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TIFF CLASS F March 1, 1992 TIFF Class F was defined in late 1989 by Joe Campbell of Everex Systems, Inc. from the results of a poll of the facsimile industry. The goal was to define a file format that is simultaneously suitable for native use in Group 3 computer facsimile products, and as a file interchange medium with the outside world. Since that time, there have been only three minor revisions, mostly editorial in nature. Those wishing to participate in the revision and upkeep of TIFF Class F should read the section "Revising the TIFF Class F Specification," at the end of this document.The revision history of the specification is at the end of this document. TIFF Class F defines a subset (a "Class") of existing TIFF tags, necessary to support Group 3 facsimile data. In many cases, the values and sizes of these tags are also defined. Three new optional tags are also defined. TIFF Classes reduce the information burden on TIFF readers and writers that wish to support narrow applications. For example, Appendix G-1 of TIFF 5.0 states that classes enable TIFF readers "to know when they can stop adding TIFF features." In other words, defining a Class enables applications interested only in reading that Class to give up if the characteristic tags and values are not present. Therefore, TIFF Class F insists on a rather narrow definition of tags. In a general TIFF file, for example, the writer would be free to create single-page documents without the NewSubFileType and PageNumber tags. Not so for a Class F file, where the multi-page tag is required even for a single page. TIFF Class B (Bilevel) is a sub-class of TIFF. That is, all tags that are required in TIFF are also required in Class B. TIFF Class F (Facsimile) is a sub-class of Class B (Bilevel). That is, all tags that are required in Class B are also required in Class F. For some common tags, however, Class F limits the range of acceptable values. The YResolution tag, for example, is a Class B tag, but its Class F value is limited to either 98 or 196 dpi. Such tags are listed in "Required Class F Tags." Other Class B tags have a slightly eccentric meaning when applied to facsimile images. These are discussed in the section "Bilevel Required." There are also tags that may be helpful but are not required. These are listed in the "Recommended Tags" section. A brief list of all the tags required by TIFF Class F, grouped by class, is in the section "Required Facsimile Tags Grouped By Class." Finally, technical topics are discussed in the sections "Technical Points" and "Warnings." REFERENCES A machine-readable copy of this document can be downloaded from the Aldus Forum on Compuserve. Type GO ALDUS and look through the "Libraries" menu. (Make certain that you download the most recent revision.) Substantive questions about TIFF Class F can be faxed to its author: Joe Campbell, Everex Systems, Inc: (510) 540-5835 or (510) 841-5441, or via Compuserve Mail 71331,1237. Internet users can contact the author through the Compuserve gateway as 71331.1237@CompuServe.COM." Group 3 facsimile is described in the "Blue Book," Volume VII, Fascicle VII.3, Terminal Equipment and Protocols for Telematic Services, The International Telegraph and Telephone Consultative Committee (CCITT), Melbourne, 1988. CLASS F REQUIRED Compression = 3 or 4. SHORT. 3 Group 3, one-dimensional encoding with "byte-aligned" EOL's. An EOL is said to be byte-aligned when 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: xxxx0000 00000001. The data in a Class F image is not terminated with an RTC. Please see items 4 and 5 in the "Warnings" section. For Group 3 two-dimensional encoding, set bit 1 in Group3Options. Please see item 2 in the "Warnings" section. 4 Group 4 two-dimensional encoding. MMR (Modified- Modified READ) compression, formerly found only in Group 4 facsimile, are now available in Group 3 devices. When this option is used, bits zero and one in the Group3Options tag are ignored. FillOrder = 1, 2. SHORT. TIFF Class F readers must be able to read data in both bit orders, but the vast majority of facsimile products store data LSB first, exactly as it appears on the telephone line. 1 Most Significant Bit first. 2 Least Significant Bit first. Group3Options = 4,5. LONG. Data may be one- or two-dimensional, but EOL's must be byte-aligned. Uncompressed data is not allowed. When Group 4 compression is used, bits zero and one in the Group3Options tag should be ignored. bit 0 0 for 1-Dimensional, 1 for 2-Dimensional bit 1 Must be 0 (uncompressed data not allowed) bit 2 1 for byte-aligned EOL's ImageWidth = 864, 1216, 1728, 2048, 2432. SHORT or LONG. LONG recommended. These are the fixed page widths in pixels defined in CCITT Group 3. NewSubFileType = 2. LONG. The value 2 identifies a single page of a multi-page image, even if the document contains only one page. PageNumber. SHORT/SHORT. This tag specifies the page numbers in the fax document. The tag comprises two SHORT values: the first value is the page number, the second is the total number of pages. The number of the first page is zero. Single-page documents therefore use 00000001 hex. The total number of pages is required only in the PageNumber tag associated with the first IFD, and is optional in subsequent IFD's. Writers utilizing this option should set the page count portion of the subsequent PageNumber tags to zero. (Please remember that the first IFD is not necessarily page one.) ResolutionUnit = 2,3. SHORT. The units of measure for resolution: 2 Inch 3 Centimeter XResolution = 204, 300, 400 (inches).RATIONAL. The horizontal resolution of the image expressed in pixels per resolution unit. See "Technical Point #6," below. YResolution = 98, 196, 300, 400 (inches).RATIONAL. The vertical resolution of the image expressed in pixels per resolution unit. See "Technical Point #6," below. BILEVEL REQUIRED Although these tags are already required in Class B (Bi- Level) files, an explanation of their usage for facsimile images may be helpful. BitsPerSample = 1. SHORT. Since facsimile is a black-and-white medium, this must be 1 (the default) for all files. ImageLength. SHORT or LONG. LONG Recommended. The total number of scan lines in the image. PhotometricInterp = 0,1. SHORT. This tag allows notation of an inverted ("negative") image: 0 Normal 1 Inverted RowsPerStrip. SHORT or LONG. LONG Recommended. The number of scan lines per strip. When a page is expressed as one large strip, this is the same as the ImageLength tag. SamplesPerPixel = 1. SHORT. The value of 1 denotes a bi-level, gray scale, or palette color image. StripByteCounts. SHORT or LONG. SHORT Recommended. For each strip, the number of bytes in that strip. If a page is expressed as one large strip, this is the total number of bytes in the page after compression. StripOffsets. SHORT or LONG. For each strip, the offset of that strip. The offset is measured from the beginning of the file. If a page is expressed as one large strip, there is one such entry per page. NEW TAGS There are only three new tags for Class F. All three tags describe page quality. The information contained in these tags is usually obtained from the receiving facsimile hardware, but since not all devices are capable of reporting this information, the tags are optional. Some applications need to understand exactly the error content of the data. For example, a CAD program might wish to verify that a file has a low error level before importing it into a high-accuracy document. Because Group 3 facsimile devices do not necessarily perform error correction on the image data, the quality of a received page must be inferred from the pixel count of decoded scan lines. A "good" scan line is defined as a line that, when decoded, contains the correct number of pixels. Conversely, a "bad" scan line is defined as a line that, when decoded, comprises an incorrect number of pixels. BadFaxLines Tag = 326 (146 hex) Type = SHORT or LONG This tag reports the number of scan lines with an incorrect number of pixels encountered by the facsimile during reception (but not necessarily in the file). Note: PercentBad = (BadFaxLines/ImageLength) * 100 CleanFaxData Tag = 327 (147 hex) Type = SHORT 0 The data is "pure": it contains no lines with incorrect pixel counts and no substituted lines. Computer-generated files should always have a value of 0. 1 The receiving device substituted good lines for lines having an incorrect pixel count. 2 Lines with an incorrect pixel count exist in the data. Many facsimile receiving devices do not actually output bad lines. Instead, when a bad line is encountered, the receiver substitutes a good line. An variety of methods are employed to derive the pixel content of the substituted line. The most common are: 1. Fixed-pattern substitution (for example, an all-white or all-black line). 2. Substitution of a previous good line. 3. Artificial intelligence may be employed to reconstruct the line based upon context. Although line substitution usually results in a visual improvement in the image, the image data is nevertheless corrupted. The CleanFaxData tag describes the error content of the data. That is, when the BadFaxLines and ImageLength tags indicate that the facsimile device encountered lines with an incorrect number of pixels during reception, the CleanFaxData tag indicates whether these lines are actually in the data or if the receiving facsimile device replaced them with substitute lines. ConsecutiveBadFaxLines Tag = 328 (148 hex) Type = LONG or SHORT This tag reports the maximum number of consecutive lines containing an incorrect number of pixels encountered by the facsimile device during reception (but not necessarily in the file). The BadFaxLines and ImageLength data indicate only the quantity of such lines. The ConsecutiveBadFaxLines tag is an indicator of their distribution and may therefore be a better general indicator of perceived image quality. RECOMMENDED TAGS BadFaxLines. LONG. The number of "bad" scan lines encountered by the facsimile during reception. ConsecutiveBadFaxLines. LONG or SHORT. The maximum number of consecutive scan lines with incorrect pixel count encountered by the facsimile device reception. DateTime. ASCII. Date and time in the format YYYY:MM:DD HH:MM:SS, in 24-hour format. String length including NUL byte is 20 bytes. Space between DD and HH. DocumentName. ASCII. This is the name of the document from which the document was scanned. ImageDescription. ASCII. This is an ASCII string describing the contents of the image. Orientation. SHORT. This tag might be useful for displayers that always want to show the same orientation, regardless of the image. The default value of 1 is "0th row is visual top of image, and 0th column is the visual left." An 180-degree rotation is 3. See TIFF 5.0 for an explanation of other values. Software. ASCII. The optional name and release number of the software package that created the image. REQUIRED TAGS GROUPED CLASS Required Tags, all TIFF: NewSubFileType, ImageWidth, ImageLength, StripOffsets, RowsPerStrip, StripByteCounts, XResolution, YResolution, ResolutionUnit Required Tags, Class B: BitsPerSample, Compression, PhotometricInterp, SamplesPerPixel Required Tags, Class F: FillOrder, Group3Options, PageNumber FILE INTERCHANGEABILITY File portability among various TIFF F applications, regardless of platform or operating system, is a primary goal of TIFF Class F. The following tag values should be used to assure maximum portability: 1. FillOrder is 2 (least-significant bit first). 2. Group3Options = 4 (one-dimensional encoding). 3. ImageWidth is 1728 (that is, an A4 page). 4. ImageLength must not exceed 1084 for 98 dpi documents and 2167 for 196 dpi documents (that is, an A4 page). See Note 2, below. 5. PhotometricInterp is 0 (normal). 6. ResolutionUnit = 2 (inches). 7. XResolution is 204. 8. YResolution tag is 98 or 196. TECHNICAL POINTS 1. Strips Those new to TIFF may not be familiar with the concept of "strips" embodied in the three tags RowsPerStrip, StripByteCount, StripOffsets. In general, third-party applications that read and write TIFF files expect the image to be divided into horizontal "strips," also known as "bands." Each strip contains a few lines of the image. By using strips, a TIFF reader need not load the entire image into memory, thus enabling it to fetch and decompress small random portions of the image as necessary. The dimensions of a strip are described by the RowsPerStrip and StripByteCount tags. The location in the TIFF file of each strip is contained in the StripOffsets tag. Is is perfectly acceptable for a Class F file to store an entire page in a single strip. In addition to strips, TIFF also permits image data to be divided into rectangular tiles. Class F images may not be organized as tiles. 2. EOL Placement in Strips As illustrated in FIGURE 1/T.4 in Recommendation T.4 (the "Blue Book"), facsimile documents begin with an EOL (End- of-Line) code. The last line of the image is not terminated by an EOL. Expressed differently, EOL's are actually BOL's (Beginning-of-Line). When a page is stored as a multi-strip image, one must consider where to divide scanline data. With the RTC not included, treating EOL codes like BOL codes permits all strips to have a consistent format: RowsPerStrip EOL- prefixed lines of data. Consequently, multi-strip Class F images must break data such that each strip begins with an EOL code. This is easily done if these codes are treated like BOL codes. 3. Bit Order Although the TIFF 5.0 documentation lists the FillOrder tag in the category "No Longer Recommended," Class F resurrects it. Facsimile data appears on the phone line in bit-reversed order relative to its description in CCITT Recommendation T.4. Therefore, a wide majority of facsimile applications choose this natural order for storage. Nevertheless, TIFF Class F readers must be able to read data in both bit orders. 4. Multi-Page Many existing applications already read Class F-like files, but do not support the multi-page tag. Since a multi-page format greatly simplifies file management in fax application software, Class F specifies multi-page documents (NewSubfileType = 2). A "multi-page document" may contain only one page. 5. Two-dimensional Encoding PC Fax applications that wish to support two-dimensional encoding may do so by setting Bit 0 in the Group3Options tag. 6. Two-Dimensional Encoding EOL Tag Bits When two-dimensional encoding is used, the tag bit that specifies whether the next line is one- or two- dimensionally encoded is part of the byte that follows the byte-aligned EOL code. That is, the tag bit is logically considered to be part of the scan line that it describes. 7. Example Use of Page-quality Tags Here are examples for writing the CleanFaxData, BadFaxLines, and ConsecutiveBadFaxLines tags: * Facsimile hardware does not provide page quality information: write no tags. * Facsimile hardware provides page quality information, but reports no bad lines. Write only BadFaxLines = 0. * Facsimile hardware provides page quality information, and reports bad lines. Write both BadFaxLines and ConsecutiveBadFaxLines. Also write CleanFaxData = 1 or 2 if you know whether the hardware can replace bad lines. * Computer generated file: write CleanFaxData = 0. 8. High Resolution Although 300 and 400 dpi are, strictly speaking, Group 4 resolutions, it is virtually certain that they will soon be added to Group 3 and, more important, are already in common use today capabilities through Group 3's NSF mechanism. Only the following resolutions are valid (horizontal x vertical): 204 x 98, 204 x 196, 300 x 300, 400 x 400. Those who choose to store images at the "Group 4" resolutions risk incompatibility with other fax applications. 9. Plain Paper Fax Many plain-paper printing mechanisms such as those found on laser printers are unable to print on the entire surface of the paper. The amount of unusable space (referred to as the "grabber") varies from device to device, but as a general rule, allow about six-tenths of an inch. Failure to reduce the image accordingly may cause the receiving fax machine to shrink the image to make it fit on one page (thus changing its scale), or to print it on two sheets of paper. The standard paper size for America (8.5 inches by 11.0 inches), is still not supported by CCITT specifications. Therefore, if you wish your images to print at scale on American plain paper fax machines, you must limit the number of lines per page to 2050 in high resolution and 1025 in low resolution. 10. Minimum TIFF Class F Support Fax applications that do not wish to embrace TIFF Class F as a native format may elect to support it as import/export medium: * Export The simplest form of support is a Class F writer that produces individual single-page Class F files with the proper NewSubFile and PageNumber tags. * Import A Class F reader must be able to handle a Class F file containing multiple pages. WARNINGS 1. Class F requires the ability to read and write at least one-dimensional T.4 Huffman ("compressed") data. Due to the disruptive effect to application software of line- length errors and because such errors are likely in everyday facsimile transmissions, uncompressed data is not allowed. In other words, "Uncompressed" bit in Group3Options must be 0. 2. Since two-dimensional encoding is not required for Group 3 compatibility, Class F readers may decline to read such files. Therefore, for maximum portability write only one- dimensional files. Although the same argument technically holds for "fine" (196 dpi) vertical resolution, only a tiny fraction of facsimile products support only 98 dpi. Therefore, 196-dpi files are quite portable in the real world. 3. In the spirit of TIFF, all EOL's in data must be byte- aligned. An EOL is said to be byte-aligned when 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 one byte preceded by a zero nibble: xxxx0000 00000001. Recall that Huffman encoding encodes bits, not bytes. This means that the end-of-line token may end in the middle of a byte. In byte alignment, extra zero bits (Fill) are added so that the first bit of data following an EOL begins on a byte boundary. In effect, byte alignment relieves application software of the burden of bit- shifting every byte while parsing scan lines for line- oriented image manipulation (such as writing a TIFF file). 4. Aside from EOL's, TIFF Class F files contain only image data. This means that the Return-to-Control sequence (RTC) is specifically prohibited. Exclusion of RTC's not only makes possible the simple concatenation of images, it eliminates the mischief failed communications and unreadable images that their mistreatment inevitably produces. REVISING THE TIFF CLASS F SPECIFICATION Changes in the specification that reflect changes in the underlying CCITT specifications as well as non-technical changes are incorporated by editorial fiat. Before substantial modifications are allowed, however, the author will consult members of the facsimile industry. The main goal in revision is to retain a specification that fulfills the original goals and serves the facsimile industry. It is especially important that Class F not be modified to accommodate unrelated goals. For example, there have been proposals to relax Class F tag requirements to make it more compatible with other flavors of TIFF. In particular, non-facsimile users seem to be vexed by the necessity to byte-align EOL's and to support the multi-page format. Such proposals inevitably originate with users outside the mainstream of facsimile vendors, in whose applications both of these features are vitally important. Non-facsimile users who find Class F too restrictive might better be served by designing a new TIFF classes to accomplish their ends. MANUSCRIPT OF PROPOSED REVISION Any person or group that wishes to propose an amendment to TIFF Class F should prepare the following manuscript: 1. Name of the person or group making the request and their affiliation (business, academic, etc.). 2. The revision date from which you are working. 3. The reason for the request. 4. A list of changes exactly as you propose that they appear in the specification. Do not submit an edited version of the entire specification. Use inserts, callouts, or other obvious editorial techniques to indicate areas of change and number each change. 5. Referring to each change number, discuss its potential effect on other standards that incorporate TIFF Class F (e.g., FaxBios). 6. A list of phone numbers of persons outside your company who may support your position. Include their affiliation (business, academic, etc.). This manuscript may be faxed to Joe Campbell, Everex Systems, Inc: (510) 540-5835 or (510) 841-5441, or via Compuserve Mail 71331,1237. REVISION HISTORY 11/17/89: Initial Publication 4/20/90 : First Revision PageNumber tag was incorrectly illustrated as page one. The correct number for the first page is zero. 5/1/91: Second Revision 1. Added 300 and 400 valid values to to the XResolution and YResolution tags. 2. Software tag moved from Bi-level Required (not true), to Recommended. 3. ImageWidth tag values of 2482 was corrected to 2432. 4. New ImageWidth tag values added to conform to the "Blue Book,": 864, 1216. 5. Corrected miscellaneous typographical errors. 6. Added summary of required tags. 10/1/91: Third Revision, 1. Total page count needed only in first tag after IFD. 2. Added MMR compression, modification procedure. 3/1/92: Fourth Revision: EDITORIAL 1. Bad fax lines are now said to be "substituted," instead of "regenerated." This generalized approach accommodates techniques besides regeneration. 2. Clarification of the encoding of EOL 2-d tag bits. 3. How to break up data in stripped images. 4. Prohibition of using TIFF tiles. 5. Deletion of discussion of minimum strip size. 6. Added suggestions for adjusting number of scan lines for destinations that use plain-paper fax and destinations that use U.S. letter-sized paper.