Ghostscript's external font and text interface

Table of contents

For other information, see the Ghostscript overview.


Introduction

Ghostscript can use the character rasterizer provided by the underlying operating system and window system; specifically, Adobe Type Manager (ATM) or a TrueType rasterizer under MS Windows, or the facilities provided by X Windows. This ability augments, but does not replace, Ghostscript's own Type 1 rasterizer: Ghostscript may still use its own rasterizer for very large characters, characters that are clipped or transformed in unusual ways, and for output to devices other than the screen.

Ghostscript connects to these platform facilities through a driver-like interface called the xfont (external font) interface. Current xfont implementations are associated directly with device drivers; in a future release, Ghostscript may separate them, so that (for example) it will be possible to use the platform rasterizer when writing to a file.

Please note that from this point, this file is likely to be useful only to a small number of Ghostscript porters and implementors.


Types

Type    Declared /
defined in
  Represents

gs_char   gsccode.h   A character code that appears in a string. Currently it is always a single byte, but composite fonts or Unicode may require it to be wider in the future.
gs_glyph   gsccode.h   A character name like "period" or "epsilon". From the xfont implementation's point of view, it is just a handle; when necessary, Ghostscript provides a gs_proc_glyph_name_t procedure to convert it to a string name.
gs_proc_glyph_name_t   gsccode.h   A procedure that maps a gs_glyph to its string name; see the char_xglyph procedure.
gx_xglyph   gsxfont.h   A character or glyph code that can be used with a specific platform font. Normally it will be a character code that the implementation of render_char will turn into a 1-character string and give to the platform's "display string" operation.
gx_xfont_procs   gsxfont.h, gxxfont.h   The xfont analogue of gx_device_procs, the type of the procedure record that defines an xfont implementation.
gx_xfont   gsxfont.h, gxxfont.h   The gxfont analogue of gx_device, the type of the basic structure for an xfont.
(encoding_index)       Not really a type, although it probably should be: an int used to indicate the Encoding used by a font. Defined values are
0
StandardEncoding
1
ISOLatin1Encoding
2
SymbolEncoding
3
DingbatsEncoding
-1
Other encoding

Implementation procedures

All the procedures that return int results return 0 on success, or an appropriate negative error code for error conditions; these error codes are defined in gserrors.h. The relevant ones are the same as for drivers, and as with drivers, if an implementation procedure returns an error, it should use the return_error macro -- defined in gx.h, which is automatically included by gdevprn.h but not by gserrors.h -- rather than a simple return statement, for instance

return_error(gs_error_VMerror);

Font-level procedures

lookup_font

gx_xfont *(*lookup_font)(P7(gx_device *dev, const byte *fname, uint len, int encoding_index, const gs_uid *puid, const gs_matrix *pmat, const gs_memory_procs *mprocs))
Look up a font name, UniqueID, and matrix, and return an xfont, or NULL if no suitable xfont exists. Use mprocs to allocate the xfont and any subsidiary data structures. The matrix is the FontMatrix concatenated with the CTM, so (roughly speaking) the font size in pixels is pmat -> yy × 1000 for a normal Type 1 font.

Note that this is the only implementation procedure that does not take an xfont * as its first argument. In fact, callers of lookup_font must use the get_xfont_device driver procedure to get the correct device to pass as the first argument to lookup_font.

char_xglyph

gx_xglyph (*char_xglyph)(P5(gx_xfont *xf, gs_char chr, int encoding_index, gs_glyph glyph, gs_proc_glyph_name_t glyph_name))
Convert a character name to an xglyph code. In the case of glyphshow, chr may be gs_no_char; for an ordinary show operation, if the character code is invalid, glyph may be gs_no_glyph.

char_metrics

int (*char_metrics)(P5(gx_xfont *xf, gx_xglyph xg, int wmode, gs_int_point *pwidth, gs_int_rect *pbbox))
Get the metrics for a character. If the metrics are unavailable, return 1.

render_char

int (*render_char)(P7(gx_xfont *xf, gx_xglyph xg, gx_device *target, int x, int y, gx_color_index color, int required))
Render a character. (x,y) corresponds to the character origin. The target may be any Ghostscript device. A good implementation will check whether the target can handle this type of xfont directly (for instance by checking the target name), and if so, will render the character directly; otherwise, it will do what has to be done in the general case, namely, get a bitmap for the character and use the target's copy_mono operation. If required is false, the procedure should return an error if the rendering operation would be expensive, since in this case Ghostscript has already cached the bitmap and metrics from a previous call with required=true. If the operation cannot be done, return 1.

release

int (*release)(P2(gx_xfont *xf, const gs_memory_procs *mprocs))
Release any external resources associated with an xfont. If mprocs is not NULL, also free any storage allocated by lookup_font (including the xfont itself).

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Ghostscript version 8.63, 1 August 2008