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diff --git a/tiff/html/TIFFTechNote2.html b/tiff/html/TIFFTechNote2.html deleted file mode 100644 index 92bace4..0000000 --- a/tiff/html/TIFFTechNote2.html +++ /dev/null @@ -1,707 +0,0 @@ -<pre> -DRAFT TIFF Technical Note #2 17-Mar-95 -============================ - -This Technical Note describes serious problems that have been found in -TIFF 6.0's design for embedding JPEG-compressed data in TIFF (Section 22 -of the TIFF 6.0 spec of 3 June 1992). A replacement TIFF/JPEG -specification is given. Some corrections to Section 21 are also given. - -To permit TIFF implementations to continue to read existing files, the 6.0 -JPEG fields and tag values will remain reserved indefinitely. However, -TIFF writers are strongly discouraged from using the 6.0 JPEG design. It -is expected that the next full release of the TIFF specification will not -describe the old design at all, except to note that certain tag numbers -are reserved. The existing Section 22 will be replaced by the -specification text given in the second part of this Tech Note. - - -Problems in TIFF 6.0 JPEG -========================= - -Abandoning a published spec is not a step to be taken lightly. This -section summarizes the reasons that have forced this decision. -TIFF 6.0's JPEG design suffers from design errors and limitations, -ambiguities, and unnecessary complexity. - - -Design errors and limitations ------------------------------ - -The fundamental design error in the existing Section 22 is that JPEG's -various tables and parameters are broken out as separate fields which the -TIFF control logic must manage. This is bad software engineering: that -information should be treated as private to the JPEG codec -(compressor/decompressor). Worse, the fields themselves are specified -without sufficient thought for future extension and without regard to -well-established TIFF conventions. Here are some of the significant -problems: - -* The JPEGxxTable fields do not store the table data directly in the -IFD/field structure; rather, the fields hold pointers to information -elsewhere in the file. This requires special-purpose code to be added to -*every* TIFF-manipulating application, whether it needs to decode JPEG -image data or not. Even a trivial TIFF editor, for example a program to -add an ImageDescription field to a TIFF file, must be explicitly aware of -the internal structure of the JPEG-related tables, or else it will probably -break the file. Every other auxiliary field in the TIFF spec contains -data, not pointers, and can be copied or relocated by standard code that -doesn't know anything about the particular field. This is a crucial -property of the TIFF format that must not be given up. - -* To manipulate these fields, the TIFF control logic is required to know a -great deal about JPEG details, for example such arcana as how to compute -the length of a Huffman code table --- the length is not supplied in the -field structure and can only be found by inspecting the table contents. -This is again a violation of good software practice. Moreover, it will -prevent easy adoption of future JPEG extensions that might change these -low-level details. - -* The design neglects the fact that baseline JPEG codecs support only two -sets of Huffman tables: it specifies a separate table for each color -component. This implies that encoders must waste space (by storing -duplicate Huffman tables) or else violate the well-founded TIFF convention -that prohibits duplicate pointers. Furthermore, baseline decoders must -test to find out which tables are identical, a waste of time and code -space. - -* The JPEGInterchangeFormat field also violates TIFF's proscription against -duplicate pointers: the normal strip/tile pointers are expected to point -into the larger data area pointed to by JPEGInterchangeFormat. All TIFF -editing applications must be specifically aware of this relationship, since -they must maintain it or else delete the JPEGInterchangeFormat field. The -JPEGxxTables fields are also likely to point into the JPEGInterchangeFormat -area, creating additional pointer relationships that must be maintained. - -* The JPEGQTables field is fixed at a byte per table entry; there is no -way to support 16-bit quantization values. This is a serious impediment -to extending TIFF to use 12-bit JPEG. - -* The 6.0 design cannot support using different quantization tables in -different strips/tiles of an image (so as to encode some areas at higher -quality than others). Furthermore, since quantization tables are tied -one-for-one to color components, the design cannot support table switching -options that are likely to be added in future JPEG revisions. - - -Ambiguities ------------ - -Several incompatible interpretations are possible for 6.0's treatment of -JPEG restart markers: - - * It is unclear whether restart markers must be omitted at TIFF segment - (strip/tile) boundaries, or whether they are optional. - - * It is unclear whether the segment size is required to be chosen as - a multiple of the specified restart interval (if any); perhaps the - JPEG codec is supposed to be reset at each segment boundary as if - there were a restart marker there, even if the boundary does not fall - at a multiple of the nominal restart interval. - - * The spec fails to address the question of restart marker numbering: - do the numbers begin again within each segment, or not? - -That last point is particularly nasty. If we make numbering begin again -within each segment, we give up the ability to impose a TIFF strip/tile -structure on an existing JPEG datastream with restarts (which was clearly a -goal of Section 22's authors). But the other choice interferes with random -access to the image segments: a reader must compute the first restart -number to be expected within a segment, and must have a way to reset its -JPEG decoder to expect a nonzero restart number first. This may not even -be possible with some JPEG chips. - -The tile height restriction found on page 104 contradicts Section 15's -general description of tiles. For an image that is not vertically -downsampled, page 104 specifies a tile height of one MCU or 8 pixels; but -Section 15 requires tiles to be a multiple of 16 pixels high. - -This Tech Note does not attempt to resolve these ambiguities, so -implementations that follow the 6.0 design should be aware that -inter-application compatibility problems are likely to arise. - - -Unnecessary complexity ----------------------- - -The 6.0 design creates problems for implementations that need to keep the -JPEG codec separate from the TIFF control logic --- for example, consider -using a JPEG chip that was not designed specifically for TIFF. JPEG codecs -generally want to produce or consume a standard ISO JPEG datastream, not -just raw compressed data. (If they were to handle raw data, a separate -out-of-band mechanism would be needed to load tables into the codec.) -With such a codec, the TIFF control logic must parse JPEG markers emitted -by the codec to create the TIFF table fields (when writing) or synthesize -JPEG markers from the TIFF fields to feed the codec (when reading). This -means that the control logic must know a great deal more about JPEG details -than we would like. The parsing and reconstruction of the markers also -represents a fair amount of unnecessary work. - -Quite a few implementors have proposed writing "TIFF/JPEG" files in which -a standard JPEG datastream is simply dumped into the file and pointed to -by JPEGInterchangeFormat. To avoid parsing the JPEG datastream, they -suggest not writing the JPEG auxiliary fields (JPEGxxTables etc) nor even -the basic TIFF strip/tile data pointers. This approach is incompatible -with implementations that handle the full TIFF 6.0 JPEG design, since they -will expect to find strip/tile pointers and auxiliary fields. Indeed this -is arguably not TIFF at all, since *all* TIFF-reading applications expect -to find strip or tile pointers. A subset implementation that is not -upward-compatible with the full spec is clearly unacceptable. However, -the frequency with which this idea has come up makes it clear that -implementors find the existing Section 22 too complex. - - -Overview of the solution -======================== - -To solve these problems, we adopt a new design for embedding -JPEG-compressed data in TIFF files. The new design uses only complete, -uninterpreted ISO JPEG datastreams, so it should be much more forgiving of -extensions to the ISO standard. It should also be far easier to implement -using unmodified JPEG codecs. - -To reduce overhead in multi-segment TIFF files, we allow JPEG overhead -tables to be stored just once in a JPEGTables auxiliary field. This -feature does not violate the integrity of the JPEG datastreams, because it -uses the notions of "tables-only datastreams" and "abbreviated image -datastreams" as defined by the ISO standard. - -To prevent confusion with the old design, the new design is given a new -Compression tag value, Compression=7. Readers that need to handle -existing 6.0 JPEG files may read both old and new files, using whatever -interpretation of the 6.0 spec they did before. Compression tag value 6 -and the field tag numbers defined by 6.0 section 22 will remain reserved -indefinitely, even though detailed descriptions of them will be dropped -from future editions of the TIFF specification. - - -Replacement TIFF/JPEG specification -=================================== - -[This section of the Tech Note is expected to replace Section 22 in the -next release of the TIFF specification.] - -This section describes TIFF compression scheme 7, a high-performance -compression method for continuous-tone images. - -Introduction ------------- - -This TIFF compression method uses the international standard for image -compression ISO/IEC 10918-1, usually known as "JPEG" (after the original -name of the standards committee, Joint Photographic Experts Group). JPEG -is a joint ISO/CCITT standard for compression of continuous-tone images. - -The JPEG committee decided that because of the broad scope of the standard, -no one algorithmic procedure was able to satisfy the requirements of all -applications. Instead, the JPEG standard became a "toolkit" of multiple -algorithms and optional capabilities. Individual applications may select -a subset of the JPEG standard that meets their requirements. - -The most important distinction among the JPEG processes is between lossy -and lossless compression. Lossy compression methods provide high -compression but allow only approximate reconstruction of the original -image. JPEG's lossy processes allow the encoder to trade off compressed -file size against reconstruction fidelity over a wide range. Typically, -10:1 or more compression of full-color data can be obtained while keeping -the reconstructed image visually indistinguishable from the original. Much -higher compression ratios are possible if a low-quality reconstructed image -is acceptable. Lossless compression provides exact reconstruction of the -source data, but the achievable compression ratio is much lower than for -the lossy processes; JPEG's rather simple lossless process typically -achieves around 2:1 compression of full-color data. - -The most widely implemented JPEG subset is the "baseline" JPEG process. -This provides lossy compression of 8-bit-per-channel data. Optional -extensions include 12-bit-per-channel data, arithmetic entropy coding for -better compression, and progressive/hierarchical representations. The -lossless process is an independent algorithm that has little in -common with the lossy processes. - -It should be noted that the optional arithmetic-coding extension is subject -to several US and Japanese patents. To avoid patent problems, use of -arithmetic coding processes in TIFF files intended for inter-application -interchange is discouraged. - -All of the JPEG processes are useful only for "continuous tone" data, -in which the difference between adjacent pixel values is usually small. -Low-bit-depth source data is not appropriate for JPEG compression, nor -are palette-color images good candidates. The JPEG processes work well -on grayscale and full-color data. - -Describing the JPEG compression algorithms in sufficient detail to permit -implementation would require more space than we have here. Instead, we -refer the reader to the References section. - - -What data is being compressed? ------------------------------- - -In lossy JPEG compression, it is customary to convert color source data -to YCbCr and then downsample it before JPEG compression. This gives -2:1 data compression with hardly any visible image degradation, and it -permits additional space savings within the JPEG compression step proper. -However, these steps are not considered part of the ISO JPEG standard. -The ISO standard is "color blind": it accepts data in any color space. - -For TIFF purposes, the JPEG compression tag is considered to represent the -ISO JPEG compression standard only. The ISO standard is applied to the -same data that would be stored in the TIFF file if no compression were -used. Therefore, if color conversion or downsampling are used, they must -be reflected in the regular TIFF fields; these steps are not considered to -be implicit in the JPEG compression tag value. PhotometricInterpretation -and related fields shall describe the color space actually stored in the -file. With the TIFF 6.0 field definitions, downsampling is permissible -only for YCbCr data, and it must correspond to the YCbCrSubSampling field. -(Note that the default value for this field is not 1,1; so the default for -YCbCr is to apply downsampling!) It is likely that future versions of TIFF -will provide additional PhotometricInterpretation values and a more general -way of defining subsampling, so as to allow more flexibility in -JPEG-compressed files. But that issue is not addressed in this Tech Note. - -Implementors should note that many popular JPEG codecs -(compressor/decompressors) provide automatic color conversion and -downsampling, so that the application may supply full-size RGB data which -is nonetheless converted to downsampled YCbCr. This is an implementation -convenience which does not excuse the TIFF control layer from its -responsibility to know what is really going on. The -PhotometricInterpretation and subsampling fields written to the file must -describe what is actually in the file. - -A JPEG-compressed TIFF file will typically have PhotometricInterpretation = -YCbCr and YCbCrSubSampling = [2,1] or [2,2], unless the source data was -grayscale or CMYK. - - -Basic representation of JPEG-compressed images ----------------------------------------------- - -JPEG compression works in either strip-based or tile-based TIFF files. -Rather than repeating "strip or tile" constantly, we will use the term -"segment" to mean either a strip or a tile. - -When the Compression field has the value 7, each image segment contains -a complete JPEG datastream which is valid according to the ISO JPEG -standard (ISO/IEC 10918-1). Any sequential JPEG process can be used, -including lossless JPEG, but progressive and hierarchical processes are not -supported. Since JPEG is useful only for continuous-tone images, the -PhotometricInterpretation of the image shall not be 3 (palette color) nor -4 (transparency mask). The bit depth of the data is also restricted as -specified below. - -Each image segment in a JPEG-compressed TIFF file shall contain a valid -JPEG datastream according to the ISO JPEG standard's rules for -interchange-format or abbreviated-image-format data. The datastream shall -contain a single JPEG frame storing that segment of the image. The -required JPEG markers within a segment are: - SOI (must appear at very beginning of segment) - SOFn - SOS (one for each scan, if there is more than one scan) - EOI (must appear at very end of segment) -The actual compressed data follows SOS; it may contain RSTn markers if DRI -is used. - -Additional JPEG "tables and miscellaneous" markers may appear between SOI -and SOFn, between SOFn and SOS, and before each subsequent SOS if there is -more than one scan. These markers include: - DQT - DHT - DAC (not to appear unless arithmetic coding is used) - DRI - APPn (shall be ignored by TIFF readers) - COM (shall be ignored by TIFF readers) -DNL markers shall not be used in TIFF files. Readers should abort if any -other marker type is found, especially the JPEG reserved markers; -occurrence of such a marker is likely to indicate a JPEG extension. - -The tables/miscellaneous markers may appear in any order. Readers are -cautioned that although the SOFn marker refers to DQT tables, JPEG does not -require those tables to precede the SOFn, only the SOS. Missing-table -checks should be made when SOS is reached. - -If no JPEGTables field is used, then each image segment shall be a complete -JPEG interchange datastream. Each segment must define all the tables it -references. To allow readers to decode segments in any order, no segment -may rely on tables being carried over from a previous segment. - -When a JPEGTables field is used, image segments may omit tables that have -been specified in the JPEGTables field. Further details appear below. - -The SOFn marker shall be of type SOF0 for strict baseline JPEG data, of -type SOF1 for non-baseline lossy JPEG data, or of type SOF3 for lossless -JPEG data. (SOF9 or SOF11 would be used for arithmetic coding.) All -segments of a JPEG-compressed TIFF image shall use the same JPEG -compression process, in particular the same SOFn type. - -The data precision field of the SOFn marker shall agree with the TIFF -BitsPerSample field. (Note that when PlanarConfiguration=1, this implies -that all components must have the same BitsPerSample value; when -PlanarConfiguration=2, different components could have different bit -depths.) For SOF0 only precision 8 is permitted; for SOF1, precision 8 or -12 is permitted; for SOF3, precisions 2 to 16 are permitted. - -The image dimensions given in the SOFn marker shall agree with the logical -dimensions of that particular strip or tile. For strip images, the SOFn -image width shall equal ImageWidth and the height shall equal RowsPerStrip, -except in the last strip; its SOFn height shall equal the number of rows -remaining in the ImageLength. (In other words, no padding data is counted -in the SOFn dimensions.) For tile images, each SOFn shall have width -TileWidth and height TileHeight; adding and removing any padding needed in -the edge tiles is the concern of some higher level of the TIFF software. -(The dimensional rules are slightly different when PlanarConfiguration=2, -as described below.) - -The ISO JPEG standard only permits images up to 65535 pixels in width or -height, due to 2-byte fields in the SOFn markers. In TIFF, this limits -the size of an individual JPEG-compressed strip or tile, but the total -image size can be greater. - -The number of components in the JPEG datastream shall equal SamplesPerPixel -for PlanarConfiguration=1, and shall be 1 for PlanarConfiguration=2. The -components shall be stored in the same order as they are described at the -TIFF field level. (This applies both to their order in the SOFn marker, -and to the order in which they are scanned if multiple JPEG scans are -used.) The component ID bytes are arbitrary so long as each component -within an image segment is given a distinct ID. To avoid any possible -confusion, we require that all segments of a TIFF image use the same ID -code for a given component. - -In PlanarConfiguration 1, the sampling factors given in SOFn markers shall -agree with the sampling factors defined by the related TIFF fields (or with -the default values that are specified in the absence of those fields). - -When DCT-based JPEG is used in a strip TIFF file, RowsPerStrip is required -to be a multiple of 8 times the largest vertical sampling factor, i.e., a -multiple of the height of an interleaved MCU. (For simplicity of -specification, we require this even if the data is not actually -interleaved.) For example, if YCbCrSubSampling = [2,2] then RowsPerStrip -must be a multiple of 16. An exception to this rule is made for -single-strip images (RowsPerStrip >= ImageLength): the exact value of -RowsPerStrip is unimportant in that case. This rule ensures that no data -padding is needed at the bottom of a strip, except perhaps the last strip. -Any padding required at the right edge of the image, or at the bottom of -the last strip, is expected to occur internally to the JPEG codec. - -When DCT-based JPEG is used in a tiled TIFF file, TileLength is required -to be a multiple of 8 times the largest vertical sampling factor, i.e., -a multiple of the height of an interleaved MCU; and TileWidth is required -to be a multiple of 8 times the largest horizontal sampling factor, i.e., -a multiple of the width of an interleaved MCU. (For simplicity of -specification, we require this even if the data is not actually -interleaved.) All edge padding required will therefore occur in the course -of normal TIFF tile padding; it is not special to JPEG. - -Lossless JPEG does not impose these constraints on strip and tile sizes, -since it is not DCT-based. - -Note that within JPEG datastreams, multibyte values appear in the MSB-first -order specified by the JPEG standard, regardless of the byte ordering of -the surrounding TIFF file. - - -JPEGTables field ----------------- - -The only auxiliary TIFF field added for Compression=7 is the optional -JPEGTables field. The purpose of JPEGTables is to predefine JPEG -quantization and/or Huffman tables for subsequent use by JPEG image -segments. When this is done, these rather bulky tables need not be -duplicated in each segment, thus saving space and processing time. -JPEGTables may be used even in a single-segment file, although there is no -space savings in that case. - -JPEGTables: - Tag = 347 (15B.H) - Type = UNDEFINED - N = number of bytes in tables datastream, typically a few hundred -JPEGTables provides default JPEG quantization and/or Huffman tables which -are used whenever a segment datastream does not contain its own tables, as -specified below. - -Notice that the JPEGTables field is required to have type code UNDEFINED, -not type code BYTE. This is to cue readers that expanding individual bytes -to short or long integers is not appropriate. A TIFF reader will generally -need to store the field value as an uninterpreted byte sequence until it is -fed to the JPEG decoder. - -Multibyte quantities within the tables follow the ISO JPEG convention of -MSB-first storage, regardless of the byte ordering of the surrounding TIFF -file. - -When the JPEGTables field is present, it shall contain a valid JPEG -"abbreviated table specification" datastream. This datastream shall begin -with SOI and end with EOI. It may contain zero or more JPEG "tables and -miscellaneous" markers, namely: - DQT - DHT - DAC (not to appear unless arithmetic coding is used) - DRI - APPn (shall be ignored by TIFF readers) - COM (shall be ignored by TIFF readers) -Since JPEG defines the SOI marker to reset the DAC and DRI state, these two -markers' values cannot be carried over into any image datastream, and thus -they are effectively no-ops in the JPEGTables field. To avoid confusion, -it is recommended that writers not place DAC or DRI markers in JPEGTables. -However readers must properly skip over them if they appear. - -When JPEGTables is present, readers shall load the table specifications -contained in JPEGTables before processing image segment datastreams. -Image segments may simply refer to these preloaded tables without defining -them. An image segment can still define and use its own tables, subject to -the restrictions below. - -An image segment may not redefine any table defined in JPEGTables. (This -restriction is imposed to allow readers to process image segments in random -order without having to reload JPEGTables between segments.) Therefore, use -of JPEGTables divides the available table slots into two groups: "global" -slots are defined in JPEGTables and may be used but not redefined by -segments; "local" slots are available for local definition and use in each -segment. To permit random access, a segment may not reference any local -tables that it does not itself define. - - -Special considerations for PlanarConfiguration 2 ------------------------------------------------- - -In PlanarConfiguration 2, each image segment contains data for only one -color component. To avoid confusing the JPEG codec, we wish the segments -to look like valid single-channel (i.e., grayscale) JPEG datastreams. This -means that different rules must be used for the SOFn parameters. - -In PlanarConfiguration 2, the dimensions given in the SOFn of a subsampled -component shall be scaled down by the sampling factors compared to the SOFn -dimensions that would be used in PlanarConfiguration 1. This is necessary -to match the actual number of samples stored in that segment, so that the -JPEG codec doesn't complain about too much or too little data. In strip -TIFF files the computed dimensions may need to be rounded up to the next -integer; in tiled files, the restrictions on tile size make this case -impossible. - -Furthermore, all SOFn sampling factors shall be given as 1. (This is -merely to avoid confusion, since the sampling factors in a single-channel -JPEG datastream have no real effect.) - -Any downsampling will need to happen externally to the JPEG codec, since -JPEG sampling factors are defined with reference to the full-precision -component. In PlanarConfiguration 2, the JPEG codec will be working on -only one component at a time and thus will have no reference component to -downsample against. - - -Minimum requirements for TIFF/JPEG ----------------------------------- - -ISO JPEG is a large and complex standard; most implementations support only -a subset of it. Here we define a "core" subset of TIFF/JPEG which readers -must support to claim TIFF/JPEG compatibility. For maximum -cross-application compatibility, we recommend that writers confine -themselves to this subset unless there is very good reason to do otherwise. - -Use the ISO baseline JPEG process: 8-bit data precision, Huffman coding, -with no more than 2 DC and 2 AC Huffman tables. Note that this implies -BitsPerSample = 8 for each component. We recommend deviating from baseline -JPEG only if 12-bit data precision or lossless coding is required. - -Use no subsampling (all JPEG sampling factors = 1) for color spaces other -than YCbCr. (This is, in fact, required with the TIFF 6.0 field -definitions, but may not be so in future revisions.) For YCbCr, use one of -the following choices: - YCbCrSubSampling field JPEG sampling factors - 1,1 1h1v, 1h1v, 1h1v - 2,1 2h1v, 1h1v, 1h1v - 2,2 (default value) 2h2v, 1h1v, 1h1v -We recommend that RGB source data be converted to YCbCr for best compression -results. Other source data colorspaces should probably be left alone. -Minimal readers need not support JPEG images with colorspaces other than -YCbCr and grayscale (PhotometricInterpretation = 6 or 1). - -A minimal reader also need not support JPEG YCbCr images with nondefault -values of YCbCrCoefficients or YCbCrPositioning, nor with values of -ReferenceBlackWhite other than [0,255,128,255,128,255]. (These values -correspond to the RGB<=>YCbCr conversion specified by JFIF, which is widely -implemented in JPEG codecs.) - -Writers are reminded that a ReferenceBlackWhite field *must* be included -when PhotometricInterpretation is YCbCr, because the default -ReferenceBlackWhite values are inappropriate for YCbCr. - -If any subsampling is used, PlanarConfiguration=1 is preferred to avoid the -possibly-confusing requirements of PlanarConfiguration=2. In any case, -readers are not required to support PlanarConfiguration=2. - -If possible, use a single interleaved scan in each image segment. This is -not legal JPEG if there are more than 4 SamplesPerPixel or if the sampling -factors are such that more than 10 blocks would be needed per MCU; in that -case, use a separate scan for each component. (The recommended color -spaces and sampling factors will not run into that restriction, so a -minimal reader need not support more than one scan per segment.) - -To claim TIFF/JPEG compatibility, readers shall support multiple-strip TIFF -files and the optional JPEGTables field; it is not acceptable to read only -single-datastream files. Support for tiled TIFF files is strongly -recommended but not required. - - -Other recommendations for implementors --------------------------------------- - -The TIFF tag Compression=7 guarantees only that the compressed data is -represented as ISO JPEG datastreams. Since JPEG is a large and evolving -standard, readers should apply careful error checking to the JPEG markers -to ensure that the compression process is within their capabilities. In -particular, to avoid being confused by future extensions to the JPEG -standard, it is important to abort if unknown marker codes are seen. - -The point of requiring that all image segments use the same JPEG process is -to ensure that a reader need check only one segment to determine whether it -can handle the image. For example, consider a TIFF reader that has access -to fast but restricted JPEG hardware, as well as a slower, more general -software implementation. It is desirable to check only one image segment -to find out whether the fast hardware can be used. Thus, writers should -try to ensure that all segments of an image look as much "alike" as -possible: there should be no variation in scan layout, use of options such -as DRI, etc. Ideally, segments will be processed identically except -perhaps for using different local quantization or entropy-coding tables. - -Writers should avoid including "noise" JPEG markers (COM and APPn markers). -Standard TIFF fields provide a better way to transport any non-image data. -Some JPEG codecs may change behavior if they see an APPn marker they -think they understand; since the TIFF spec requires these markers to be -ignored, this behavior is undesirable. - -It is possible to convert an interchange-JPEG file (e.g., a JFIF file) to -TIFF simply by dropping the interchange datastream into a single strip. -(However, designers are reminded that the TIFF spec discourages huge -strips; splitting the image is somewhat more work but may give better -results.) Conversion from TIFF to interchange JPEG is more complex. A -strip-based TIFF/JPEG file can be converted fairly easily if all strips use -identical JPEG tables and no RSTn markers: just delete the overhead markers -and insert RSTn markers between strips. Converting tiled images is harder, -since the data will usually not be in the right order (unless the tiles are -only one MCU high). This can still be done losslessly, but it will require -undoing and redoing the entropy coding so that the DC coefficient -differences can be updated. - -There is no default value for JPEGTables: standard TIFF files must define all -tables that they reference. For some closed systems in which many files will -have identical tables, it might make sense to define a default JPEGTables -value to avoid actually storing the tables. Or even better, invent a -private field selecting one of N default JPEGTables settings, so as to allow -for future expansion. Either of these must be regarded as a private -extension that will render the files unreadable by other applications. - - -References ----------- - -[1] Wallace, Gregory K. "The JPEG Still Picture Compression Standard", -Communications of the ACM, April 1991 (vol. 34 no. 4), pp. 30-44. - -This is the best short technical introduction to the JPEG algorithms. -It is a good overview but does not provide sufficiently detailed -information to write an implementation. - -[2] Pennebaker, William B. and Mitchell, Joan L. "JPEG Still Image Data -Compression Standard", Van Nostrand Reinhold, 1993, ISBN 0-442-01272-1. -638pp. - -This textbook is by far the most complete exposition of JPEG in existence. -It includes the full text of the ISO JPEG standards (DIS 10918-1 and draft -DIS 10918-2). No would-be JPEG implementor should be without it. - -[3] ISO/IEC IS 10918-1, "Digital Compression and Coding of Continuous-tone -Still Images, Part 1: Requirements and guidelines", February 1994. -ISO/IEC DIS 10918-2, "Digital Compression and Coding of Continuous-tone -Still Images, Part 2: Compliance testing", final approval expected 1994. - -These are the official standards documents. Note that the Pennebaker and -Mitchell textbook is likely to be cheaper and more useful than the official -standards. - - -Changes to Section 21: YCbCr Images -=================================== - -[This section of the Tech Note clarifies section 21 to make clear the -interpretation of image dimensions in a subsampled image. Furthermore, -the section is changed to allow the original image dimensions not to be -multiples of the sampling factors. This change is necessary to support use -of JPEG compression on odd-size images.] - -Add the following paragraphs to the Section 21 introduction (p. 89), -just after the paragraph beginning "When a Class Y image is subsampled": - - In a subsampled image, it is understood that all TIFF image - dimensions are measured in terms of the highest-resolution - (luminance) component. In particular, ImageWidth, ImageLength, - RowsPerStrip, TileWidth, TileLength, XResolution, and YResolution - are measured in luminance samples. - - RowsPerStrip, TileWidth, and TileLength are constrained so that - there are an integral number of samples of each component in a - complete strip or tile. However, ImageWidth/ImageLength are not - constrained. If an odd-size image is to be converted to subsampled - format, the writer should pad the source data to a multiple of the - sampling factors by replication of the last column and/or row, then - downsample. The number of luminance samples actually stored in the - file will be a multiple of the sampling factors. Conversely, - readers must ignore any extra data (outside the specified image - dimensions) after upsampling. - - When PlanarConfiguration=2, each strip or tile covers the same - image area despite subsampling; that is, the total number of strips - or tiles in the image is the same for each component. Therefore - strips or tiles of the subsampled components contain fewer samples - than strips or tiles of the luminance component. - - If there are extra samples per pixel (see field ExtraSamples), - these data channels have the same number of samples as the - luminance component. - -Rewrite the YCbCrSubSampling field description (pp 91-92) as follows -(largely to eliminate possibly-misleading references to -ImageWidth/ImageLength of the subsampled components): - - (first paragraph unchanged) - - The two elements of this field are defined as follows: - - Short 0: ChromaSubsampleHoriz: - - 1 = there are equal numbers of luma and chroma samples horizontally. - - 2 = there are twice as many luma samples as chroma samples - horizontally. - - 4 = there are four times as many luma samples as chroma samples - horizontally. - - Short 1: ChromaSubsampleVert: - - 1 = there are equal numbers of luma and chroma samples vertically. - - 2 = there are twice as many luma samples as chroma samples - vertically. - - 4 = there are four times as many luma samples as chroma samples - vertically. - - ChromaSubsampleVert shall always be less than or equal to - ChromaSubsampleHoriz. Note that Cb and Cr have the same sampling - ratios. - - In a strip TIFF file, RowsPerStrip is required to be an integer - multiple of ChromaSubSampleVert (unless RowsPerStrip >= - ImageLength, in which case its exact value is unimportant). - If ImageWidth and ImageLength are not multiples of - ChromaSubsampleHoriz and ChromaSubsampleVert respectively, then the - source data shall be padded to the next integer multiple of these - values before downsampling. - - In a tiled TIFF file, TileWidth must be an integer multiple of - ChromaSubsampleHoriz and TileLength must be an integer multiple of - ChromaSubsampleVert. Padding will occur to tile boundaries. - - The default values of this field are [ 2,2 ]. Thus, YCbCr data is - downsampled by default! -</pre> |