Re: \fontsize in staff-space units

[Aaron Hill]

On 2020-02-05 3:34 pm, Paolo Prete wrote:
> is it possible to have a text with height set in staff-space units?
>
> For example, I would like that the height of  " \markup { foo } " is X 
> * (1
> staff-space)
> where X can be integer of decimal.

The default font size--11/20 of the staff height--results in an x-height 
for the default font being just about one staff space.  As such, you 
should only need to use \fontsize with magnification->font-size to 
adjust the font by a simple scaling factor (i.e. number of staff 
spaces):

%%%%
\version "2.19.83"
% Make the text x-height about three staff spaces:
\markup \fontsize #(magnification->font-size 3) "hello"
%%%%

As this default behavior is not precise and since fonts vary in their 
x-height, it may be necessary to measure text to obtain the actual 
dimensions and scale accordingly:

%%%%
\version "2.19.83"

#(define-markup-command
  (staff-fontsize layout props size arg)
  (number? markup?)
  (let* ((staff-space (ly:output-def-lookup layout 'staff-space 1))
         (sten (interpret-markup layout props (markup "x")))
         (yex (ly:stencil-extent sten Y))
         (height (interval-length yex))
         (magnification (* size (/ staff-space height)))
         (font-size (magnification->font-size magnification)))
    (interpret-markup layout props
      (markup (#:fontsize font-size arg)))))

test =
\new Staff
  \with {
    \omit Clef
    \omit TimeSignature
    \override NoteHead.stencil =
      #(lambda (grob)
        (grob-interpret-markup grob #{
          \markup {
            \staff-fontsize #1 "bxq"
            \staff-fontsize #2 "bxq"
            \staff-fontsize #3 "bxq"
            \staff-fontsize #4 "bxq"
          } #}))
  }
  { e'1 }

\score { \test \layout { #(layout-set-staff-size 12) } }
\score { \test \layout { #(layout-set-staff-size 20) } }
\score { \test \layout { #(layout-set-staff-size 28) } }
%%%%

'x' usually has a flat top and bottom, making it a stable reference.  If 
the x-height is not your interest, you can substitute 'x' for something 
else.  Just be aware that some glyphs often have larger physical 
dimensions than their ideal bounds as a way to compensate for optic 
effects.  This is most noticeable with rounded or pointed features.  If 
you wanted to measure 'o', for instance, you would need to adjust the 
height computation:

%%%%
         (height (+ (interval-length yex) (* 2 (car yex))))
%%%%

The principle here is to assume that the extent with which the character 
descends is the optic adjustment, so we subtract it from the top and 
bottom to get the effective height.  This only works with glyphs that 
are vertically symmetric in this regard.  An 'A', for instance, usually 
has a flat bottom but the point at the top often extends slightly beyond 
its nominal height.


-- Aaron Hill