The outcome of shaping a run of text depends on the contents of a specific font file (such as the substitutions and positioning moves in the 'GSUB' and 'GPOS' tables), so HarfBuzz makes accessing those internals fast.

An hb_face_t represents a face in HarfBuzz. This data type is a wrapper around an hb_blob_t blob that holds the contents of a binary font file. Since HarfBuzz supports TrueType Collections and OpenType Collections (each of which can include multiple typefaces), a HarfBuzz face also requires an index number specifying which typeface in the file you want to use. Most of the font files you will encounter in the wild include just a single face, however, so most of the time you would pass in 0 as the index when you create a face:

      hb_blob_t* blob = hb_blob_create_from_file(file);
      ...
      hb_face_t* face = hb_face_create(blob, 0);
    

On its own, a face object is not quite ready to use for shaping. The typeface must be set to a specific point size in order for some details (such as hinting) to work. In addition, if the font file in question is an OpenType Variable Font, then you may need to specify one or variation-axis settings (or a named instance) in order to get the output you need.

In HarfBuzz, you do this by creating a font object from your face.

Font objects also have the advantage of being considerably lighter-weight than face objects (remember that a face contains the contents of a binary font file mapped into memory). As a result, you can cache and reuse a font object, but you could also create a new one for each additional size you needed. Creating new fonts incurs some additional overhead, of course, but whether or not it is excessive is your call in the end. In contrast, face objects are substantially larger, and you really should cache them and reuse them whenever possible.

You can create a font object from a face object:

      hb_font_t* hb_font = hb_font_create(hb_face);
    

After creating a font, there are a few properties you should set. Many fonts enable and disable hints based on the size it is used at, so setting this is important for font objects. hb_font_set_ppem(font, x_ppem, y_ppem) sets the pixels-per-EM value of the font. You can also set the point size of the font with hb_font_set_ptem(font, ptem). HarfBuzz uses the industry standard 72 points per inch.

HarfBuzz lets you specify the degree subpixel precision you want through a scaling factor. You can set horizontal and vertical scaling factors on the font by calling hb_font_set_scale(font, x_scale, y_scale).

There may be times when you are handed a font object and need to access the face object that it comes from. For that, you can call

      hb_face = hb_font_get_face(hb_font);
    

You can also create a font object from an existing font object using the hb_font_create_sub_font() function. This creates a child font object that is initiated with the same attributes as its parent; it can be used to quickly set up a new font for the purpose of overriding a specific font-functions method.

All face objects and font objects are lifecycle-managed by HarfBuzz. After creating a face, you increase its reference count with hb_face_reference(face) and decrease it with hb_face_destroy(face). Likewise, you increase the reference count on a font with hb_font_reference(font) and decrease it with hb_font_destroy(font).

You can also attach user data to face objects and font objects.