TIFF CHANGE INFORMATION
This document describes the changes made to the software between the
previous and current versions (see above).
If you don't find something listed here, then it was not done in this
timeframe, or it was not considered important enough to be mentioned.
The following information is located here:
MAJOR CHANGES:
BigTIFF support changes:
- The options parameter in the TIFFOpen and TIFFClientOpen funcs has been
extended. When creating new files, you can add option '4' to specify you
want to create a ClassicTIFF file, though that is the default and the option
is not strictly necessary. (As such, old calling code will continue to
function and create ClassicTIFF files.) Or you can add option '8' to specify
you want to create a BigTIFF file instead. This new option is also reflected
in some of the tools we already upgraded. For instance, you can use the -8
option on tiffcp to have tiffcp produce BigTIFF files instead of the default
ClassicTIFF. (Whilst on additional option is provided for version selection when
creating new files, no such option is necessary when reading TIFF files.
LibTiff reads ClassicTIFF and BigTIFF both, and the application does not
need to be aware which TIFF version an opened file is.)
- Allthough the tag count in BigTIFF is 64bit, we restricted the count in
the implementation to a much more reasonable size. This is necessary in
current implementation, because all tag data gets read automatically in the
IFD reading stage, so if there's half a dozen private tags with multiple
gigabytes of data that causes considerable overhead even if the application
level is never interested in these tags. Our choice to ignore tags with data
longer then a certain sanity value is much needed as things stand. We also
recommend to step away from writing tiles that are 8 kilobyte in their
uncompressed form, or writing single-line strips, in really big files,
resulting in mega's of tiles or strips. It's much more efficient to choose
bigger tile or strip sizes, up to several megabyte if needed, and have a few
kilo of tiles or strips instead.
- Allthough it's rare, some application code does directly access file
offsets. Some of these are automatically upgraded because they used the
toff_t type, others need to be aware that the datatype changed and need to
start using toff_t or uint64. This impacts access to tags like the EXIF IFD
tag, for example, or the SubIfds tag, or to StripOffsets or TileOffsets,
the return type of functions like TIFFCurrentDirOffset, and a parameter type to
functions like TIFFSetSubDirectory.
- Allthough it's rare, some application code does use structures like
TIFFHeader or TIFFDirEntry that used to be an exact binary representation of
TIFF structures. These need to change. The old TIFFHeader structure is
replaced by the new TIFFHeaderClassic, TIFFHeaderBig, and TIFFHeaderCommon
structures that are an exact binary representation of the ClassicTIFF and
BigTIFF header, and of the part that is common to both. There is no new
equivalent for the old TIFFDirEntry structure (or more precisely, there is
still a TIFFDirEntry structure, but it is changed, moved to library-private
definition, and no longer an exact binary representation of the tag
structure of either TIFF version).
- Sizer functions, like TIFFTileSize or TIFFScanlineSize and the like,
return a tmsize_t value (tmsize_t is defined as int32 on 32bit machines,
and int64 on 64bit machines, and as such it is meant to represent signed
memory sizes). This is because we figure 98% of the calling code
uses the return value as sizes in allocations and the like. So, any overflow
that is theoretically possible with BigTIFF when LibTiff is running on a
32bit system, is best detected inside the sizer functions and it is best to
return a type that makes sense as a memory size. If your calling code is the
exception and is interested in actual file size, you best use the newer
TIFFTileSize64 or TIFFScanlineSize64 function that returns an uint64 type.
Other important backward incompatible changes in the public API:
- TIFFReassignTagToIgnore() function and TIFFIgnoreSense enumeration
have been removed. They was unused and never been used properly.
Should be unneeded for high-level applications.
- TIFFTagValue structure removed from the public tiffio.h
to private tif_dir.h and not accessible anymore. It should be unneeded
for high-level applications.
CHANGES IN THE SOFTWARE CONFIGURATION:
CHANGES IN LIBTIFF:
- There is considerable change in some files
like tif_dirread and tif_dirwrite. These changes don't impact backwards
compatibility, they are mostly a clean rewrite that does allow BigTIFF
support as well as somewhat more
robust reading of the unexpected already and will also serve future API
extension but does not impact current API or functionality in a negative way
that you need to know about.
- Allthough there is still a functional definition for types like toff_t
(file offset), tstrip_t (strip index number), etc, we recommend against
using these in newer code. We have learned that it is next to impossible to
use these consistently and make real abstraction of the binary format of
these types. Instead, at a certain level we always end up doing casts
anyway, and taking the exact binary format into account, so these types are
nothing but dangerously misleading and obfuscating. You do not need to
update calling code that uses them, as 99.9% of such code will continue to
work. But we recommend against using them in newer calling code, and we
started replacing them with binary clear types like uint16, uint32 and such
in the library.
- We do use and will continue to use one functional type that is an
exception to the above rule, being tmsize_t. This is a signed memory size
type, i.e. it is int32 on 32bit machines, or int64 on 64bit machines.
CHANGES IN THE TOOLS:
CHANGES IN THE CONTRIB AREA:
Last updated $Date: 2007-07-05 14:15:30 $.