Most of the functionality in systemfonts is intended to be used from compiled code to help e.g. graphic devices to resolve font specifications to a font file prior to rendering. systemfonts provide key functionality to get called at the C level by putting systemfonts in the
LinkingTo field in the description and adding
#include <systemfonts.h> to your C code. Make sure systemfonts is loaded before using it, e.g. by having
match_font() imported into your package namespace. The different functionality will be discussed below
The C equivalent of the
match_font() R function is
locate_font() with the following signature:
It takes a UTF-8 encoded string with the font family name, an int setting both italic and bold styles along with a char pointer to be filled with the located path and the maximum length it can hold. The return value is an int giving the index of the font in the font file.
With the advent of systemfonts 0.3.0 fonts can now also have OpenType features attached to them through the use of
register_variant(). If you wish to support such features you can use an alternative to the above:
FontSettings struct will contain both the font location and index along with any OpenType feature settings. The struct (along with its
FontFeature struct dependency) is shown below and is pretty self-documenting.
Do not cache the
FontSettings struct as the
features array may be cleared at any time after the call has ended. systemfonts itself takes care of caching so this is not something you should be concerned with in your code.
The C equivalent of
glyph_metrics() with the following signature:
It takes the glyph to measure as an int giving the UTF code of the glyph, along with a fontfile and index to identify the font to measure with. Further it takes a size in pt and a resolution in ppi. It will write the ascent, descent, and width in pts to the pointers passed in, and return
0 if the operation was successful.
The C equivalent of the
string_width() R function is also called
string_width() with the following signature:
This function calculates the width of a string, ignoring any newlines (these are automatically being handled by the graphic engine). It takes a UTF-8 encoded string, along with a fontfile and index identifying the font to use for the calculation. It also take a size in pt and a res in ppi for setting the size. In addition it takes an include_bearing flag to control whether the bearings of the first and last character should be taken into account (this is recommended by the graphic engine). It will write the width in pts to the passed in pointer and return 0 if successful.
A parred down version of
shape_string() is accessible at the C level with
string_shape(). It behaves more or less like
string_width() above, but instead returns the location to write each glyph at relative to a (0, 0) origin.
string_shape() behaves more or less like
string_width() above, but instead returns the location to write each glyph at relative to a (0, 0) origin. It takes a UTF-8 encoded string, along with a fontfile and index identifying the font to use for the calculation. It also take a size in pt and a res in ppi for setting the size. In addition it takes an include_bearing flag to control whether the bearings of the first and last character should be taken into account (this is recommended by the graphic engine). It will write the x and y location of each glyph in pts to the passed in arrays, stopping before the provided max_length and return 0 if successful.
A heavy part of text layouting is reading and parsing font files. systemfonts contains its own cache to make sure that parsing is kept at a minimum. If you want to use this cache to load and cache freetype face object (FT_Face) you can use
get_cached_face(). This resides in a separate header (
systemfonts-ft.h) because it requires FreeType to be linked in your package, which the rest of the C api does not. It will look in the cache for a face and size that matches your request and return that if found. If not, it will load it for you and add it to the cache, before returning it to you.
get_cached_face() sets the passed in error pointer to 0 if successful.
Freetype uses reference counting to keep track of objects and the count is increased by a call to
get_cached_face(). It is the responsiblity of the caller to decrease it once the face is no longer needed using
When rendering text it is not given that all the requested characters have a glyph in the given font. While one can elect to render a “missing glyph” glyph (often either an empty square or a questionmark in a tilted square) a better approach is often to find a font substitute that does contain the character and use that for rendering it. This function allows you to find a fallback font for a given string and font. The string should be stripped of characters that you already know how to render. The fallback font is returned as a FontSettings object, though features are always empty.
When encoding text with CSS it may be necessary to know the exact weight of the font given by a file so that it may be reflected in the style sheet. This function takes a path and an index and returns the weight (100-900 in steps of 100) or 0 if it is undefined by the font.
It may be beneficial to know the family name from a given path and index into a font. This can be obtained with
get_font_family() which will write the name to the provided
char* argument. It will return 0 if it was somehow unsuccessful.
Figuring out which character in a string should be treated as an emoji is non-trivial due to the existence of emojis with text representation default etc. systemfonts allow you to get the embedding of emojis in a string based on the correct rules.