libwmf-0.2.0 Tutorial 1: The API
Contents
#intro
Introduction
#headers
Header Files
#api
Creating The
API
#devinit
Acknowledging The Device Layer
#scan
Scanning The Metafile
#devsetup
Setting Up The Device Layer
#play
Playing The Metafile
#font
Fonts & libwmf
#fns
Other
API
Functions
Introduction
The figure below illustrates the tripartite division of
application
,
interpreter
, and
device layer
. The internal workings of each
of the three is irrelevant; all that matters is the three interfaces which
are separate. The core of
libwmf
is the interpreter, and as such
only the interfaces between (a) the interpreter and the application - the
API
- and (b) the interpreter and the device layer - the
IPA
- are defined. Applications that wish to implement their own new device
layer are free therefore to define whatever interface is suitable.
libwmf
includes two device layers:
eps
for conversion to
EPS
(Encapsulated Postscript) or
PS
(Postscript); and
gd
for conversion to
PNG
or
JPEG
image formats.
Included with
libwmf
are converters for these:
wmf2eps
and
wmf2gd
respectively. The source-code for these is an excellent
place to see how to use the various features of the
API
and device-layer
interfaces.
Header Files
All libwmf header files are now in a
libwmf
subdirectory. In general
you will need to include two. Unless you are writing a new device layer, which
will be discussed later, you should include only
libwmf/api.h
and the
header for whichever device layer you are interested in, for example:
#include <libwmf/api.h>
#include <libwmf/gd.h>
The
gd
device layer uses the
GD
library, included with
libwmf
, to output images in
PNG
(or possibly in
JPEG
)
format. Alternatively, if you wish the output to be in
PS
or
EPS
format, you would include:
#include <libwmf/api.h>
#include <libwmf/eps.h>
Creating The API
For each metafile you wish to process and convert to a given image type, you
must create an instance of the
API
. This can be sophisticated or simple.
In its simplest form, the only thing that needs to be specified is which
device layer you wish to use. For example, to use
GD
:
wmfAPI_Options options;
wmfAPI* API;
wmf_error_t error;
unsigned long flags;
/* */
flags = WMF_OPT_FUNCTION;
options.function = wmf_gd_function;
/* Other Options */
error = wmf_api_create (&API,flags,&options);
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
/* */
The library performs all memory allocation with respect to the
API
, and
destroying the instance will free up all associated memory:
error = wmf_api_destroy (API);
There are a number of different options which can be used when creating the
API
. The structure of
wmfAPI_Options
is:
typedef struct _wmfAPI_Options wmfAPI_Options;
struct _wmfAPI_Options
{	void* context;
void* (*malloc)  (void* context,size_t size);
void* (*realloc) (void* context,void* mem,size_t size);
void  (*free)    (void* context,void* mem);
int    argc;
char** argv;
char** fontdirs; /* NULL-terminated list of directories to search for font files */
struct
{	wmfFontMap* wmf; /* {0,*}-terminated list: wmf-font-name -> ps-font-name */
wmfMapping* sub; /* {0,*}-terminated list: wmf-font-name substring equiv */
wmfMapping* ps;  /* {0,*}-terminated list: ps-font-name -> pfb-file-name */
} font;
char* sys_fontmap_file;
char* xtra_fontmap_file;
void (*function) (wmfAPI*);
char*  module;
char** dirs;
FILE* debug_out;
FILE* error_out;
};
WMF_OPT_ALLOC
If you wish to provide your own memory allocation routines for
libwmf
to use instead of
malloc
,
realloc
and
free
, these
can be specified:
flags |= WMF_OPT_ALLOC;
options.context = my_memory_manager_data;
options.malloc  = my_malloc;
options.realloc = my_realloc;
options.free    = my_free;
WMF_OPT_ARGS
libwmf
can be allowed to check for certain command line arguments:
--wmf-ignore-nonfatal     ignore any non-critical errors or unsupported features
--wmf-error=no            suppress error reports
--wmf-debug=no            suppress debug reports
--wmf-diagnostics         emit diagnostic information (for analysis of metafiles)
--wmf-fontdir=<path>      add <path> to list of directories searched for fonts
--wmf-sys-fonts           use system fonts, if any found
--wmf-sys-fontmap=<file>  use system xml-fontmap file <file> (implies --wmf-sys-fonts)
--wmf-xtra-fonts          use non-system fonts, if any found
--wmf-xtra-fontmap=<file> use non-system xml-fontmap file <file> (implies --wmf-xtra-fonts)
where the command line is specified by:
int main (int argc,char** argv)
{	/* */
flags |= WMF_OPT_ARGS;
options.argc = argc;
options.argv = argv;
WMF_OPT_FONTDIRS
A
NULL
-terminated list of directories to be searched for fonts can be
specified by:
char* dirs[3] = { "/usr/share/fonts", "/usr/local/share/fonts", NULL };
/* */
flags |= WMF_OPT_FONTDIRS;
options.fontdirs = dirs;
WMF_OPT_FONTMAP
[UNTESTED]
This is a system for specifying font mappings; see section on fonts.
WMF_OPT_SYS_FONTS
Use any system fonts found, not just standard 13.
WMF_OPT_SYS_FONTMAP
[UNTESTED]
Use specified system XML fontmap file:
flags |= WMF_OPT_SYS_FONTS;
flags |= WMF_OPT_SYS_FONTMAP;
options.sys_fontmap_file = "/usr/local/share/fonts/fontmap";
WMF_OPT_XTRA_FONTS
Use any system fonts found, not just standard 13.
WMF_OPT_XTRA_FONTMAP
[UNTESTED]
Use specified non-system XML fontmap file:
flags |= WMF_OPT_XTRA_FONTS;
flags |= WMF_OPT_XTRA_FONTMAP;
options.xtra_fontmap_file = "/usr/local/share/libwmf/fonts/fontmap";
WMF_OPT_FUNCTION
This is how to initialize a device-layer with a supplied function. (Currently
this is the only way to initialize a device layer since modules are not yet
implemented.) The function is device-specific. See above.
WMF_OPT_MODULE
[UNIMPLEMENTED]
initialize device-layer with specified module
WMF_OPT_MODULE_DIRS
[UNIMPLEMENTED]
check for module also in specified directories
WMF_OPT_IGNORE_NONFATAL
[HIGHLY RECOMMENDED]
Ignore (some) non-fatal errors, equivalent to --wmf-ignore-nonfatal
flags |= WMF_OPT_IGNORE_NONFATAL;
WMF_OPT_NO_ERROR
Suppress all error reports, equivalent to --wmf-error=no
flags |= WMF_OPT_NO_ERROR;
WMF_OPT_NO_DEBUG
Suppress all debug reports, equivalent to --wmf-debug=no
flags |= WMF_OPT_NO_DEBUG;
WMF_OPT_LOG_ERROR
[UNTESTED]
Divert error reports to specified stream:
FILE* e_out;
/* */
e_out = fopen ("error.log","w");
if (e_out)
{	flags |= WMF_OPT_LOG_ERROR;
options.error_out = e_out;
}
WMF_OPT_LOG_DEBUG
[UNTESTED]
Divert debug reports to specified stream:
FILE* d_out;
/* */
d_out = fopen ("debug.log","w");
if (d_out)
{	flags |= WMF_OPT_LOG_DEBUG;
options.debug_out = d_out;
}
Acknowledging The Device Layer
In addition to creating the
API
,
wmf_api_create
sets up the
link with the device layer. Device layer data can be accessed through
API->device_data
, but the implementation will probably provide a
convenient interface. For example, with
GD
:
wmfAPI_Options options;
wmfAPI* API;
wmf_error_t error;
wmf_gd_t* ddata;
unsigned long flags;
/* */
flags = WMF_OPT_FUNCTION | WMF_OPT_IGNORE_NONFATAL;
options.function = wmf_gd_function;
/* Other Options */
error = wmf_api_create (&API,flags,&options);
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
ddata = WMF_GD_GetData (API);
if ((ddata->flags & WMF_GD_SUPPORTS_JPEG) == 0)
{	fprintf (stderr,"libwmf does not support JPEG conversion!\n");
wmf_api_destroy (API);
/* */
}
/* */
At this point there is little need to acknowledge the device layer, except if
(as in the above example) the device layer indicates possible restrictions.
Scanning The Metafile
Before
playing
the metafile (i.e., rendering the image) it is necessary
first to open the metafile and second to
scan
it. To open a metafile
that is a file on disc is simple:
error = wmf_file_open (API,"file.wmf");
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
The file needs to be closed later by calling:
error = wmf_file_close (API);
Alternatively, if the metafile is preloaded in memory, it can be opened with:
error = wmf_mem_open (API,(unsigned char*) buffer,(long) buffer_length);
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
and closed later by calling:
error = wmf_mem_close (API);
A third, more generic method is available as well, but requires more work. For
example, to `open' an already-open file stream:
int  my_wmfRead (void*);
int  my_wmfSeek (void*,long);
long my_wmfTell (void*);
/* User redefinable functions for input-stream access:
* wmfRead: returns unsigned char cast to int, or EOF
*/
int my_wmfRead (void* context)
{	return (fgetc ((FILE*) context));
}
/* User redefinable functions for input-stream access:
* wmfSeek: returns (-1) on error, else 0
*/
int my_wmfSeek (void* context,long position)
{	return (fseek ((FILE*) context,position,SEEK_SET));
}
/* User redefinable functions for input-stream access:
* wmfTell: returns (-1) on error, else pos
*/
long my_wmfTell (void* context)
{	return (ftell ((FILE*) context));
}
/* */
error = wmf_bbuf_input (API,my_wmfRead,my_wmfSeek,my_wmfTell,(void*) in);
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
Once the metafile has been opened, it must be scanned. Scanning parses the
metafile without any interaction with the device layer, and its purpose is
(a) determine whether it is a valid metafile, and (b) estimate the dimensions
of the metafile.
wmfD_Rect bbox;
/* */
error = wmf_scan (API,0,&bbox);
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
A metafile needs to be scanned, but only once.
wmf_scan
will reject
any further attempts to scan;
wmf_play
(see below) will fail unless
the metafile has been scanned.
Setting Up The Device Layer
Metafiles are basically a vector-graphic format and as such they can be scaled
to any size and aspect ratio. The interpreter describes the metafile in a
scaleable manner to the device layer, and the device layer is free to scale
the metafile in any manner it chooses. As such, the device layer will almost
certainly require (a) the dimensions of the metafile found by
wmf_scan
(see above), and (b) the image width and height desired by the application.
wmfD_Rect
describes a bounding box in the interpreter's device
coordinates, and is defined as:
typedef struct _wmfD_Coord wmfD_Coord;
typedef struct _wmfD_Rect  wmfD_Rect;
struct _wmfD_Coord
{	float x;
float y;
};
struct _wmfD_Rect
{	wmfD_Coord TL;
wmfD_Coord BR;
};
and so an application can determine the image's aspect ratio by:
float aspect_ratio;
/* */
aspect_ratio = (bbox.BR.y - bbox.TL.y) / (bbox.BR.x - bbox.TL.x);
Note: By definition bbox.BR.y > bbox.TL.y and bbox.BR.x > bbox.TL.x
Putting this all together, and again using
GD
as an example:
wmfAPI_Options options;
wmfAPI* API;
wmfD_Rect bbox;
wmf_error_t error;
wmf_gd_t* ddata;
unsigned long flags;
FILE* out;
/* */
flags = WMF_OPT_FUNCTION | WMF_OPT_IGNORE_NONFATAL;
options.function = wmf_gd_function;
/* Other Options */
error = wmf_api_create (&API,flags,&options);
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
ddata = WMF_GD_GetData (API);
if ((ddata->flags & WMF_GD_SUPPORTS_JPEG) == 0)
{	fprintf (stderr,"libwmf does not support JPEG conversion!\n");
wmf_api_destroy (API);
/* */
}
error = wmf_file_open (API,"file.wmf");
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
error = wmf_scan (API,0,&bbox);
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
out = fopen ("file.jpeg","w");
if (out == 0)
{	wmf_api_destroy (API);
/* */
}
ddata->type = wmf_gd_jpeg;
ddata->flags |= WMF_GD_OUTPUT_FILE;
ddata->file = out;
ddata->bbox = bbox;
ddata->width  = (unsigned int) ceil (ddata->bbox.BR.x - ddata->bbox.TL.x);
ddata->height = (unsigned int) ceil (ddata->bbox.BR.y - ddata->bbox.TL.y);
The above example doesn't do anything intelligent with the size of the image.
The converter
wmf2gd
sets a maximum size and scales the image
accordingly.
Playing The Metafile
The final step is simply to play the metafile:
error = wmf_play (API,0,0);
if (error != wmf_E_None)
{	wmf_api_destroy (API);
/* */
}
This can be done any number of times, with varying device layer settings if
desired.
Fonts & libwmf
There are a number of issues with fonts and
libwmf
to complicate
matters: the first is the need to translate font names specified into font
names recognized by
libwmf
; the second is to find the corresponding
font files; a third is to work out what encoding (char. set) to use. Described
below is
libwmf
's native mechanism for dealing with these issues.
While versatile, unfortunately it requires significant effort on the part of an
application that seeks to extend the limited font mapping provided by default.
A second (experimental and optional) mechanism is provided also, and this reads
an XML file containing font information (file locations and font names); on
RedHat Linux systems this file is
/usr/share/fonts/fontmap
. No doubt
this is an area that will see considerable future development.
Chances are, the metafile will require fonts that are not available to
libwmf
, and there must be a mechanism, therefore, for translating
metafile fontnames into usable fonts. As noted above, the
wmfAPI_Options
structure passed to
wmf_api_create
has the
following entries:
typedef struct _wmfAPI_Options wmfAPI_Options;
struct _wmfAPI_Options
{	/* */
struct
{	wmfFontMap* wmf; /* {0,*}-terminated list: wmf-font-name -> ps-font-name */
wmfMapping* sub; /* {0,*}-terminated list: wmf-font-name substring equiv */
wmfMapping* ps;  /* {0,*}-terminated list: ps-font-name -> pfb-file-name */
} font;
/* */
}
The two mapping types are defined:
typedef struct _wmfMapping             wmfMapping;
typedef struct _wmfFontMap             wmfFontMap;
struct _wmfMapping
{	char* name;
char* mapping;
FT_Encoding encoding;
};
struct _wmfFontMap
{	char* name;       /* wmf font name */
char* normal;     /* postscript font names */
char* italic;
char* bold;
char* bolditalic;
};
Internally
libwmf
has the following default definitions:
/* postscript (standard 13) font name mapping to type 1 font file
*/
static wmfMapping PSFontMap[13] = {
{	"Courier",		"n022003l.pfb",	ft_encoding_adobe_standard	},
{	"Courier-Oblique",	"n022023l.pfb",	ft_encoding_adobe_standard	},
{	"Courier-Bold",		"n022004l.pfb",	ft_encoding_adobe_standard	},
{	"Courier-BoldOblique",	"n022024l.pfb",	ft_encoding_adobe_standard	},
{	"Helvetica",		"n019003l.pfb",	ft_encoding_adobe_standard	},
{	"Helvetica-Oblique",	"n019023l.pfb",	ft_encoding_adobe_standard	},
{	"Helvetica-Bold",	"n019004l.pfb",	ft_encoding_adobe_standard	},
{	"Helvetica-BoldOblique","n019024l.pfb",	ft_encoding_adobe_standard	},
{	"Times-Roman",		"n021003l.pfb",	ft_encoding_adobe_standard	},
{	"Times-Italic",		"n021023l.pfb",	ft_encoding_adobe_standard	},
{	"Times-Bold",		"n021004l.pfb",	ft_encoding_adobe_standard	},
{	"Times-BoldItalic",	"n021024l.pfb",	ft_encoding_adobe_standard	},
{	"Symbol",		"s050000l.pfb",	ft_encoding_adobe_custom	}};
/* exact WMF font name to postscript (standard 13) equivalent...
* well, yeah, I know. :-(
*/
static wmfFontMap WMFFontMap[] = {
{ "Courier",		"Courier",     "Courier-Oblique",   "Courier-Bold",   "Courier-BoldOblique"   },
{ "Helvetica",		"Helvetica",   "Helvetica-Oblique", "Helvetica-Bold", "Helvetica-BoldOblique" },
{ "Modern",		"Courier",     "Courier-Oblique",   "Courier-Bold",   "Courier-BoldOblique"   },
{ "Monotype Corsiva",	"Courier",     "Courier-Oblique",   "Courier-Bold",   "Courier-BoldOblique"   },
{ "News Gothic",	"Helvetica",   "Helvetica-Oblique", "Helvetica-Bold", "Helvetica-BoldOblique" },
{ "Symbol",		"Symbol",      "Symbol",            "Symbol",         "Symbol"                },
{ "System",		"Courier",     "Courier-Oblique",   "Courier-Bold",   "Courier-BoldOblique"   },
{ "Times",		"Times-Roman", "Times-Italic",      "Times-Bold",     "Times-BoldItalic"      }};
/* Sub-string match if not in the above list;
*/
static wmfMapping SubFontMap[] = {
{	"Arial",		"Helvetica"	},
{	"Courier",		"Courier"	},
{	"Fixed",		"Courier"	},
{	"Helvetica",		"Helvetica"	},
{	"Sans",			"Helvetica"	},
{	"Sym",			"Symbol"	},
{	"Terminal",		"Courier"	},
{	"Times",		"Times"		},
{	"Wingdings",		"Symbol"	}};
/* If all else fails, assume Times
*/
static char* DefaultFontMapping = "Times";
Bundled with
libwmf
are the thirteen standard
ghostscript
fonts. Unless the application specifies other fonts,
libwmf
will
attempt to map the metafile's font name to one of these default fonts.
First an exact match is sought with the first argument of each font in
WMFFontMap
. For example, "News Gothic" would be an exact match;
"Arial" would not. If there is no exact match, then a substring search is
performed against the first argument of each font in
SubFontMap
.
For example, this time "Arial" would match; "Zapf Chancery" would not. If
there is still no exact match then "Times" is assumed.
Once a font name is found, this is converted to an equivalent name, assumed to
be the postscript font name, using
WMFFontMap
. And finally the
equivalent name is searched for in
PSFontMap
. For example,
"Zapf Chancery" in italics would be mapped to "Times" in italics, then to
"Times-Italic", and finally to "n021023l.pfb".
libwmf
searches through the list of font directories for this file
and loads it using
freetype (2)
with the encoding specified by the
third argument of the
PSFontMap
entry.
These three lists can be prepended by using the
WMF_OPT_FONTMAP
option.
The font files can be any format supported by
freetype (2)
.
After the metafile has been scanned,
API->fonts
is a
NULL
-terminated list of (postscript) font names corresponding to the
fonts selected by the interpreter.
Other API Functions
libwmf
performs all memory allocation with respect to the
API
.
When the
API
is destroyed, all associated memory is freed. The routines
for allocating and freeing such memory are:
extern void* wmf_malloc (wmfAPI*,size_t);
extern void* wmf_calloc (wmfAPI*,size_t,size_t);
extern void* wmf_realloc (wmfAPI*,void*,size_t);
extern void  wmf_free (wmfAPI*,void*);
These behave similarly to the standard
malloc
, etc. Similarly:
extern char* wmf_strdup (wmfAPI*,char*);
is an internal implementation of
strdup
which allocates the copied
string w.r.t. the
API
. On a related note,
extern char* wmf_strstr (const char*,const char*);
is an internal implementation of
strstr
and may be just a wrapper for
it. This is a special case of a function which does not refer to the
API
.
Messages can be added to the
error
and
debug
streams using:
extern void wmf_error (wmfAPI* API,char* srcfilename,int linenumber,char* message);
extern void wmf_debug (wmfAPI* API,char* srcfilename,int linenumber,char* message);
extern void wmf_assert (wmfAPI* API,char* srcfilename,int linenumber);
but these functions should be used via the macros
WMF_ERROR(API,"mesg")
,
WMF_DEBUG(API,"mesg")
, and
WMF_ASSERT(API,<expr>)
.
Finally, a call-back function can be set. This is called after each metafile
record with an estimate of the proportion of the metafile read; if the return
value is non-zero then the player will exit prematurely.
typedef int (*wmfStatus) (void* context,float proportion);
extern void wmf_status_function (wmfAPI* API,void* context,wmfStatus function);
For example, the following status function is used by
wmf2gd
:
int wmf2gd_status (void* context,float p)
{	int percent;
percent = (int) (p * 100);
if (percent
99) percent = 99;
fprintf (stderr,"%2d%%\b\b\b",percent);
}
Although there are other functions in the
API
, their use is
not
recommended.
Copyright 2001 wvWare/libwmf
http://www.wvware.com/
http://www.wvware.com/
