pango:cairo-font-map-resolution (fontmap)

fontmap -- a pango:cairo-font-map object
dpi -- a number coerced to a double float for the resolution in dots per inch, physical inches are not actually involved, the terminology is conventional

The pango:cairo-font-map-resolution function gets the resolution for the font map. The (setf pango:cairo-font-map-resolution) function sets the resolution. This is a scale factor between points specified in a pango:font-description instance and Cairo units. The default value is 96, meaning that a 10 point font will be 13 units high, that is (10 * 96 / 72 = 13.3).

(pango:cairo-font-map-resolution (pango:cairo-font-map-default))
=> 96.0d0