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| author | Qt by Nokia <qt-info@nokia.com> | 2011-04-27 12:05:43 +0200 |
|---|---|---|
| committer | axis <qt-info@nokia.com> | 2011-04-27 12:05:43 +0200 |
| commit | 38be0d13830efd2d98281c645c3a60afe05ffece (patch) | |
| tree | 6ea73f3ec77f7d153333779883e8120f82820abe /src/3rdparty/libtiff/html/addingtags.html | |
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Branched from the monolithic repo, Qt master branch, at commit
896db169ea224deb96c59ce8af800d019de63f12
Diffstat (limited to 'src/3rdparty/libtiff/html/addingtags.html')
| -rw-r--r-- | src/3rdparty/libtiff/html/addingtags.html | 292 |
1 files changed, 292 insertions, 0 deletions
diff --git a/src/3rdparty/libtiff/html/addingtags.html b/src/3rdparty/libtiff/html/addingtags.html new file mode 100644 index 0000000000..452821c2d0 --- /dev/null +++ b/src/3rdparty/libtiff/html/addingtags.html @@ -0,0 +1,292 @@ +<HTML> +<HEAD> +<TITLE> +Modifying The TIFF Library +</TITLE> +</HEAD> +<BODY BGCOLOR=white> +<FONT FACE="Arial, Helvetica, Sans"> + +<H1> +Defining New TIFF Tags +</H1> + +Libtiff has built-in knowledge of all the standard TIFF tags, as +well as extentions. The following describes how to add knowledge of +new tags as builtins to libtiff, or how to application specific tags can +be used by applications without modifying libtiff. +<p> + +<h2>TIFFFieldInfo</h2> + +How libtiff manages specific tags is primarily controlled by the +definition for that tag value stored internally as a TIFFFieldInfo structure. +This structure looks like this: +<p> + +<pre> +typedef struct { + ttag_t field_tag; /* field's tag */ + short field_readcount; /* read count/TIFF_VARIABLE/TIFF_SPP */ + short field_writecount; /* write count/TIFF_VARIABLE */ + TIFFDataType field_type; /* type of associated data */ + unsigned short field_bit; /* bit in fieldsset bit vector */ + unsigned char field_oktochange;/* if true, can change while writing */ + unsigned char field_passcount;/* if true, pass dir count on set */ + char *field_name; /* ASCII name */ +} TIFFFieldInfo; +</pre> + +<ul> +<li> <b>field_tag</b>: the tag number. For instance 277 for the +SamplesPerPixel tag. Builtin tags will generally have a #define in +tiff.h for each known tag. <p> + +<li> <b>field_readcount</b>: The number of values which should be read. +The special value TIFF_VARIABLE (-1) indicates that a variable number of +values may be read. The special value TIFFTAG_SPP (-2) indicates that there +should be one value for each sample as defined by TIFFTAG_SAMPLESPERPIXEL. +The special value TIFF_VARIABLE2 (-3) is presumably similar to TIFF_VARIABLE +though I am not sure what the distinction in behaviour is. This field +is TIFF_VARIABLE for variable length ascii fields.<p> + +<li> <b>field_writecount</b>: The number of values which should be written. +Generally the same as field_readcount. A few built-in exceptions exist, but +I haven't analysed why they differ. <p> + +<li> <b>field_type</b>: Type of the field. One of TIFF_BYTE, TIFF_ASCII, +TIFF_SHORT, TIFF_LONG, TIFF_RATIONAL, TIFF_SBYTE, TIFF_UNDEFINED, +TIFF_SSHORT, TIFF_SLONG, TIFF_SRATIONAL, TIFF_FLOAT, TIFF_DOUBLE or +TIFF_IFD. Note that some fields can support more than one type (for +instance short and long). These fields should have multiple TIFFFieldInfos. +<p> + +<li> <b>field_bit</b>: Built-in tags stored in special fields in the +TIFF structure have assigned field numbers to distinguish them (ie. +FIELD_SAMPLESPERPIXEL). New tags should generally just use +FIELD_CUSTOM indicating they are stored in the generic tag list.<p> + +<li> <b>field_oktochange</b>: TRUE if it is OK to change this tag value +while an image is being written. FALSE for stuff that must be set once +and then left unchanged (like ImageWidth, or PhotometricInterpretation for +instance).<p> + +<li> <b>field_passcount</b>: If TRUE, then the count value must be passed +in TIFFSetField(), and TIFFGetField(), otherwise the count is not required. +This should generally be TRUE for non-ascii variable count tags unless +the count is implicit (such as with the colormap).<p> + +<li> <b>field_name</b>: A name for the tag. Normally mixed case (studly caps) +like "StripByteCounts" and relatively short. <p> + +</ul> + +A TIFFFieldInfo definition exists for each built-in tag in the tif_dirinfo.c +file. Some tags which support multiple data types have more than one +definition, one per data type supported. <p> + +Various functions exist for getting the internal TIFFFieldInfo definitions, +including _TIFFFindFieldInfo(), and _TIFFFindFieldInfoByName(). See +tif_dirinfo.c for details. There must be some mechanism to get the whole +list, though I don't see it off hand.<p> + +<h2>Default Tag Auto-registration</h2> + +In libtiff 3.6.0 a new mechanism was introduced allowing libtiff to +read unrecognised tags automatically. When an unknown tags is encountered, +it is automatically internally defined with a default name and a type +derived from the tag value in the file. Applications only need to predefine +application specific tags if they need to be able to set them in a file, or +if particular calling conventions are desired for TIFFSetField() and +TIFFGetField().<p> + +When tags are autodefined like this the <b>field_readcount</b> and +<b>field_writecount</b> values are always TIFF_VARIABLE. The +<b>field_passcount</b> is always TRUE, and the <b>field_bit</b> is +FIELD_CUSTOM. The field name will be "Tag %d" where the %d is the tag +number.<p> + +<h2>Defining Application Tags</h2> + +For various reasons, it is common for applications to want to define +their own tags to store information outside the core TIFF specification. +This is done by calling TIFFMergeFieldInfo() with one or more TIFFFieldInfos. +<p> + +The libgeotiff library provides geospatial information extentions within +a TIFF file. First, a set of TIFFFieldInfo's is prepared with information +on the new tags:<p> + +<pre> +static const TIFFFieldInfo xtiffFieldInfo[] = { + + /* XXX Insert Your tags here */ + { TIFFTAG_GEOPIXELSCALE, -1,-1, TIFF_DOUBLE, FIELD_CUSTOM, + TRUE, TRUE, "GeoPixelScale" }, + { TIFFTAG_GEOTRANSMATRIX, -1,-1, TIFF_DOUBLE, FIELD_CUSTOM, + TRUE, TRUE, "GeoTransformationMatrix" }, + { TIFFTAG_GEOTIEPOINTS, -1,-1, TIFF_DOUBLE, FIELD_CUSTOM, + TRUE, TRUE, "GeoTiePoints" }, + { TIFFTAG_GEOKEYDIRECTORY, -1,-1, TIFF_SHORT, FIELD_CUSTOM, + TRUE, TRUE, "GeoKeyDirectory" }, + { TIFFTAG_GEODOUBLEPARAMS, -1,-1, TIFF_DOUBLE, FIELD_CUSTOM, + TRUE, TRUE, "GeoDoubleParams" }, + { TIFFTAG_GEOASCIIPARAMS, -1,-1, TIFF_ASCII, FIELD_CUSTOM, + TRUE, FALSE, "GeoASCIIParams" } +}; +</pre> + +In order to define the tags, we call TIFFMergeFieldInfo() on the +desired TIFF handle with the list of TIFFFieldInfos.<p> + +<pre> +#define N(a) (sizeof (a) / sizeof (a[0])) + + /* Install the extended Tag field info */ + TIFFMergeFieldInfo(tif, xtiffFieldInfo, N(xtiffFieldInfo)); +</pre> + +The tags need to be defined for each TIFF file opened - and when reading +they should be defined before the tags of the file are read, yet a valid +TIFF * is needed to merge the tags against. In order to get them +registered at the appropriate part of the setup process, it is necessary +to register our merge function as an extender callback with libtiff. +This is done with TIFFSetTagExtender(). We also keep track of the +previous tag extender (if any) so that we can call it from our extender +allowing a chain of customizations to take effect. <P> + +<pre> +static TIFFExtendProc _ParentExtender = NULL; + +static +void _XTIFFInitialize(void) +{ + static int first_time=1; + + if (! first_time) return; /* Been there. Done that. */ + first_time = 0; + + /* Grab the inherited method and install */ + _ParentExtender = TIFFSetTagExtender(_XTIFFDefaultDirectory); +} +</pre> + +The extender callback is looks like this. It merges in our new fields +and then calls the next extender if there is one in effect.<p> + +<pre> +static void +_XTIFFDefaultDirectory(TIFF *tif) +{ + /* Install the extended Tag field info */ + TIFFMergeFieldInfo(tif, xtiffFieldInfo, N(xtiffFieldInfo)); + + /* Since an XTIFF client module may have overridden + * the default directory method, we call it now to + * allow it to set up the rest of its own methods. + */ + + if (_ParentExtender) + (*_ParentExtender)(tif); +} +</pre> + +The above approach ensures that our new definitions are used when reading +or writing any TIFF file. However, since on reading we already have +default definitions for tags, it is usually not critical to pre-define them. +If tag definitions are only required for writing custom tags, you can just +call TIFFMergeFieldInfo() before setting new tags. The whole extender +architecture can then be avoided.<p> + +<A NAME=AddingTags><P><H2>Adding New Builtin Tags</H2></A> + +A similar approach is taken to the above. However, the TIFFFieldInfo +should be added to the tiffFieldInfo[] list in tif_dirinfo.c. Ensure that +new tags are added in sorted order by the tag number.<p> + +Normally new built-in tags should be defined with FIELD_CUSTOM; however, if +it is desirable for the tag value to have it's own field in the TIFFDirectory +structure, then you will need to #define a new FIELD_ value for it, and +add appropriate handling as follows: + + +<OL> +<LI>Define the tag in <B>tiff.h</B>. +<LI>Add a field to the directory structure in <B>tif_dir.h</B> + and define a <TT>FIELD_*</TT> bit (also update the definition of + <TT>FIELD_CODEC</TT> to reflect your addition). +<LI>Add an entry in the <TT>TIFFFieldInfo</TT> array defined at the top of + <B>tif_dirinfo.c</B>. + Note that you must keep this array sorted by tag + number and that the widest variant entry for a tag should come + first (e.g. <TT>LONG</TT> before <TT>SHORT</TT>). +<LI>Add entries in <TT>_TIFFVSetField()</TT> and <TT>_TIFFVGetField()</TT> + for the new tag. +<LI>(<I>optional</I>) If the value associated with the tag is not a scalar value + (e.g. the array for <TT>TransferFunction</TT>) and requires + special processing, + then add the appropriate code to <TT>TIFFReadDirectory()</TT> and + <TT>TIFFWriteDirectory()</TT>. You're best off finding a similar tag and + cribbing code. +<LI>Add support to <TT>TIFFPrintDirectory()</TT> in <B>tif_print.c</B> + to print the tag's value. +</OL> + +<P> +If you want to maintain portability, beware of making assumptions +about data types. Use the typedefs (<TT>uint16</TT>, etc. when dealing with +data on disk and <TT>t*_t</TT> when stuff is in memory) and be careful about +passing items through printf or similar vararg interfaces. + +<A NAME=AddingCODECTags><P><H2>Adding New Codec-private Tags</H2></A> + +To add tags that are meaningful <EM>only when a particular compression +algorithm is used</EM> follow these steps: + +<OL> +<LI>Define the tag in <B>tiff.h</B>. +<LI>Allocate storage for the tag values in the private state block of + the codec. +<LI>Insure the state block is created when the codec is initialized. +<LI>At <TT>TIFFInitfoo</TT> time override the method pointers in the + TIFF structure + for getting, setting and printing tag values. For example, +<PRE> + sp->vgetparent = tif->tif_vgetfield; + tif->tif_vgetfield = fooVGetField; /* hook for codec tags */ + sp->vsetparent = tif->tif_vsetfield; + tif->tif_vsetfield = fooVSetField; /* hook for codec tags */ + tif->tif_printdir = fooPrintDir; /* hook for codec tags */ +</PRE> + (Actually you may decide not to override the + <TT>tif_printdir</TT> method, but rather just specify it). +<LI>Create a private <TT>TIFFFieldInfo</TT> array for your tags and + merge them into the core tags at initialization time using + <TT>_TIFFMergeFieldInfo</TT>; e.g. +<PRE> + _TIFFMergeFieldInfo(tif, fooFieldInfo, N(fooFieldInfo)); +</PRE> + (where <TT>N</TT> is a macro used liberaly throughout the distributed code). +<LI>Fill in the get and set routines. Be sure to call the parent method + for tags that you are not handled directly. Also be sure to set the + <TT>FIELD_*</TT> bits for tags that are to be written to the file. Note that + you can create ``pseudo-tags'' by defining tags that are processed + exclusively in the get/set routines and never written to file (see + the handling of <TT>TIFFTAG_FAXMODE</TT> in <B>tif_fax3.c</B> + for an example of this). +<LI>Fill in the print routine, if appropriate. +</OL> + +Note that space has been allocated in the <TT>FIELD_*</TT> bit space for +codec-private tags. Define your bits as <TT>FIELD_CODEC+<offset></TT> to +keep them away from the core tags. If you need more tags than there +is room for, just increase <TT>FIELD_SETLONGS</TT> at the top of +<B>tiffiop.h</B>. + +<HR> + +Last updated: $Date: 2004/09/10 14:43:18 $ + +</BODY> + +</HTML> |