The Infinality bytecode interpreter is removed in favor of the new v40 TrueType
interpreter. In the past, the Infinality interpreter provided support for
ClearType-style hinting instructions while the default interpreter (then v35)
provided support only for original TrueType-style instructions. The v40
interpreter corrects this deficiency, so the Infinality interpreter is no longer
necessary.
To understand why the Infinality interpreter is no longer necessary, we should
understand how ClearType differs from TrueType and how the v40 interpreter
works. The following is a summary of information available on the FreeType
website [1] mixed with my own editorializing.
TrueType instructions use horizontal and vertical hints to improve glyph
rendering. Before TrueType, fonts were only vertically hinted; horizontal hints
improved rendering by snapping stems to pixel boundaries. Horizontal hinting is
a risk because it can significantly distort glyph shapes and kerning. Extensive
testing at different resolutions is needed to perfect the TrueType
hints. Microsoft invested significant effort to do this with its "Core fonts for
the Web" project, but few other typefaces have seen this level of attention.
With the advent of subpixel rendering, the effective horizontal resolution of
most displays increased significantly. ClearType eschews horizontal hinting in
favor of horizontal supersampling. Most fonts are designed for the Microsoft
bytecode interpreter, which implements a compatibility mode with
TrueType-style (horizontal and vertical) instructions. However, applying the
full horizontal hints to subpixel-rendered fonts leads to color fringes and
inconsistent stem widths. The Infinality interpreter implements several
techniques to mitigate these problems, going so far as to embed font- and
glyph-specific hacks in the interpreter. On the other hand, the v40 interpreter
ignores the horizontal hinting instructions so that glyphs render as they are
intended to on the Microsoft interpreter. Without the horizontal hints, the
problems of glyph and kerning distortion, color fringes, and inconsistent stem
widths--the problems the Infinality interpreter was created to solve--simply
don't occur in the first place.
There are also security concerns which motivate removing the Infinality patches.
Although there is an updated version of the Infinality interpreter for FreeType
2.7, the lack of a consistent upstream maintainer is a security concern. The
interpreter is a Turing-complete virtual machine which has had security
vulnerabilities in the past. While the default interpreter is used in billions
of devices and is maintained by an active developer, the Infinality interpreter
is neither scrutinized nor maintained. We will probably never know if there are
defects in the Infinality interpreter, and if they were discovered they would
likely never be fixed. I do not think that is an acceptable situtation for a
core library like FreeType.
Dropping the Infinality patches means that font rendering will be less
customizable. I think this is an acceptable trade-off. The Infinality
interpreter made many compromises to mitigate the problems with horizontal
hinting; the main purpose of customization is to tailor these compromises to the
user's preferences. The new interpreter does not have to make these compromises
because it renders fonts as their designers intended, so this level of
customization is not necessary.
The Infinality-associated patches are also removed from cairo. These patches
only set the default rendering options in case they aren't set though
Fontconfig. On NixOS, the rendering options are always set in Fontconfig, so
these patches never actually did anything for us!
The Fontconfig test suite is patched to account for a quirk in the way PCF fonts
are named.
The fontconfig option `hintstyle` is no longer configurable in NixOS. This
option selects the TrueType interpreter; the v40 interpreter is `hintslight` and
the older v35 interpreter is `hintmedium` or `hintfull` (which have actually
always been the same thing). The setting may still be changed through the
`localConf` option or by creating a user Fontconfig file.
Users with HiDPI displays should probably disable hinting and antialiasing: at
best they have no visible effect.
The fontconfig-ultimate settings are still available in NixOS, but they are no
longer the default. They still work, but their main purpose is to set rendering
quirks which are no longer necessary and may actually be
detrimental (e.g. setting `hintfull` for some fonts). Also, the vast array of
font substitutions provided is not an appropriate default; the default setting
should be to give the user the font they asked for.
[1]. https://www.freetype.org/freetype2/docs/subpixel-hinting.html
Fixes#9110. Fontconfig recommends different precedence for rendering
settings and generic alias settings. To comply with the recommendations,
`98-nixos.conf` has been separated into `10-nixos-rendering.conf` and
`60-nixos-generic-alias.conf`.
Details:
* The option `fonts.enableFontConfig` has (finally) been renamed
`fonts.fontconfig.enable`.
* Configurations are loaded in this order: first the Fontconfig-upstream
configuration is loaded, then the NixOS-specific font directories are
set, the system-wide default configuration is loaded, and finally the
user configuration is loaded (if enabled).
* The NixOS options `fonts.fontconfig.defaultFonts.monospace`,
`fonts.fontconfig.defaultFonts.sansSerif` and
`fonts.fontconfig.defaultFonts.serif` are added to allow setting the
default system-wide font used for these generic faces. The defaults
are the appropriate faces from the DejaVu collection because of their
comprehensive Unicode coverage, clean rendering, and excellent
legibility.
* The NixOS option `fonts.fontconfig.antialias` can be used to disable
antialiasing (it is enabled by default).
* The options `fonts.fontconfig.subpixel.rgba` and
`fonts.fontconfig.subpixel.lcdfilter` control the system-wide default
settings for subpixel order and LCD filtering algorithm,
respectively.
* `fonts.fontconfig.hinting.enable` can be used to disable TrueType font
hinting (it is enabled by default).
`fonts.fontconfig.hinting.autohint` controls the FreeType autohinter.
`fonts.fontconfig.hinting.style` controls the hint style; it is "full"
by default.
* User configurations can be disabled system-wide by setting
`fonts.fontconfig.includeUserConf = false`. They are enabled by
default so users can set Fontconfig options in the desktop environment
of their choice.
Any reasonably new version of fontconfig does search that path by default,
and setting this globally causes problems, as 2.10 and 2.11 need
incompatible configs.
Tested: slim+xfce desktop, chrootenv-ed steam.
I have no idea why we were setting the global variable;
e.g., neither Fedora nor Ubuntu does that.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
