forked from mirrors/nixpkgs
65592837b6
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 |
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*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.