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LilyPond's Feta and Parmesan fonts
==================================
[The name `feta' is based on a non-translatable Dutch pun (`font-en-tja').
Given that it is the name of a cheese type, the font for old music symbols
is called `parmesan', and the OpenType font that merges these two fonts is
called `emmentaler'.]
This is a font of music symbols. All MetaFont sources are original. Most
of the documentation is in comments in the MetaFont code.
Non-square pixels are not supported; with other words, the horizontal and
vertical resolution of the output device must be the same.
Conversion from MetaFont to PostScript
--------------------------------------
The MetaFont code gets directly converted to PostScript Type 1 font with a
tool called `mf2pt1', which in turn calls `FontForge' to postprocess the
output (mainly to remove outline overlaps and to add hints).
The recommended calling sequence of mf2pt1 is
mf2pt1 --rounding=0.0001 <other options> <font>
You need mf2pt1 version 2.1 or newer.
Here some guidelines to assure a clean conversion.
. Never use `---'. Replace it with `--' together with explicit path
directions (if necessary).
. Don't use self-intersecting outlines in general since they can confuse
mf2pt1's algorithm to get the orientation of a closed path. Note that
MetaPost's implementation of the `turningnumber' primitive (which would
immediately give the orientation of a path) is severely broken before
version 1.0 of MetaPost, thus some hand-made code in mf2pt1.mp is used
to work around this bug.
. If outlines intersect, avoid grazing intersections. In case two outlines
intersect in an explicitly defined point, include this point in both
intersecting paths to avoid problems due to rounding errors.
. Don't use `draw' with a polygonal pen but for straight lines (consisting
of exactly two points). In most cases it is quite easy to emulate `draw'
with an explicit definition of the outline or with `penstroke'.
. Don't apply transformations after calling `fill' -- for example, don't
mirror `currentpicture'. Instead, transform the path and call `fill'
afterwards. This ensures that mf2pt1 gets the correct outline directions,
which is a necessary prerequisite for FontForge's algorithm to remove
overlaps.
Glyph name rules
----------------
. Most glyph names have the form <group>.<name>, where <group> is defined
with the `fet_begingroup' command, and <name> is given with
`fet_beginchar' (within a `fet_begingroup' block). Example:
`clefs.vaticana.fa'.
. Sometimes it would be sensible to use negative numbers in glyph names.
However, the `-' character shouldn't be used in a glyph name. Replace it
with `M'. For example, write `rests.M3mensural' instead of
`rests.-3mensural'.
. Glyphs that exist in both an `up' and `down' version should start the
<name> part with either `u' or `d', respectively. Example: `flags.d3',
`flags.u3'. Glyphs that are neutral w.r.t. the direction, and where
members of the glyph group exist that have `up' and `down' versions,
should start with an `s'. Example: `noteheads.s0re'.
Design rules
------------
. Always use smooth curve transitions. Since this is difficult to see in
MetaFont proof sheets (which don't show the tangents) I recommend to call
mf2pt1 like this
FONTFORGE=foo mf2pt1 ...
(`foo' should be a non-existent program; this avoids the default
postprocessing). Then call FontForge to inspect the outlines.
. Use rounded corners.
Hints for stem attachment
-------------------------
. Stem attachment of glyphs is controlled by two special variables called
`charwx' and `charwy'. Stems can be regarded as (very oblonged)
rectangles with slightly rounded corners. For stems pointing upwards the
lower right corner of this rectangle is attached to the glyph at position
(charwx, charwy). For stems pointing downwards it works analogously but
with the upper left corner, where the position of the attachment point is
additionally reflected horizontally about the center of the glyph -- this
ensures that in most cases `charwx' and `charwy' can be set to the same
values for up and down stems even though these are attached at the
right/left end of the note, respectively. To make this more precise, the
upper left corner of a down stem is attached at position
(charwd/2 - charwx, charwy) ,
where `charwd' is an internal MetaFont variable representing the glyph
width as specified by the `set_char_box' command.
. In case different stem attachments for upward and downward pointing stems
are needed, two separate glyphs must be defined in the MetaFont file; of
course, this also applies if two entirely different shapes are needed.
These have the same name but are prefixed by `u' and `d', respectively
(for `up' and `down', obviously). In each of these glyphs the variables
`charwx' and `charwy' must be set accordingly. If, on the other hand, the
attachment point is the `same' for both directions (with the
abovementioned horizontal reflection taken into account), then the prefix
`s' (for `symmetric') should be used. See the existing files for
examples. The numbers in the glyph names refer to the duration of the
note; e.g., `s0cross' in feta-noteheads.mf defines the notehead for a
whole cross-shaped note (similarly, `s1cross' and `s2cross' are for half
and quarter notes, respectively).
Proofing
--------
The proofing tool for MetaFont to inspect its output is `gftodvi',
converting a font in MetaFont's native GF font format to a DVI document,
which can then be viewed with DVI viewers like `xdvi'. The `gftodvi'
program needs two special fonts, `gray' and `black'. Assuming that you are
using TeXLive, say
mktextfm gray
mktextfm black
on the command line to generate the needed metric files (from its source
files), which are then stored within your local TEXMF tree.
Here is a shell script that you can use to produce two DVI files,
`foo.proof.dvi' and `foo.ljfour.dvi', showing the glyphs of font `foo.mf' in
proofing mode and a 600dpi rasterization for a LaserJet IV printer.
#!/bin/sh
mf "\mode:=proof; input $1"
gftodvi $1.2602gf && mv $1.dvi $1.proof.dvi
mf "\mode:=ljfour; input $1"
echo "grayfont black
" | gftodvi $1.600gf/ \
&& mv $1.dvi $1.ljfour.dvi
Assuming that you name this script `makefeta.sh', you can call it with e.g.
sh makefeta.sh feta23
The main importance of the proofing DVI output is to show points marked in
the MetaFont source code with the macros `penlabels' and `labels'. In most
cases, those points are used for constructing the shapes only and are thus
not present in the output after the conversion with mf2pt1.
Rasterization
-------------
Finally, some rules to assure that rasterization at low resolutions
(produced directly with MetaFont) gives good results. Today, this is a
minor issue, but in some cases it might show design flaws.
. Use `define_whole_pixels' and friends where appropriate.
. Use `hround' and `vround' consistently. A lot of auxiliary macros are
defined in feta-macros.mf.
. If a path element is duplicated or shifted, use an integer value for the
offset.
. Add `eps' for mirrored paths to assure correct MetaFont rasterization.
See the comment and the variables at the end of `feta-params.mf' for
details how vertical symmetry should be achieved.
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