Network Working Group A. Katz Request for Comments: 1314 D. Cohen ISI April 1992
A File Format for the Exchange of Images in the Internet
Status of This Memo
This document specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document defines a standard file format for the exchange of fax-like black and white images within the Internet. It is a product of the Network Fax Working Group of the Internet Engineering Task Force (IETF).
The standard is:
** The file format should be TIFF-B with multi-page files supported. Images should be encoded as one TIFF strip per page.
** Images should be compressed using MMR when possible. Images may also be MH or MR compressed or uncompressed. If MH or MR compression is used, scan lines should be "byte-aligned".
** For maximum interoperability, image resolutions should either be 600, 400, or 300 dpi; or else be one of the standard Group 3 fax resolutions (98 or 196 dpi vertically and 204 dpi horizontally).
Note that this specification is self contained and an implementation should be possible without recourse to the TIFF references, and that only the specific TIFF documents cited are relevant to this specification. Updates to the TIFF documents do not change this specification.
Experimentation with this file format specified here is encouraged.
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1. Introduction
The purpose of this document is to define a standard file format for exchange of black and white images using the Internet. Since many organizations have already started to accumulate and exchange scanned documents it is important to reach agreement about an interchange file format in order to promote and facilitate the exchange and distribution of such documents. These images may originate from scanners, software, or facsimile (fax) machines. They may be manipulated by software, communicated, shared, duplicated, displayed, printed by laser printers, or faxed.
This file format provides for the uniform transfer of high quality images at a reasonable cost and with reasonable speed whether these files are generated by scanners, totally by software (e.g., text-to- fax, bitmap-to-fax, OCR, etc), or by fax. Also the intent of this document is to remain compatible with future moves to multi-level (i.e., gray-scale), higher resolution, or color images. The format proposed here is supported by both commercially available hardware and commercial and public domain software for most popular platforms in current use.
The file format for images is a totally separate issue from how such files are to be communicated. For example, FTP or SMTP could be used to move an image file from one host to another, although there are complications in the use of SMTP as currently implemented due to file size and the need to move binary data. (There is currently a proposal for removing these limitations from SMTP and in particular extending it to allow binary data. See reference [1].)
One major potential application of the communications format defined here is to allow images to be sent to fax machines using the Internet. It is intended that one or more separate companion documents will be formulated to address the issues of standardization in the areas of protocols for transmitting images through the Internet and the issues of addressing fax machines and routing faxes. Just as the exchange format is separate from the transmission mechanism, it is also separate from how hosts store images.
This document specifies a common exchange format; it does not require a host to store images in the format specified here, only to convert between the host's local image storage formats and the exchange format defined here for the purpose of exchanging images with other hosts across the network.
This standard specifies the use of TIFF (Tagged Image File Format, see below) as a format for exchange of image files. This is not a specific image encoding, but a framework for many encoding
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techniques, that can be used within the TIFF framework. For example, within TIFF it is possible to use MMR (the data encoding of CCITT Group 4 fax, see below), MH or MR (the data encodings of CCITT Group 3 fax), or other encoding methods.
Which encoding technique to use is not specified here. Instead, with time the encoding schemes used by most document providers will emerge as the de-facto standard. Therefore, we do not declare any as "the standard data encoding scheme," just as we do not declare that English is the standard publication language. (However, we expect that most document providers will use MMR in the immediate future because it offers much better compression ratios than MH or MR.)
Similarly, TIFF does not require that an image be communicated at a specific resolution. Resolution is a parameter in the TIFF descriptive header. We do suggest that images now be sent using one of a set of common resolutions in the interests of interoperability, but the format accommodates other resolutions that may be required by specialized applications or changing technologies.
Occasionally, image files will have to be converted, such as in the case where a document that was scanned at 400 dpi is to be printed on a 300 dpi printer. This conversion could be performed by the document provider, by the consumer, or by a third party. This document specifies neither who performs the conversion, nor which algorithms should be used to accomplish it.
Note that this standard does not attempt to define an exchange format for all image types that may be transmitted in the Internet. Nothing in this standard precludes it from being used for other image type such as gray-scale (e.g., JPEG) or color images but, for the purposes of standardization, the scope of this document is restricted to monochromatic bitmapped images.
The developers of this standard recognize that it may have a limited lifespan as Office Document Architecture (ODA) matures and comes into use in the Internet; ultimately the class of images covered by this standard will likely be subsumed by the more general class of images supported by the ODA standards. However, at present, there does not appear to be a sufficient installed base of ODA compliant software and the ODA standards are not fully mature. This standard is intended to fill the need for a common image transfer format until ODA is ready. Finally, we believe that it should be possible to automatically map images encoded in the format specified here into a future ODA-based image interchange format, thus providing a reasonable transition path to these future standards.
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2. Relationship to Fax
Transmission of facsimile (fax) images over phone lines is becoming increasingly widespread. The standard of most fax machines in the U.S. is CCITT Group 3 (G3), specified in Recommendations T.4 and T.30 [2] and in EIA Standards EIA-465 and EIA-466. G3 faxes are 204 dots per inch (dpi) horizontally and 98 dpi (196 dpi optionally, in fine-detail mode) vertically. Since G3 neither assumes error free transmission nor retransmits when errors occur, the encoding scheme used is differential only over small segments never exceeding 2 lines at standard resolution or 4 lines for fine-detail. (The incremental G3 encoding scheme is called two-dimensional and the number of lines so encoded is specified by a parameter called k.)
CCITT Group 4 fax (G4) is defined by the T.400 and T.500 series of Recommendations as well as Recommendation T.6 [2]. It provides for 400 dpi (both vertical and horizontal) and is a fully two-dimensional encoding scheme (k is infinite) called MMR (Modified Modified READ, where READ stands for: Relative Element Address Designate). G4 assumes an error free transmission medium (generally an X.25 Public Data Network, or PDN). Because of this, G4 is not in widespread use in the U.S. today.
The traditional fax bundles together four independent issues:
(1) Data presentation and compression; (2) Data transmission; (3) Image input from paper ("scanning"); and (4) Image output to paper ("printing").
This bundling supports, for example, the high quality CCITT Group 4 (G4) images (400x400 dpi) but only over X.25 public data networks with error correction, and similarly it supports the mid-quality CCITT Group 3 (204x98 and 204x196 dpi) but only over phone voice circuits (the Switched Telephone Network, or STN) without error correction. This bundling does not support the use of any other data transmission capabilities (e.g., FTP over LANs and WANs), nor asynchrony between the scanning and the printing, nor image storage, nor the use of the popular laser printers for output (even though they are perfectly capable of doing so).
In conventional fax, images are never stored. In today's computer network environment, a better model is:
(1) Images are scanned into files or created by software; (2) These image files are stored, manipulated, or communicated; (3) Images in a file are printed or displayed.
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The only feature of the CCITT fax that should be used is the encoding technique (preferably MMR, but with MR or MH allowed) which may be implemented with a variety of fax-oriented chips at low cost due to the popularity of fax.
"Sending a fax" means both encoding (and decoding) the fax images as well as transmitting the data. Since the Internet ALREADY provides several mechanisms for data transmission (in particular, FTP for general file transmission), it is unnecessary to use the data transmission methods specified in the CCITT standard. Within the Internet, each fax image should be stored in a file and these files could be transferred (e.g., using FTP, SMTP, RPC-based methods, etc.).
Fax machines should be considered just as scanners and printers are, as I/O devices between paper and files; but not as a transmission means. Higher quality Group 4 images are thus supported at low cost, while enjoying the freedom to use any computerized file transfer and duplication mechanism, standard laser printers, multiple printing (possibly at multiple remote sites) of the same image without having to rescan it physically, and a variety of software for various processing of these images, such as OCR and various drawing programs. We should be able to interoperate with files created by fax machines, scanners, or software and to be able to print all of them on fax machines or on laser printers.
The CCITT Recommendations assume realtime communications between fax machines and do not therefore specify any kind of fax file format. We propose using TIFF [3] which seems to be emerging as a standard, for encapsulation of encoded images. Because they assume realtime communications, the CCITT fax protocols require negotiations to take place between the sender and receiver. For example, they negotiate whether to use two-dimensional coding (and with what k parameter) and what (if any) padding there is between scan lines.
In our approach, the image in the file is already compressed in a particular manner. If it is to be sent to an ordinary fax machine using a fax board/modem, that board will perform the negotiations with the receiving fax machine. In the cases where the receiver cannot handle the type of compression used in the file, it will be necessary to convert the image to another compression scheme before transmission. (Most fax cards seem to either store images using the default values of the parameters which are negotiated or in a format which can quickly be converted to this. With currently available hardware and software, any necessary format conversion should be easy to accomplish.)
In conventional fax, if the compression used for a particular image
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is "negative" (i.e., the compressed form is larger than the uncompressed form, something that happens quite frequently with dithered photographic images), the larger compressed form of the image is still sent. If the images are first scanned into files, this problem could be recognized and the smaller, uncompressed file sent instead. (Also, Recommendations T.4 and T.6 [2] allow for an "uncompressed mode." Thus, lines which have negative compression may each be sent uncompressed. However, very few G3 fax machines support this mode.)
3. Image File Format
Image files should be in the TIFF-B format which is the bi-level subclass of TIFF. TIFF and TIFF-B are described in reference [3], cited at the end of this document. Images should be compressed using MMR (the G4 compression scheme) because it offers superior compression ratios. However, images may also be compressed using MH or MR (the G3 methods). MMR offers much better compression ratios than these (which are used in G3 fax because of the lack of an error-free communications path).
TIFF-F, described in [4], is the proposed subclass of TIFF-B for fax images. However, since TIFF-F was intended for use with G3, it recommends against certain features we recommend. Specifically, it suggests not using MMR or MR compression (we recommend MMR and allow MR) and prohibits uncompressed mode (which we allow and suggest for some photographic images). Apart from these, the TIFF-F restrictions should be followed. (Complete compatibility between the format specified here and TIFF-F can only be guaranteed for MH compressed images.)
[NOTE: Aldus Corp., the TIFF Developer, considers fax applications to be outside the scope of mainstream TIFF since it is not a part of general publishing which is what TIFF was originally designed for. They specify the LZW [5] compression scheme rather than MMR. We, however, are concerned with the transmission and storage of images rather than publishing. Therefore, we are more concerned with compression ratios and compatibility with CCITT fax than Aldus is.]
TIFF itself allows for gray-scale and color images. Image files should be restricted to TIFF-B for now because most of the currently available hardware is bi-level (1 bit per pixel). In the future, when gray-scale or color scanners, printers, and fax becomes available, the file format suggested here can already accommodate it. (For example, though JPEG is not currently a TIFF defined compression type, work is currently underway for including it as such.)
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[NOTE: In this document, we will use the term "reader" or "TIFF reader" to refer to the process or device which reads and parses a TIFF file.]
3.A. TIFF File Format
Figure 1 below (reproduced here from Figure 1 of reference [3]) depicts the structure of a TIFF file.
TIFF files start with a file header which specifies the byte order used in the file (i.e., Big or Little Endian), the TIFF version number, and points to the first "Image File Directory" (IFD). If the first two bytes are hex 4D4D, the byte order is from most to least significant for both 16 and 32 bit integers (Big Endian). If the first two bytes are hex 4949, the byte order is from least to most significant (Little Endian). In both formats, character strings are stored into sequential bytes and are null terminated.
The next two bytes (called the TIFF Version) must be 42 (hex 002A). This does not refer to the current TIFF revision number. The following 4 bytes contain the offset (in bytes from the beginning of the file) to the first IFD.
An IFD contains a 2 byte count of the number of entries in the IFD, a sequence of 12 byte directory entries, and a 4 byte pointer to the next IFD. One of these fields (StripOffsets) points to (parts of) an image in the file. There may be more than one image in the file (e.g., a "multi-page" TIFF file) and therefore more then one IFD. IFD field entries may appear in any order.
Each directory entry is 12 bytes and consists of a tag, its type, a length, and an offset to its value. If the value can fit into 4 bytes (i.e., if the type is BYTE, SHORT, or LONG), the actual value rather than an offset is given. If the value is less than 4 bytes (i.e., if the type is BYTE or SHORT), it is left-justified within the 4 byte value offset. More details about directory entries and the possible tags will be given in Section 3.C.
All pointers (called offsets in the TIFF reference [3]) are the number of bytes from the beginning of the file and are 4 bytes long. The first byte of the file has an offset of 0. In the case of only one image per file, there should therefore be only one IFD. The last IFD's pointer to the next IFD is set to hex 00000000 (32 bits).
The entries in an IFD must be sorted in ascending order by Tag.
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Header +--------+--------+ Directory Entry 0 | | | Byte Order +--------+--------+ +--------+--------| X | | | Tag 2 | | | Version(42) +--------+--------| +--------+--------| X+2 | | | Type 4 | | | Offset of +--------+--------| +- - A - -+ 0th IFD X+4 | | | 6 | | | +- -+ Length +--------+--------+ | | | | +--------+--------+ | X+8 | | | Value | +- - Y - -+ or V | | | Value +--------+--------+ Offset IFD +--------+--------+ | A | - B - | Entry Count | +--------+--------| | | | | V A+2 Entry 0 | | | +--------+--------+ +--------+--------+ | | | | | | Y Value A+14 Entry 1 | | | | | | +--------+--------| +--------+--------+ | | | A+26 Entry 2 | | | +--------+--------+ | | | . . | | | . +--------+--------+ | | | Entry B-1 | | | +--------+--------+ | | | Offset of A+2+B*12 - C - + Next IFD | | | +--------+--------+ | V (next IFD)
Figure 1: The Structure of a TIFF File
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3.B. Image Format and Encoding Issues
Images in TIFF files are organized as horizontal strips for fast access to individual rows. One can specify how many rows there are in each strip and all of the strips are the same size (except possibly the last one). Each strip must begin on a byte boundary but successive rows are not so required. For two-dimensional G3 compression (MR), each strip must begin with an "absolute" one- dimensional line. For MMR (G4) compression, each strip must be encoded as if it were a separate image.
For a variety of reasons, each page must be a single strip (e.g., not broken up into multiple strips).
One problem with multiple strips per page is that images which come from G4 fax machines as well as most scanned images will be generated as a single strip per page. These would have to be decoded and re- encoded as multiple strips (remember that for MMR compression, each strip must be start with a one-dimensionally encoded line).
Another problem with multiple strips per page arises in MR compression. Here, there MAY be at most k-1 two-dimensionally encoded lines following a one-dimensionally encoded line, but this is not required. It is possible to have one-dimensional lines more frequently than every k lines. However, since each strip (except possibly the last one) is required to be the same size, it may be necessary to re-encode the image to insure that each strip starts with a one-dimensional line. This is not a problem if each page is a single strip.
[NOTE: The TIFF document [3] suggests using strips which are about 8K bytes long. However, TIFF-F [4] recommends that each page be a single strip regardless of its size. The format specified in this document follows the TIFF-F recommendation.]
Also, as TIFF-F recommends, all G3 encoded images (MH and MR) should be "byte-aligned." This means that extra zero bits (fill bits) are added before each EOL (end-of-line) so that every line starts on a byte boundary.
In addition, as in the TIFF-F specification, the RTC (Return to Control signal which consists of 6 continuous EOL's) of G3 shall not be included at the end of G3 encoded documents. RTC is to be considered part of the G3 transmission protocol and not part of the encoding. Most, if not all, G3 fax modems attach RTC to outgoing images and remove it from incoming ones.
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For MMR (G4) encoded files, readers should be able to read images with only one EOFB (End Of Facsimile Block) at the end of the page and should not assume that Facsimile Blocks are of any particular size. (It has been reported that some MMR readers assume that all Facsimile Blocks are the maximum size.)
Systems may optionally choose to store the entire image uncompressed if the compression increases the size of the image file. Also, uncompressed mode (specified in Group3Options or Group4Options, see below) allows portions of the image to be uncompressed.
The multi-page capability of TIFF is supported and should be used for multi-page documents. TIFF files which have multiple pages have an IFD for each page of the document each of which describes and points to a single page image. (Note: though the current TIFF specification does not specifically prohibit having a single IFD point to an image which is actually multiple pages, with one strip for each page, most if not all TIFF readers would probably not be able to read such a file. Therefore, this should not be done.)
[A NOTE ON TIFF AND MULTI-PAGE DOCUMENTS:
Since most publications (e.g., reports, books, and magazine articles) are composed of more than a single page, multi-page TIFF files should be used where appropriate. However, many current TIFF implementations now only handle single-page files.
It is hoped that in the future, more TIFF implementations will handle multi-page files correctly. In the meantime, it would be useful to develop a utility program which could join several single-page TIFF files into a single multi-page file and also separate a multi-page TIFF file into several single page files.
For example, the utility could take a single TIFF file with N pages, called doc.tif, and create the files doc.000, doc.001, doc.002, ..., doc.N. doc.000 would be an ASCII listing of the files created. This naming scheme is compatible with that used by the image systems we have seen which only handle single page files.
In going the other way, the N+1 single page files could be combined into a single multi-page TIFF file. In this case, if the file doc.000 exists but contains information contrary to what is found in looking for the files doc.001, doc.002, ..., the program would notify the user.]
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3.C. TIFF Fields
TIFF is tag or field based. The various fields and their format are listed in [3]. There are Basic Fields (common to all TIFF files), Informational Fields (which provide useful information to a user), Facsimile Fields (used here), and Private Fields.
Each directory entry contains:
The Tag for the field (2 bytes)
The field Type (2 bytes)
The field Length (4 bytes) (This is in terms of the data type, not in bytes. For example, a single 16-bit word or SHORT has a Length of 1, not 2)
The Value Offset (4 bytes) (Pointer to the actual value, which must begin on a word boundary. Therefore, this offset will always be an even number. If the Value fits into 4 bytes, the Value Offset contains the Value instead. If the Value takes less than 4 bytes, it is left justified)
The allowed types and their codes are:
1 = BYTE 8-bit unsigned integer (1 byte)
2 = ASCII 8-bit ASCII terminated with a null (variable length)
3 = SHORT 16-bit unsigned integer (2 bytes)
4 = LONG 32-bit unsigned integer (4 bytes)
5 = RATIONAL Two LONGs (64 bits) representing the numerator and denominator of a fraction. In this document, RATIONAL's will be written as numerator/denominator. (8 bytes)
For ASCII, the Length specifies the number of characters and includes the null. It does not, however, include padding if such is necessary.
(Note that ASCII strings of length 3 or less may be stored in the Value Offset field instead of being pointed to.)
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The following fields should be used in a TIFF image file. Only the Basic Fields are mandatory; the others are optional (except that for MH and MR encoded files, the Group3Options Facsimile Field is mandatory). The optional fields have default values which are given in the TIFF specification. (Note that the TIFF reference [3] recommends not relying on the default values.)
Some fields contain one or more flag bits all stored as one value. In these cases, the bit labeled 0 is the least significant bit (i.e., Little Endian order). Where there is more than one suggested value for a tag, the possible values are separated by |.
Note that some fields (such as ImageLength or ImageWidth) can be of more than one type.
It would be useful to develop a TIFF viewer and editor which would allow one to read, add, and edit the fields in a TIFF file. Such an editor would display fields in sorted order and force the inclusion of all mandatory fields. Also, resolution and position should always be displayed or specified together with their units.
3.C.1. Basic Fields (Mandatory)
Basic Fields are those which are fundamental to the pixel architecture or visual characteristics of an image. The following Basic Fields should be included in a TIFF image file:
FIELD NAME (TAG in hex, TYPE) VALUE DESCRIPTION ------------------ ----- -----------
BitsPerSample 1 Number of bits (0102, SHORT) per pixel (bi-level for now, but may allow more later)
Compression 4 Type of Compression (0103, SHORT) (could also be 1 = Uncompressed 1 or 3) 3 = G3 (MH or MR) 4 = G4 (MMR) Use 4 if possible
ImageLength <image's length> Length of the Image (0101, SHORT in scan lines or LONG)
ImageWidth <image's width> Width of the Image (0100, SHORT in pixels
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or LONG)
NewSubFileType 0 usually Flag bits indicating (00FE, LONG) bit 0: 1 if the kind of image. reduced (see the TIFF resolution of reference [3]) another image bit 1: 1 if single page of a multi-page image bit 2: 1 if image defines a transparency mask
Photometric- 0 for positive Interpretation image (0 imaged (0106, SHORT) as white, 1 as black) 1 means reverse black and white
RowsPerStrip <Number of Rows> Number of Rows in (0116, SHORT Each Strip. Each or LONG) page should be a single strip.
SamplesPerPixel 1 (since are Bi-level (0115, SHORT) images)
StripByteCounts count1, count2... Number of Bytes in (0117, SHORTs each strip of the or LONGs) images. (The Value is an offset which points to a series of counts, each of which is the same Type, LONG or SHORT. The Length is the same as the number of strips.)
StripOffsets off1, off2,... Pointers to the strips (0111, SHORTs of the image (remember, or LONGs) one strip per page). (The Value is an offset which points to a series of offsets,
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each of which points to the actual image data for the strip.)
ResolutionUnit 2 | 3 Units of Resolution (0128, SHORT) See Below, 3.C.6 2: Inches 3: Centimeters
XResolution See Below, 3.C.6 Resolution in the X (011A, RATIONAL) direction in pixels per ResolutionUnit (we suggest 400 dots per inch when possible)
YResolution See Below, 3.C.6 Resolution in the Y (011B, RATIONAL) direction in pixels per ResolutionUnit (we suggest 400 dots per inch when possible)
3.C.2. Informational Fields (Optional)
The following Informational Fields are optional. They provide useful information to a user. All Field values are ASCII strings.
NAME (TAG in hex) DESCRIPTION ---------------- -----------
Artist (013B) Person Who Created the Image
DateTime (0132) Date and Time of Image Creation
HostComputer (013C) Name of Computer Image was Created On
ImageDescription A Short Text Description (010E)
Make (010F) Manufacturer of Hardware (Scanner) Used
Model (0110) Model Number of Hardware (Scanner) Used
Software (0131) Software Package that Created the Image
3.C.3. Facsimile Fields (Optional, Mandatory for G3 Compression)
In addition to the above, the Facsimile Fields below should be used. The TIFF document recommends that they not be used for interchange between applications, but they are now in wide enough
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use for just that. These fields are optional and default to 0 (all bits off).
FIELD NAME (TAG in hex, TYPE) VALUE DESCRIPTION ------------------ ----- -----------
Group3Options bit 0: 1 for Flag bits indicating (0124, LONG) 2-dimensional Options for G3 coding (i.e., MR with k > 1) bit 1: 1 if uncompressed mode MAY be used, 0 if uncompressed mode IS NOT used. bit 2: 1 if fill (As allowed by the G3 bits have been protocol, fill bits added may be added between each line of data and the EOL. Since fill bits are used to "byte-align" G3 image files, bit 2 should be set to 1 for these images.)
Group4Options bit 0: unused Flag bits indicating (0125, LONG) bit 1: 1 if Options for G4 uncompressed mode MAY be used, if this bit is 0 it means that uncompressed mode IS NOT used.
3.C.4. Storage and Retrieval Fields (Optional)
The following fields are optional and may be useful for document storage and retrieval.
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FIELD NAME (TAG in hex, TYPE) DESCRIPTION ------------------ -----------
DocumentName Name of the Document (010D, ASCII)
PageName Name of the Page (011D, ASCII)
PageNumber Page Number in a Multi-Page Document (0129, SHORTs) Two SHORT Values are specified, the first is the page number and the second is the total number of pages in the document. The first page is page 0. (NOTE: This does not necessarily correspond to page numbers which may be printed in the image.)
XPosition X Offset of the Left Side of (011E, RATIONAL) the Image, in ResolutionUnits
YPosition Y Offset of the Top of (011F, RATIONAL) the Image, in ResolutionUnits
3.C.5. TIFF-F Fields (NOT Recommended)
TIFF-F defines the following new fields for G3 (MH) encoded images. Since these fields are not defined in TIFF-B itself, their use is not recommended. However, since TIFF-F files may include these tags for image data which came from a G3 fax machine, readers should be prepared for them.
These three fields deal with corrupted image data which is due to the fact that G3 devices may not perform error correction on bad data.
FIELD NAME (TAG in hex, TYPE) DESCRIPTION ------------------ -----------
BadFaxLines Number of Bad fax scan lines (0146, SHORT or LONG) encountered during fax reception (but not necessarily in the file)
CleanFaxData 0 means no bad lines received (0147, SHORT) 1 means bad lines were regenerated
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by the receiving device 2 means bad lines were detected but not regenerated
ConsecutiveBadFaxLines The maximum number of consecutive (0148, SHORT or LONG) bad fax lines (but not necessarily in the file)
3.C.6. More on Representing Resolutions
The tags XResolution and YResolution are both RATIONALs, i.e., the ratio of two LONGS. G3 fax resolutions are actually specified in dots (or lines) per mm while G4 is in dots per inch (actually, dots per 25.4 mm).
For example, G3 horizontal resolution is defined to be 1728 dots per 215 mm which comes out to 80.4 dots per cm or about 203 dots per inch. It is frequently referred to as just 200 dpi. To avoid any possibility of problems due to round off error, this should be represented by having XResolution = 17280/215 and ResolutionUnit = 3 (cm). However when reading, 204/1 or even 200/1 with ResolutionUnit = 2 (inches) should be recognized as representing the same resolution.
For G4, on the other hand, the resolution 400 dots/inch should be represented by an XResolution of 400/1 and ResolutionUnit = 2.
The following table shows various ways of representing the standard resolutions in order of preference:
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G4 200 dpi 2 200/1 200/1
G4 300 dpi 2 300/1 300/1
Other 300 dpi 2 300/1 300/1
G4 400 dpi 2 400/1 400/1
600 dpi 2 600/1 600/1
It is suggested that Image readers be able to handle all of the above representations.
4. A Sample TIFF Image File
Below is a sample of what might be in a TIFF file for an MMR (G4) encoded single image which is about 100K bytes compressed at 400 dpi. A generic outline is given first, followed by a more detailed hex listing.
4.A. Sample File
Comments are to the right and are preceded by a semicolon. Note that tags must be sorted in order of the tag codes.
0:, IFDADDR:, and STRIP0: are addresses within the file and denote the number of bytes from the beginning of the file.
Header:
0: Byte Order= hex 4D4D ;first bytes of the file, from ;most significant bit to least ;significant (big endian) Version= 42 (hex 002A) ;Must be 42 First IFD= IFDADDR ;Address of first (and only) IFD
Image File Directory (the only one in this example):
IFDADDR:
IFD Entry Count= 24 ;(NOT A TAG) Count of ; Number of IFD Entries
NewSubFileType= 0 ImageWidth= 3400 ;8.5 inches at 400 dpi ImageLength= 4400 ;11 inches at 400 dpi BitsPerSample= 1 ;Bi-Level
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Compression= 4 ;MMR Photometric- Interpretation= 0 DocumentName= "LAMap1" ImageDescription= "A map of Los Angeles" Make= "Fujitsu" Model= "M3093E" StripOffsets= <STRIP0> ;There is only one strip in ;this example. However, note ;that strips can be in any ;order. (Offsets are from the ;beginning of the TIFF file.)
SamplesPerPixel= 1 ;Bi-Level RowsPerStrip= 4400 ;Entire image in 1 strip StripByteCounts= <COUNT0> ;Byte count of entire ;compressed image
XResolution= 400/1 YResolution= 400/1 XPosition= 0/1 ;position of left side of image YPosition= 0/1 ;position of top of image Group4Options= hex 00000002 ;bit 1 on means uncompressed ;mode MAY be used
Next IFD Pointer= hex 00000000 ;(NOT A TAG) Indicates no ; more IFDs in this file
Image Data:
<STRIP0>: <actual compressed image data>
[end of TIFF file]
In this example there is only one strip. Note that if there were more than one, the TIFF specification does not require them to be in any particular order. Strips may be given in any order and TIFF readers must use the StripOffsets to locate them.
Also, the TIFF document recommends not relying on the default values of the tags.
Katz & Cohen [Page 19] RFC 1314 Image Exchange Format April 1992
4.B. Detailed Hex Listing
All offsets and values are represented by hex except for ASCII strings which are double quoted. Remember that Value Offsets must always be an even number since the value it points to must always be on a 16-bit word boundary.
Entries in the Name column are for reference and are not actually a part of the TIFF file.
Offset Name Value ---- ------------------- ------------------------------------- Header (first byte is Offset 0): 0000 Byte Order 4D4D 0002 Version 002A 0004 1st. IFD pointer 00000010
Fields Offsets Point to: 0136 DocumentName "LAMap1" 013E ImageDescription "A map of Los Angeles"
Katz & Cohen [Page 20] RFC 1314 Image Exchange Format April 1992
0154 Make "Fujitsu" 015C Model "M3093E" 0164 X,Y Resolution 00000190 00000001 016C X,Y Position 00000000 00000001 0174 Software "Xionics" 017C DateTime "1990:10:05 15:00:00" 0190 Artist "Joe Pro" 0198 HostComputer "Tardis.Isi.Edu"
Image Data (<STRIP0> from above is here 01A8) 01A8 Compressed Data for single strip, of length <COUNT0> bytes
[end of TIFF file]
NOTE: Since in this example there is only a single strip, there is only one count for StripByteCounts and one offset for StripOffsets. Thus, each of these only takes 4 bytes and will fit in the Value Offset instead of being pointed to.
5. Conclusions
Bitmapped images transferred within the Internet should be in the following format:
1. The file format should be TIFF-B with multi-page files supported. Images should be encoded as one TIFF strip per page.
2. Images should be compressed using MMR when possible. Images may also be MH or MR compressed or uncompressed. If MH or MR compression is used, scan lines should be "byte-aligned".
3. For maximum interoperability, image resolutions should either be 600, 400, or 300 dpi; or else be one of the standard Group 3 fax resolutions (98 or 196 dpi vertically and 204 dpi horizontally).
Note that this specification is self contained and an implementation should be possible without recourse to the TIFF references, and that only the specific TIFF documents cited are relevant to this specification. Updates to the TIFF documents do not change this specification.
Existing commercial off-the-shelf products are available which can handle images in the above format. ISI would be delighted to help those interested in assembling a system.
Katz & Cohen [Page 21] RFC 1314 Image Exchange Format April 1992
6. Acknowledgments
Many contributions to this work were made by members of the IETF Network Fax Working Group especially by its chairman, Mark Needleman and by Clifford Lynch of the University of California Office of the President, Library Automation. Also, Kiyo Inaba of Ricoh Co. Ltd. made a number of helpful suggestions.
7. References
[1] Borenstein, N., and N. Freed, "Mechanisms for Specifying and Describing the Format of Internet Message Bodies", RFC in preparation.
[2] International Telegraph and Telephone Consultative Committee (CCITT), Red Book, October, 1984.
[3] Aldus Corp., Microsoft Corp., "Tag Image File Format Specification", Revision 5.0, Final, 1988.
[4] Cygnet Corporation, "The Spirit of TIFF Class F, 1990", available from Cygnet Technologies, 2560 9th., Suite 220, Berkeley, CA 94710, FAX: (415) 540-5835.
[5] Welch, T., "A Technique for High Performance Data Compression", IEEE Computer, Vol. 17, No. 6, Page 8, June 1984.
8. Security Considerations
While security issues are not directly addressed by this document, it is important to note that the file format described in this document is intended for the communications of files between systems and across networks. Thus the same precautions and cares should be applied to these files as would be to any files received from remote and possibly unknown systems.
Katz & Cohen [Page 22] RFC 1314 Image Exchange Format April 1992
9. Authors' Addresses
Alan Katz USC Information Sciences Institute 4676 Admiralty Way #1100 Marina Del Rey, CA 90292-6695
