Kit.


Appendix E: Numerical List of TIFF Tags

SubfileType
Tag     = 255  (FF)
Type    = SHORT
N       = 1

ImageWidth
Tag     = 256  (100)
Type    = SHORT
N       = 1

ImageLength
Tag     = 257  (101)
Type    = SHORT
N       = 1

BitsPerSample
Tag     = 258  (102)
Type    = SHORT
N       = SamplesPerPixel

Compression
Tag     = 259  (103)
Type    = SHORT
N       = SamplesPerPixel for PlanarConfiguration equal to 1 or 2.

PhotometricInterpretation
Tag     = 262  (106)
Type    = SHORT
N       = 1

Threshholding
Tag     = 263  (107)
Type    = SHORT
N       = 1

CellWidth
Tag     = 264  (108)
Type    = SHORT
N       = 1

CellLength
Tag     = 265  (109)
Type    = SHORT
N       = 1

FillOrder
Tag     = 266  (10A)
Type    = SHORT
N       = 1

DocumentName
Tag     = 269  (10D)
Type    = ASCII

ImageDescription
Tag     = 270 (10E)
Type    = ASCII

Make
Tag     = 271  (10F)
Type    = ASCII

Model
Tag     = 272  (110)
Type    = ASCII

StripOffsets
Tag     = 273  (111)
Type    = SHORT or LONG
N       = StripsPerImage for PlanarConfiguration equal to 1.
        = SamplesPerPixel * StripsPerImage for PlanarConfiguration equal to 2

Orientation
Tag     = 274 (112)
Type    = SHORT
N               = 1

SamplesPerPixel
Tag     = 277  (115)
Type    = SHORT
N       = 1

RowsPerStrip
Tag     = 278  (116)
Type    = SHORT or LONG
N       = 1

StripByteCounts
Tag     = 279  (117)
Type    = LONG
N       = StripsPerImage for PlanarConfiguration equal to 1.
        = SamplesPerPixel * StripsPerImage for PlanarConfiguration equal to 2

MinSampleValue
Tag     = 280  (118)
Type    = SHORT
N       = SamplesPerPixel

MaxSampleValue
Tag     = 281  (119)
Type    = SHORT
N       = SamplesPerPixel

XResolution
Tag     = 282  (11A)
Type    = RATIONAL
N       = 1

YResolution
Tag     = 283  (11B)
Type    = RATIONAL
N       = 1

PlanarConfiguration
Tag     = 284  (11C)
Type    = SHORT
N       = 1

PageName
Tag     = 285  (11D)
Type    = ASCII

XPosition
Tag     = 286  (11E)
Type    = RATIONAL

YPosition
Tag     = 287  (11F)
Type    = RATIONAL

FreeOffsets
Tag     = 288  (120)
Type    = LONG

FreeByteCounts
Tag     = 289  (121)
Type    = LONG

GrayResponseUnit
Tag     = 290 (122)
Type    = SHORT
N       = 1

GrayResponseCurve
Tag     = 291 (123)
Type    = SHORT
N       = 2**BitsPerSample

Group3Options
Tag     = 292  (124)
Type    = LONG
N       = 1

Group4Options
Tag     =  293  (125)
Type    = LONG
N       = 1

ResolutionUnit
Tag     = 296 (128)
Type    = SHORT
N       = 1

PageNumber
Tag     = 297  (129)
Type    = SHORT
N       = 2

ColorResponseUnit
Tag     = 300 (12C)
Type    = SHORT
N       = 1

ColorResponseCurves
Tag     = 301 (12D)
Type    = SHORT
N       = 2**BitsPerSample (for Red sample)+
                2**BitsPerSample (for Green sample)+
                2**BitsPerSample(for Blue sample)


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