Gets the font extents for the currently selected font. This will return a table
that has the following elements initialized:
Ascent-
The distance that the font extends above the baseline. Note that this is not always exactly equal to the
maximum of the extents of all the glyphs in the font, but rather is picked to express the font designer's
intent as to how the font should align with elements above it.
Descent-
The distance that the font extends below the baseline. This value is positive for typical fonts that include
portions below the baseline. Note that this is not always exactly equal to the maximum of the extents of all
the glyphs in the font, but rather is picked to express the font designer's intent as to how the font should
align with elements below it.
Height-
The recommended vertical distance between baselines when setting consecutive lines of text with the font. This
is greater than
Ascent + Descent by a quantity known as the line spacing or external leading. When space is
at a premium, most fonts can be set with only a distance of Ascent + Descent between lines.
MaxXAdvance-
The maximum distance in the X direction that the origin is advanced for any glyph in the font.
MaxYAdvance-
The maximum distance in the Y direction that the origin is advanced for any glyph in the font. This will be zero
for normal fonts used for horizontal writing (The scripts of East Asia are sometimes written vertically.)
All values are given in the current user-space coordinate system.
Because font metrics are in user-space coordinates, they are mostly, but not entirely, independent of the current
transformation matrix. If you call ccontext:Scale() with coefficients of (2.0, 2.0), text will be drawn twice as
big, but the reported text extents will not be doubled. They will change slightly due to hinting (so you can't assume
that metrics are independent of the transformation matrix), but otherwise will remain unchanged.