layout.txt - abc2ps - A powerful sheet setting tool using the simple abc notation
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layout.txt (7123B)
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1
2
3 How the notes are arranged on the staff in abc2ps
4 =================================================
5
6 This file contains more detailed information on the way in which
7 the layout is calculated in abc2ps, for those who are interested
8 or would like to change the layout.
9
10
11 ----- General points -----
12
13 When typesetting music, an important feature is presumably
14 that notes and rests are arranged in some way which indicates
15 the note durations. The following considerations entered
16 into abc2ps:
17
18 (a) The space assigned a note should be qualitatively proportional
19 to its durations, but this mapping should not be strictly linear.
20 It seems best to "saturate" the mapping so that whole notes
21 are somewhat less than twice as wide as half notes. At the other
22 end, the width allocated to 8th and 16th notes must be
23 more similar than a factor of 1/2 or the layout looks uneven.
24 Thus, for durations approaching zero the note width should go
25 to some constant. A modifiable function nwidth(len) defines
26 the mapping.
27
28 (b) Given a sequence of notes (or rests) within a bar, the space
29 between any two should reflect the duration of the first.
30 If this is done strictly, the distance along the staff becomes
31 the correct "time axis" but the output looks uneven.
32 Therefore there is an "asymmetry" parameter beta to adjust.
33 For beta=0, the internote spacing comes equally from both neighbor
34 notes, and for beta=1 only the first note of each pair is taken
35 into account.
36
37 (c) When notes are grouped under a beam, they are moved slightly
38 closer together.
39
40 (d) The spacing between a note at the start of a measure and the
41 preceding bar line is a separate case. On the one hand, the duration
42 of the note could influence the spacing. On the other hand,
43 music often seems to be written with this spacing some fixed standard
44 distance. The treatment here is to choose a standard space (eg. some
45 fraction of a full measure) and another tuning parameter beta. For beta=0
46 the standard distance is used, and for beta=1 the space is taken from
47 the note duration. Notes at the end of a measure are treated similarily.
48
49 (e) If a measure has only one note in it, one can put the note exactly
50 into the middle and calculate the measure width from the note duration.
51 Alternatively, this case can be treated like case (d) as a bar-note
52 and a note-bar case. A further parameter interpolates between these
53 alternatives.
54
55
56 ----- Puristic layout -----
57
58 To change the layout rules, the parameters in file "style.h" should
59 be modified. As a starting point, it is useful to consider a strict
60 layout defined as follows (see file "style.pure"):
61
62 (a) Make the mapping between note length and the space on the paper linear.
63
64 (b) Make the space between two notes exactly proportional to the length
65 of the first note.
66
67 (c) Do not move notes under beams closer together.
68
69 (d) Put notes at the start of a measure at a fixed distance behind the bar.
70 Put a bar behind a note at a spacing proportional to the note length.
71
72 (e) Treat a measure with a single note in it like case (d).
73
74 For this layout, the position of each note exactly gives the time when
75 the note starts. It looks more or less like what one wants, but it is
76 too strict for good readability. One way to get suitable layout
77 parameters is to start from the settings in "style.pure" and twiddle
78 on the parameters.
79
80
81 ----- Algorithm for filling out the staffs -----
82
83 This is similar to the procedure used by Knuth in TeX
84 as described in the TeXBook, chapter on "Glue".
85 Three different sets of spacings are defined: shrink, space, stretch.
86
87 shrink: the smallest acceptable spacings. These are controlled
88 by function set_sym_widths().
89 space: prefered "natural" spacings at which layout looks best.
90 All parameters in style.h ending in 'p' are relevant.
91 stretch: spacings for an expanded "stretched" layout.
92 All parameters in style.h ending in 'x' are relevant.
93
94 To set a line, the natural spacings are added together. If the sum is
95 larger than the desired staff length, the final spacings are obtained
96 by linear interpolation between the "shrink" and "space" values.
97 If the sum is smaller, the spacings are obtained by interpolating
98 between the "space" and "stretch" values. Compression does not go
99 beyond the "shrink" spacings but expansion is allowed to any width.
100 (Formal note: the definition of shrink and stretch used here is not
101 the same as in TeX; eg. the minimal spacing is here 'shrink'
102 whereas in TeX it is 'space-shrink'.)
103
104
105 ----- How to proceed when changing the layout -----
106
107 When changing the output appearance, the important thing is to modify
108 the three glue modes "shrink", "space", and "stretch" separately.
109 Since a typical music line is a mixture of these modes, it is
110 difficult to see what is happening when changing the parameters
111 in the "fill" mode. In detail:
112
113 (1) Use info field 'E:shrink' or the flag "-g shrink" to force
114 maximally compressed output. In this mode, no extra glue is put
115 between the symbols. The layout should be as compressed as possible
116 while still being acceptable. The spacings are here given by
117 the left and right widths wl,wr which are defined in routine
118 set_sym_widths(). These are presently set so that symbols
119 have a small space around them, taking into account dots, flags,
120 accidentals etc. Slightly more space is left around open notes and
121 fat bars.
122
123 (2) Next, use 'E:space' or "-g space" to force the prefered natural
124 spacing. This layout should be adjusted to get the ouput which looks
125 best. As in case (1), there are left and right widths for each symbol,
126 now called pl and pr, which are also set in set_sym_widths().
127 The important difference is that spacings around notes and rests
128 are influenced by the durations. This is done according
129 to the rules described above, using the parameters ending in 'p'
130 in style.h. The relevant routine is set_sym_poslist().
131
132 (3) Finally, use 'E:stretch' to force stretched output.
133 This output should be something like twice as wide but should still be
134 easily readable. For example, this implies that internote spacings are
135 more even than in the "space" mode. Internally, the stretched spacings
136 are calculated exactly as the prefered spacings, using left and right
137 widths xl and xr, function xwidth(len), and the parameters ending
138 in 'x' in style.h.
139
140 The idea is that after obtaining satisfactory layouts for the shrink,
141 space, and stretch modes separately, the output should look good
142 when the spacings are interpolated to fill out the staff.
143
144 If you change the style file to your tastes, you might as well
145 give the layout a new name, by changing the macro STYLE in style.h.
146 This is so that the command 'abc2ps -V' can show what style is
147 currently being used.
148
149
150 Michael Methfessel, Feb 1996.