libwmf-0.2.0 Tutorial 1: The API
Contents
#introIntroduction #headersHeader Files #apiCreating The API #devinitAcknowledging The Device Layer #scanScanning The Metafile #devsetupSetting Up The Device Layer #playPlaying The Metafile #fontFonts & libwmf #fnsOther 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: wmf2epsand 
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_ALLOCIf you wish to provide your own memory allocation routines for 
libwmfto 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_ARGSlibwmf 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_FONTDIRSA 
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_FONTSUse 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_FONTSUse 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_FUNCTIONThis 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_ERRORSuppress all error reports, equivalent to --wmf-error=no
	flags |= WMF_OPT_NO_ERROR;
WMF_OPT_NO_DEBUGSuppress 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 ghostscriptfonts. 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 notrecommended.
Copyright 2001 wvWare/libwmf 
http://www.wvware.com/http://www.wvware.com/ 