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Once you have your face and font objects configured as desired and
your input buffer is filled with the characters you need to shape,
all you need to do is call hb_shape().
HarfBuzz will return the shaped version of the text in the same buffer that you provided, but it will be in output mode. At that point, you can iterate through the glyphs in the buffer, drawing each one at the specified position or handing them off to the appropriate graphics library.
For the most part, HarfBuzz's shaping step is straightforward from the outside. But that doesn't mean there will never be cases where you want to look under the hood and see what is happening on the inside. HarfBuzz provides facilities for doing that, too.
The hb_shape() function call takes four arguments: the font
object to use, the buffer of characters to shape, an array of
user-specified features to apply, and the length of that feature
array. The feature array can be NULL, so for the sake of
simplicity we will start with that case.
Internally, HarfBuzz looks at the tables of the font file to
determine where glyph classes, substitutions, and positioning
are defined, using that information to decide which
shaper to use (ot for
OpenType fonts, aat for Apple Advanced
Typography fonts, and so on). It also looks at the direction,
script, and language properties of the segment to figure out
which script-specific shaping model is needed