Issue 5806: Tweak mf files to avoid FontForge internal overlap error

When compiling LilyPond, there were 352 error messages caused by
FontForge trying to resolve overlapping path problems:

Internal Error (overlap) in [...]

Fortunately, the resulting glyphs had been OK, though.
FontForge should be able to cope with this, but, alas, it isn't.

I've now tweaked all the mf files causing these problems so that the
annoying error messages can be avoided without while keeping the
original resulting glyph outlines.

modified:   mf/feta-arrowheads.mf

former def_set_arrow_paths (returning both open_path and close_path)
has been replaced by def_set_arrow_path returning one path arrow_path
depending on additional boolean parameter CLOSED
No need to draw two overlapping mirrored half-paths anymore.
Char definitions adapted accordingly
FIXES arrowheads.open.01
arrowheads.open.0M1
arrowheads.open.11
arrowheads.open.1M1

modified: mf/feta-braces.mf
now draw one single cyclic path rather than overlaying two
separate, mirrored half-paths
using intersection and subpaths to exaclty retain the original
slopes.
FIXES brace4 - brace507

modified: mf/feta-clefs.mf
draw_gclef: just one control point changed (hidden in overlapping area)
FIXES clefs.G
clefs.GG
clefs.tenorG
draw_tab_B: lower arc of the B in "TAB", angle slighty changed by 1°
FIXES clefs.tab

modified: mf/feta-numbers.mf
original paths in char definition re-arranged, intersected, joined,
in order to create one single outline (two glyphs concerned):
two char definitions affected:
FIXES two
FIXES seven

modified: mf/feta-scripts.mf
draw_turn: "ploop" overlay completely removed, using joined sub-
paths instead
FIXES scripts.reverseturn
scripts.turn
scripts.slashturn
"Segno" char definition: similar to draw_turn change
FIXES scripts.segno

modified: mf/feta-timesignatures.mf
draw_C: .. instead of -- (hidden in overlapping area)
FIXES timesig.C44
timesig.C22

modified: mf/parmesan-clefs.mf
draw_neomensural_c_clef: .. instead of -- (hidden in overlap area)
FIXES clefs.neomensural.c
clefs.neomensural.c_change
draw_petrucci_c_clef: .. instead of .. (hidden in overlap area)
FIXES clefs.petrucci.c1
clefs.petrucci.c2
clefs.petrucci.c2_change
clefs.petrucci.c3
clefs.petrucci.c3_change
clefs.petrucci.c4
clefs.petrucci.c5
clefs.petrucci.c5_change

modified: mf/parmesan-rests.mf
"2neomensural" char definition: one additional auxiliary point
inserted in overlap area
FIXES rests.2neomensural

RESULT: same glyphs, no FontForge errors anymore

https://codereview.appspot.com/571780043

[issues:#5806] Tweak mf files to avoid FontForge internal overlap error

Status: Started
Created: Fri Feb 28, 2020 07:57 PM UTC by Torsten Hämmerle
Last Updated: Sat Feb 29, 2020 09:01 AM UTC
Owner: Torsten Hämmerle
Attachments:

feta-braces-a.pfb (24.8 kB; application/x-font-type1)
internal-error-overlap-log.txt (15.9 kB; text/plain)

Overlapping mf paths

In some cases, overlapping mf paths will cause errors in FontForge processing, even if the resulting glyphs seem to be OK.
Currently, we have 142 error messages being produced during make, which is quite annoying..
Please find the corresponding log extract attached.

Remedy: Slightly adapt mf code (where needed), keeping the resulting glyph outline unchanged, but avoid problematic crossing angles for FontForge.
slight path adaptions inside overlapping areas (not affecting the resulting glyph outline at all!)
In some cases (braces, arowheads, segno/turns) now using one single outline rather than combinng two overlapping ones (with the exact same result).

FontForge issue

This has been a pain in the neck for many years now, but it doesn't seem to be easy to fix in FontForge, though. Werner has created a (FontForge) issue: https://github.com/fontforge/fontforge/issues/4184.

Nevertheless, it'd probably be a good idea to make our mf files a bit easier to handle.

Changes to mf files

The mf files have only been changed where needed, using all the original mf control points and directions, keeping the resulting character outline unaltered.
I'll attach PFB files of feta11.pfb and feta.braces-a.pfb so that Werner (and others, of course) can have a look at the final FontForge output.

Overview: Internal Error (overlap) in [...]

The errors are being caused by just a few glyphs in just a couple of mf files. Most of the time, there fixing one macro will fix all the glyphs using that macro.

feta-arrowheads.mf -> Internal Error (overlap) in arrowheads.*

macro set_arrow_path: now one single outline. Fixes glyphs open.01, open.0M1, open.11, open.1M1

feta-scripts.mf -> Internal Error (overlap) in scripts.*

macro draw_turn: "ploops" removed, now one single outline. Fixes glyphs reverseturn, turn, slashturn
char def "segno": similar case as in draw_turn. Fixes glyph segno

feta-timesignatures.mf -> Internal Error (overlap) in timesig.*

macro draw_C: smooth .. instead of straight -- within overlapping area. Fixes glyphs C44, C22

feta-clefs.mf -> Internal Error (overlap) in clefs.*

macro draw_gclef: Fixes glyphs G, GG, tenorG
macro draw_tab_B: Starting angle of one stroke in lower handwriting style "B" of "TAB" slightly changed (invisible difference). Fixes glyph tab

feta-braces.mf -> Internal Error (overlap) in brace*

macro draw_brace: Even simplified a bit and now drawing one single outline without any overlaps.

Examle: two overlaping outline -> single outline

In many symmetric cases, such as arrowheads braces, the glyphs had been constructed by separately drawing half and a mirrored version of itself, thus creating an overlap that might be problematic for FontForge.

Original mf code (with overlap)

   fill path;
   fill path yscaled -1;

New mf code (one single outline)
After determining the intersection point and using the appropriate subpath only, we are able to draw one single cyclic outine by joining one half part and its reversed/mirrored counterpart:
Using subpaths helps keeping the exact original control points and slopes, thus keeping the resulting glyph exactly identical to the original glyph..

   fill path
      .. reverse path yscaled -1
      .. cycle;

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From:	Auto mailings of changes to Lily Issues via Testlilyissues-auto
Subject:	[Lilypond-auto] [LilyIssues-auto] [testlilyissues:issues] #5806 Tweak mf files to avoid FontForge internal overlap error
Date:	Sat, 29 Feb 2020 09:01:49 -0000

[Lilypond-auto] [LilyIssues-auto] [testlilyissues:issues] #5806 Tweak mf