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How the notes are arranged on the staff in abc2ps
=================================================
This file contains more detailed information on the way in which
the layout is calculated in abc2ps, for those who are interested
or would like to change the layout.
----- General points -----
When typesetting music, an important feature is presumably
that notes and rests are arranged in some way which indicates
the note durations. The following considerations entered
into abc2ps:
(a) The space assigned a note should be qualitatively proportional
to its durations, but this mapping should not be strictly linear.
It seems best to "saturate" the mapping so that whole notes
are somewhat less than twice as wide as half notes. At the other
end, the width allocated to 8th and 16th notes must be
more similar than a factor of 1/2 or the layout looks uneven.
Thus, for durations approaching zero the note width should go
to some constant. A modifiable function nwidth(len) defines
the mapping.
(b) Given a sequence of notes (or rests) within a bar, the space
between any two should reflect the duration of the first.
If this is done strictly, the distance along the staff becomes
the correct "time axis" but the output looks uneven.
Therefore there is an "asymmetry" parameter beta to adjust.
For beta=0, the internote spacing comes equally from both neighbor
notes, and for beta=1 only the first note of each pair is taken
into account.
(c) When notes are grouped under a beam, they are moved slightly
closer together.
(d) The spacing between a note at the start of a measure and the
preceding bar line is a separate case. On the one hand, the duration
of the note could influence the spacing. On the other hand,
music often seems to be written with this spacing some fixed standard
distance. The treatment here is to choose a standard space (eg. some
fraction of a full measure) and another tuning parameter beta. For beta=0
the standard distance is used, and for beta=1 the space is taken from
the note duration. Notes at the end of a measure are treated similarily.
(e) If a measure has only one note in it, one can put the note exactly
into the middle and calculate the measure width from the note duration.
Alternatively, this case can be treated like case (d) as a bar-note
and a note-bar case. A further parameter interpolates between these
alternatives.
----- Puristic layout -----
To change the layout rules, the parameters in file "style.h" should
be modified. As a starting point, it is useful to consider a strict
layout defined as follows (see file "style.pure"):
(a) Make the mapping between note length and the space on the paper linear.
(b) Make the space between two notes exactly proportional to the length
of the first note.
(c) Do not move notes under beams closer together.
(d) Put notes at the start of a measure at a fixed distance behind the bar.
Put a bar behind a note at a spacing proportional to the note length.
(e) Treat a measure with a single note in it like case (d).
For this layout, the position of each note exactly gives the time when
the note starts. It looks more or less like what one wants, but it is
too strict for good readability. One way to get suitable layout
parameters is to start from the settings in "style.pure" and twiddle
on the parameters.
----- Algorithm for filling out the staffs -----
This is similar to the procedure used by Knuth in TeX
as described in the TeXBook, chapter on "Glue".
Three different sets of spacings are defined: shrink, space, stretch.
shrink: the smallest acceptable spacings. These are controlled
by function set_sym_widths().
space: prefered "natural" spacings at which layout looks best.
All parameters in style.h ending in 'p' are relevant.
stretch: spacings for an expanded "stretched" layout.
All parameters in style.h ending in 'x' are relevant.
To set a line, the natural spacings are added together. If the sum is
larger than the desired staff length, the final spacings are obtained
by linear interpolation between the "shrink" and "space" values.
If the sum is smaller, the spacings are obtained by interpolating
between the "space" and "stretch" values. Compression does not go
beyond the "shrink" spacings but expansion is allowed to any width.
(Formal note: the definition of shrink and stretch used here is not
the same as in TeX; eg. the minimal spacing is here 'shrink'
whereas in TeX it is 'space-shrink'.)
----- How to proceed when changing the layout -----
When changing the output appearance, the important thing is to modify
the three glue modes "shrink", "space", and "stretch" separately.
Since a typical music line is a mixture of these modes, it is
difficult to see what is happening when changing the parameters
in the "fill" mode. In detail:
(1) Use info field 'E:shrink' or the flag "-g shrink" to force
maximally compressed output. In this mode, no extra glue is put
between the symbols. The layout should be as compressed as possible
while still being acceptable. The spacings are here given by
the left and right widths wl,wr which are defined in routine
set_sym_widths(). These are presently set so that symbols
have a small space around them, taking into account dots, flags,
accidentals etc. Slightly more space is left around open notes and
fat bars.
(2) Next, use 'E:space' or "-g space" to force the prefered natural
spacing. This layout should be adjusted to get the ouput which looks
best. As in case (1), there are left and right widths for each symbol,
now called pl and pr, which are also set in set_sym_widths().
The important difference is that spacings around notes and rests
are influenced by the durations. This is done according
to the rules described above, using the parameters ending in 'p'
in style.h. The relevant routine is set_sym_poslist().
(3) Finally, use 'E:stretch' to force stretched output.
This output should be something like twice as wide but should still be
easily readable. For example, this implies that internote spacings are
more even than in the "space" mode. Internally, the stretched spacings
are calculated exactly as the prefered spacings, using left and right
widths xl and xr, function xwidth(len), and the parameters ending
in 'x' in style.h.
The idea is that after obtaining satisfactory layouts for the shrink,
space, and stretch modes separately, the output should look good
when the spacings are interpolated to fill out the staff.
If you change the style file to your tastes, you might as well
give the layout a new name, by changing the macro STYLE in style.h.
This is so that the command 'abc2ps -V' can show what style is
currently being used.
Michael Methfessel, Feb 1996.
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