Chip 2000 May/Chip_2000-05_cd1.bin/sharewar/FFE/GRAPH.SWG/0069_TIFF Class F.pas

Pascal/Delphi Source File | 1997-05-12 | 17KB | 429 lines

This is the most recent revision of the TIFF Class F specification from
Joe Campbell at Cygnet.

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                   THE SPIRIT OF TIFF CLASS F

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 F 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.

Finally, technical topics are discussed in the sections "Technical
Points" and "Warnings."

                   REFERENCES

Substantive questions about TIFF Class F can be faxed to:

          Cygnet Technologies
          2560 9th, Suite 220
          Berkeley, CA 94710
          Fax: [phone]

TIFF Class F is a parallel but unrelated effort to EIA Project Number
2188, an industry standards group working to standardize facsimile
hardware. For information about this standard, contact Joe Decuir at
the above address or phone [phone].

Group 3 facsimile is described in the "Red Book", Volume VII, Fascicle
VII.3, Terminal Equipment and Protocols for Telematic Services, The
International Telegraph and Telephone Consultative Committee (CCITT),
Geneva, 1985.






                        CLASS F REQUIRED

     Compression = 3.  SHORT.   Group 3, one- dimensional encoding
with "byte-aligned" EOLs.  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 two-dimensional encoding, set bit 1 in Group3Options. Please
see item 2 in the "Warnings" section.

     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 EOLs must be byte-aligned. Uncompressed data is not allowed.

        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 EOLs

     ImageWidth = 1728, 2048, 2482.  SHORT or LONG.  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.

     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. Single-
page documents therefore use 00000001 hex.

     ResolutionUnit = 2,3.  SHORT.  The units of measure for
resolution:

        2 = Inch

        3 = Centimeter

     XResolution = 204 (inches).  RATIONAL. The horizontal resolution
of the image expressed in pixels per resolution unit.

     YResolution = 98, 196 (inches).  RATIONAL. The vertical
resolution of the image expressed in pixels per resolution unit.

                   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

     Software.  ASCII.  The optional name and release number of the
software package that created the image.

     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,
grayscale, or palatte 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

            N = 0

                         0 = Data  contains  no lines  with  incorrect
                             pixel counts or regenerated lines  (i.e.,
                             computer generated)


                         1 = Lines with an incorrect pixel count  were
                             regenerated by receiving device


                         2 = Lines  with  an  incorrect  pixel   count
                             existed,  but  were  not  regenerated  by
                             receiving device

Many facsimile devices do not actually output bad lines. Instead,  the
previous good line is repeated in place of a bad  line. Although  this
substitution,  known  as  line   regeneration,  results  in  a  visual
improvement  to the image,  the 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 regenerated 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.

     CleanFaxData = 0, 1, 2.  BYTE.  This tag indicates whether
lines with incorrect pixel count are actually in the data or if
the receiving facsimile device replaced them with regenerated
lines.

                         0 = Data contains no lines with incorrect
                             pixel counts or regenerated lines (i.e.,
                             computer generated)

                         1 = Lines with an incorrect pixel count were
                             regenerated by receiving device

                         2 = Lines with an incorrect pixel count
                             existed, but were not regenerated by
                             receiving device

     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.















                   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 "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.

     The TIFF documentation suggests using strips of an arbitrary size
of about 8K.  Although various application programs assert that they
"prefer" banded images, research failed to uncover a single existing
application that could not read a single-strip page where they could
read the same file in a multi- strip format. Indeed, applications seem
to be more sensitive to the total size of the decoded image and are
not particularly fussy about banding.  This result is not surprising,
considering that most desktop publishing programs are prepared to deal
with massively larger images than those one finds in facsimile.  In
short, each page may be represented as a single strip of any length.

     In fact, there may be a compelling reason to employ a strip size
equal to the length of one A4 page (297 mm).  When a document is
imaged, it may be of any length.  Not all fax machines, however, can
accept unlimited length documents. Worse, the remote machine's page-
length capability is not known until the fax connection has been
established.  The solution is for the transmitting fax device to image
long documents into A4-size strips, then seam them together at
transmission, after the capabilities of the remote fax machine is
known.

2.  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.

3.  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).

4.  Two-dimensional Encoding  PC Fax applications that wish to support
two-dimensional encoding may do so by setting Bit 0 in the
Group3Options tag.  Please see item 2 in the "Warnings" section.



5.  Example Use of Page-quality Tags  Here are examples for writing
the CleanFaxData,  BadFaxLines, and ConsecutiveBadFaxLines tags:

     1.  Facsimile hardware does not provide page quality information:
write no tags.

     2.  Facsimile hardware provides page quality information, but
reports no bad lines.  Write only BadFaxLines = 0.

     3.  Facsimile hardware provides page quality information, and
reports bad lines.  Write both BadFaxLines and ConsecutiveBadFaxLines.
Also write CleanFaxData = 1 or 2 if the hardware's regeneration
capability is known.

     4.  Computer generated file:  write CleanFaxData = 0.


              WHAT CONSTITUTES 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 tag and the PageNumber (page  one-of-one) tag.

     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, high-resolution files are quite
portable in the real world.

3.  In the spirit of TIFF, all EOLs 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.  As illustrated in FIGURE 1/T.4 in Recommendation T.4 (the Red
Book, page 20), facsimile documents begin with an EOL (which in Class
F is byte-aligned). The last line of the image is not terminated by an
EOL.

5.  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.  (This view is reflected in the work of the EIA
PN2188 committee, where the modem device attaches the  RTC outbound
and removes it inbound.)

                        REVISION HISTORY

11/17/89: Initial Publication

4/29/90 : First revision

PageNumber tag was incorrectly illustrated as page one. The correct
number for the first page is zero.



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