Make a Thicker Latin Modern Roman
All free fonts with unicode-math support are imperfect. For my taste, Latin Modern is the most polished free OTF font with unicode math, except its thin and sharp strokes for the "modern" laser printers and the lack of lowercase script math. I spent a few nights to figure out how to automatically create a thicker version of the font.
2019-11-02 2020-02-11

Why Latin Modern

As mentioned in the previous post, there are multiply reasons to migrate to XeTeX and LuaTeX. In particular, the possibility to easily use OpenType fonts is such a good feature that I would never look back to the classical TeX if I could choose.

The following fonts are the available ones with unicode-math support on my operating system:

• XITS Math
• STIX Two Math
• Asana Math
• TeX Gyre Bonum Math
• TeX Gyre Pagella Math
• TeX Gyre Schola Math
• TeX Gyre Termes Math
• TeX Gyre DejaVu Math
• Latin Modern Math
• Libertinus Math
• Garamond Math
• Cambria Math
All of them are imperfect. Here are some points one can easily realize:
• Both STIX version 1 and 2 are basically very beautiful fonts for the text, but
• XITS/STIX1 does not distinguish the bold and regular uppercase letters obviously.
• STIX Two is still not mature enough. Especially, the Math font has misaligned accents. Plus some other mistakes, which sometimes appear and I have to figure out special tricks to solve them.
• Schola and Termes has the same non-optimal kerning for mixed italic and upright styles. For example, both much too less space before a bold italic `j`.
• Pagella has comfortable kernings, but the difference between bold variant and regular `v`, `w` are not clear enough.
• I have not used Bonum for documents, since the glyphs are slightly wider than the others, they may cause more Overfull boxes, especially when that is used in A5 pages. Bless Microtype will save you.
• The sharp and thin strokes in the Latin/Computer Modern Roman fonts was the reason, that I have never applied CM-Super to any of my document. (The original CMR might be OK, if one could perform a few tests on the target printer, but definitively not optimal for the screen reading.)
Latin Modern (LM) is in my taste the most polished font in this list except the thinness and the lack of lowercase script letters. Hence, I started to figure out an automatic way to increase its thickness. The results should not be very professional, but should save my eyes when shown on the monitors and should not have broken strokes when printed

Excluded Solutions

The Original Computer Modern

In the T3 computer modern, one can adjust the `blacker` value to make the glyphs thicker. However, that will not be well displayed by a low-DPI monitor. Typically the T3 texts would be rendered as if they were misaligned, even with a high rasterizing DPI. Besides, who would use a bitmap font nowadays?

To Rebuid CM-Super or Latin Modern

Rebuilding cm-super from a blacker T3 CM bitmaps does not seem to be a time-saving solution. On the other hand, the sources of LM Roman and its math symbols are not completely obtainable. Thus, this approach has also to be excluded.

Solution 1: Fakebold by Fontspec

TLDR, conclusion: visual effects depend on the PDF reader software

The fake bold works in both XeLaTeX and LuaLaTeX. In XeLaTeX it is written as

```\setmainfont[%
FakeBold=1,
SmallCapsFont={* Caps},
SlantedFont={* Slanted},
]{Latin Modern Roman}
```
and for the current LuaLaTeX (works in XeLaTeX as well)
```\setmainfont[%
RawFeature={embolden=1.0}
SmallCapsFont={* Caps},
SlantedFont={* Slanted},
]{Latin Modern Roman}
```
A fakebold factor of 1.0 as in the above settings makes the document look quite nice in Adobe Reader, SumatraPDF and in the Firefox browser, too. However, the visual effect depends on the PDF reader. Okular renders fakebolds imperfectly, but still acceptable; while the current evince or other readers based on poppler cairo will make the font much bolder, which becomes full bolds. Hence, this solution works only for limited PDF readers.

Solution 2: Modify the OTF

Starting from the following structure for the fontforge operations:
```(my doc folder)\$ tree fakebold/
fakebold/
└── origin
├── latinmodern-math.otf
├── lmroman10-bolditalic.otf
├── lmroman10-bold.otf
├── lmroman10-italic.otf
├── lmroman10-regular.otf
├── lmroman12-bold.otf
├── lmroman12-italic.otf
├── lmroman12-regular.otf
├── lmroman17-regular.otf
├── lmroman5-bold.otf
├── lmroman5-regular.otf
├── lmroman6-bold.otf
├── lmroman6-regular.otf
├── lmroman7-bold.otf
├── lmroman7-italic.otf
├── lmroman7-regular.otf
├── lmroman8-bold.otf
├── lmroman8-italic.otf
├── lmroman8-regular.otf
├── lmroman9-bold.otf
├── lmroman9-italic.otf
├── lmroman9-regular.otf
├── lmromancaps10-oblique.otf
├── lmromancaps10-regular.otf
├── lmromandunh10-oblique.otf
├── lmromandunh10-regular.otf
├── lmromanslant10-bold.otf
├── lmromanslant10-regular.otf
├── lmromanslant12-regular.otf
├── lmromanslant17-regular.otf
├── lmromanslant8-regular.otf
├── lmromanslant9-regular.otf
└── lmromanunsl10-regular.otf

1 directory, 37 files
```
Although the Python API of fontforge exposures more features than the native script language, those additional features will not be needed here. In addition, the advantage of the native script is that the work can be easily parallelized by launching a shell process for each OTF file as you will see later.

Approach a: Change Weight

This approach works well on the text fonts. However, I could not find a way to tune the weight of math font, without triggering the warning of (only) Adobe PDF reader. The script calls the `ChangeWeight()` function of fontforge. After the weight is enlarged, each glyph also becomes slightly wider. Therefore, a rescaling is worth to be considered. Since the vertical scaling will misalign the baseline of the letters, a horizontal shrinking is applied here.

There are two additional points to be taken into account:

1. One needs to simplify the glyphs before scaling, otherwise, the shapes of some symbols like will be destroyed, as visualized in the follow three figures

Fig. 1a:the original `multiply` symbol

Fig. 1b:directly increasing the weight by 15

Fig. 1c:increasing weight by 15 after simplification
2. Some glyphs will have distorted shapes after changing the weight, as shown in the next figure.
Fig. 2:Left side is the original glyph, right side is the one with an increased weight
The solution is to accumulatively increase the weight step by step, at which this distortion does not appear.
For this approach, the `adjust-lm-weight.pe` is
``` 1 #!/user/bin/fontforge
2
3 Open("origin/"+\$1)
4 SelectAll()
5 Simplify()
6 i = 0
7 while(i < Strtol(\$2))
8     ChangeWeight(Strtod(\$3))
9     ++i
10 endloop
11 if(\$4 == "true")
12     Scale(95, 100)
13 endif
14 RemoveOverlap()
15 RemoveHints()
16 Generate(\$1)
17 Close()
```
and `adjust-lm-weight.sh` is:
```1 #!/bin/sh
2
3 for i in origin/lmroman*.otf ;  do
4     fontforge adjust-lm.pe "\${i#origin/}" 3 5.0 true &
5 done
6
7 fontforge adjust-lm.pe latinmodern-math.otf  1 15.0 false &
8
9 wait
```

Approach b: Expand Stroke

This might be the best approach so far. Compared to the previous one, expanding the strokes will also work for the LM-Math font. This approach does not have the first issue of increasing weight, but it is still bothered by the second issue, which can be solved in the same way.

The script `adjust-lm-stroke.pe` is now

``` 1 #!/user/bin/fontforge
2
3 Open("origin/"+\$1)
4 SelectAll()
5 joinstyle = 0
6 i = 0
7 while (i < Strtod(\$2))
8     ExpandStroke(Strtol(\$3), 1, joinstyle, 0, 1)
9     ++i
10 endloop
11 RemoveOverlap()
12 ClearHints()
13 Simplify()
14 RoundToInt()
15 Generate(\$1)
16 Close()
```
and `adjust-lm-weight.sh`:
```1 #!/bin/sh
2 for i in origin/lmroman*.otf ;  do
3     fontforge adjust-lm-stroke.pe "\${i#origin/}" 3 6 &
4 done
5 # \symscr{L} would have problem with 3*6, switch to 1*18 for math
6 fontforge adjust-lm-stroke.pe latinmodern-math.otf 1 18 &
7 wait
```
The hints should be removed, as the original ones are no more optimal and `AutoHint()` from fontforge is not clever enough.

Settings for fontspec

``` 1 % Extract the fonts in 'fakebold/'
2 \usepackage{fontspec}
3 \usepackage[warnings-off={mathtools-colon,mathtools-overbracket}]{unicode-math}
4 \setmainfont[%
5   Path={fakebold/},
6   %
7   UprightFont={*-regular.otf},
8   UprightFeatures={%
9     SmallCapsFont={lmromancaps10-regular.otf},
10     SizeFeatures={%
11       {Size=     -5.5,  Font={lmroman5-regular.otf}},%
12       {Size=  5.5-6.5,  Font={lmroman6-regular.otf}},%
13       {Size=  6.5-7.5,  Font={lmroman7-regular.otf}},%
14       {Size=  7.5-8.5,  Font={lmroman8-regular.otf}},%
15       {Size=  8.5-9.5,  Font={lmroman9-regular.otf}},%
16       {Size=  9.5-11.5, Font={lmroman10-regular.otf}},%
17       {Size= 11.5-14.5, Font={lmroman12-regular.otf}},%
18       {Size= 14.5-    , Font={lmroman17-regular.otf}}%
19     }%
20   },%
21   %
22   BoldFont={lmroman10-bold.otf},
23   BoldFeatures={%
24     SizeFeatures={%
25       {Size=     -5.5,  Font={lmroman5-bold.otf}},%
26       {Size=  5.5-6.5,  Font={lmroman6-bold.otf}},%
27       {Size=  6.5-7.5,  Font={lmroman7-bold.otf}},%
28       {Size=  7.5-8.5,  Font={lmroman8-bold.otf}},%
29       {Size=  8.5-9.5,  Font={lmroman9-bold.otf}},%
30       {Size=  9.5-11.5, Font={lmroman10-bold.otf}},%
31       {Size= 11.5-    , Font={lmroman12-bold.otf}},%
32     }%
33   },%
34   %
35   ItalicFont={lmroman10-italic.otf},
36   ItalicFeatures={%
37     SizeFeatures={%
38       {Size=     -7.5,  Font={lmroman7-italic.otf}},%
39       {Size=  7.5-8.5,  Font={lmroman8-italic.otf}},%
40       {Size=  8.5-9.5,  Font={lmroman9-italic.otf}},%
41       {Size=  9.5-11.5, Font={lmroman10-italic.otf}},%
42       {Size= 11.5-    , Font={lmroman12-italic.otf}}%
43     }%
44   },%
45   BoldItalicFont={lmroman10-bolditalic.otf},%
46   %
47   SlantedFont={lmromanslant10-regular.otf},
48   SlantedFeatures={%
49     SmallCapsFont={lmromancaps10-oblique.otf},
50     SizeFeatures={%
51       {Size=     -8.5,  Font={lmromanslant8-regular.otf}},%
52       {Size=  8.5-9.5,  Font={lmromanslant9-regular.otf}},%
53       {Size=  9.5-11.5, Font={lmromanslant10-regular.otf}},%
54       {Size= 11.5-14.5, Font={lmromanslant12-regular.otf}},%
55       {Size= 14.5-    , Font={lmromanslant17-regular.otf}}%
56     }%
57   },%
58   BoldSlantedFont={lmromanslant10-bold.otf},%
59   %
60   Ligatures={TeX}
61 ]{lmroman10}%
62 \newfontfamily\upit[Path={fakebold/}]{lmromanunsl10-regular.otf}
63 \newcommand{\textupit}[1]{{\upit#1}}
64 \newcommand{\textitup}[1]{{\upit#1}}
65 \newfontfamily\dhstyle[Path={fakebold/},Scale=MatchLowercase,UprightFont={*-regular.otf},SlantedFont={*-oblique.otf}]{lmromandunh10}
66 \newcommand{\textdh}[1]{{\dhstyle#1}}
67 %
68 \setsansfont[%
69   Scale=MatchUppercase,
70   Ligatures=TeX
71 ]{Latin Modern Sans}
72 \newfontfamily{\dcstyle}[%
73   ItalicFont={lmsansdemicond10-oblique.otf},%
74   SlantedFont={lmsansdemicond10-oblique.otf},%
75 ]{lmsansdemicond10-regular.otf}
76 \newcommand{\textdc}[1]{{\dcstyle#1}}
77 %
78 \setmonofont[%
79   Scale=MatchUppercase
80 ]{Latin Modern Mono}
81 %
82 \setmathfont[%
83   Path={fakebold/},
84   bold-style=ISO,
85   partial=upright
86 ]{latinmodern-math.otf}
87 %
88 \setmathfontface\symupit[%
89   Path={fakebold/},
90 ]{lmromanunsl10-regular.otf}
91 %
92 \setmathfontface\symitup[%
93   Path={fakebold/},
94 ]{lmromanunsl10-regular.otf}
```

Known Problems

• Hints are not manually tuned, but this is with the modern PDF renders no more a critical problem.
• Sans Serif Math symbols also become improperly thicker (but acceptable).