The new v40 TrueType interpreter mode
+FreeType 2.7 ships the new v40 TrueType instructions + interpreter version enabled by default. It finally brings + DirectWrite/ClearType-like rendering to the screen, or + ‘subpixel hinting’ as some FreeType code calls + it. Actually, there is no subpixel hinting. Read on.
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For the past years, the previous v35 interpreter did as + the TrueType specification from the 90s said. The + TrueType hinting machinery was originally conceived to + make vector outlines render well as black-and-white + bitmaps on the coarse-pixeled CRTs of back in the days. + Look at screenshots of Windows 95 and you'll see pixely, + bitmappy text. In reality, the single glyphs are scalable + vector outlines beaten into the pixel grid by small + programs run on them inside the font before being handed + off to the rasterizer. Read: glyphs were designed to be + reprogrammed on the fly, per size, for sharper on-screen + display. Microsoft invested significant manpower into + what is commonly called the core web fonts (Arial, Times + New Roman, Courier New, etc.) to beat and re-beat each and + every glyph to a full pixel grid at all commonly used + sizes. If this sounds like a lot of sisyphean work, + that's because it is. To this day, only few font families + have had the same amount of work put into them.
+ +Times changed, LCDs came along and Microsoft rediscovered + subpixel rendering that exploits the physical structure of + LCD subpixels, usually RGB stripes, to increase the + horizontal resolution approximately three times (not quite + actually since you need to apply a blurrying filter to + lessen color fringes). More resolution meant that less + work had to be put into a font and instead of snapping + things to full pixels, they could now be snapped to one of + those three subpixels or a fraction of that for a much + finer appearance (‘subpixel hinting’) while + still appearing sharper than with plain old grayscale + antialiasing. Since fonts are explicitly programmed in + the TrueType model, they now had a lot of older fonts that + would need to be updated to take advantage of the new + possibilities or to even render correctly. Knowing this + would never happen, they implemented supersampling and a + compatibility mode that contained several interpreter rigs + and preventive measures for dirty hacks used in older + fonts to achieve pixel perfection. Older fonts that were + snapping things to full pixels all around could now at + least benefit a little from the increased horizontal + resolution. Beat Stamm describes this work in detail + on + his site. Microsoft later released the ClearType + collection fonts (Calibri, Cambria, Consolas, etc.) to + demonstrate the new possibilities that just so happened + to rely on this compatibility mode to render correctly. + Many new fonts did and still do.
+ +FreeType didn't implement a similar compatibility mode. + This had two side effects.
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It lead to glitches when rendering fonts that assumed + a compatibility mode with supersampler and interpreter + rigs is present, i.e., most modern fonts.
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The same can happen on Windows if you disable + ClearType in the system settings and look at random + web pages with custom fonts.
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Different fonts in a browser could look jarringly + different next to each other. Older fonts like the + core web fonts snap things to full hard pixels on both + axes, newer fonts and especially web fonts often use a + hinting strategy that snaps glyphs to the pixel grid + mainly or only vertically and with varying + strength.
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Snapping things to the grid on one axis instead of + two greatly reduces the complexity and cost of + hinting, looks smoother and comes with important + spacing benefits for horizontal (Latin!) text. And + it's just as readable.
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For some time already, FreeType shipped with a v38 + interpreter, also known as ‘Infinality’. Its + developer set out to make fonts render better than on + Windows and give users the ability to configure font + rendering to their liking. Out of the box, it made fonts + look like they were rendered through ClearType instead of + on Windows 95. It was disabled by default because it was + painfully slow and the original developer lost interest in + developing the code further. You'll find it used in + several distribution repositories maintained by community + members.
+ +My work on stem darkening (incompatible with explicit + horizontal hinting, i.e., TrueType) and frustration that + v35 was still the default (I prefer what + DirectWrite/ClearType puts on the screen) led me to strip + the v38 Infinality code to the bare minimum and remove all + configurability in the name of speed and simplicity. The + result is called v40 and it's just as fast as v35. v38 is + still there and usable, it just isn't compiled in by + default. If you compile it in, you can switch between the + different interpreters at runtime, using an environment + variable (or using the property API of FreeType). There + is currently no way to switch this from FontConfig or any + other means. Packagers of ‘Infinality’ + packages will probably jump in and update the packages + accordingly.
+ +Here is the core secret to making fonts render like + through DirectWrite/ClearType on Windows: There actually + is no subpixel hinting going on here. Shock. The code + simply ignores all horizontal hinting + instructions. That's less work than supersampling and + gets us almost identical results and additionally + prevents changes to the advance width of glyphs. This + greatly harmonizes the look of older and newer TrueType + fonts and incidentally solves glyph spacing problems of + less well instructed fonts. Switching to the new mode + might take some getting used to though, so if you think + your fonts are suddenly fat, fuzzy or weird, give your + brain some time to adjust.
+ +No upside without a downside, though. What made v38 so + slow was its attempt to implement the hacks Microsoft + describes in + a whitepaper + for their interpreter and rasterizer and a + configuration layer on top of that so fonts could be + specially handled down to single glyphs. Given the bugs + in and dirty hacks used by Arial and co., this was + necessary to make them render better than on Windows. In + my opinion, this is solid over-engineering in times where + the web is full of good-quality typefaces that display + well without horizontal hinting or don't even have it.
+ +The v40 code does not use any whitelist or other means to + handle certain fonts differently. It focuses on ignoring + horizontal hinting and preventing the dirtiest hacks that + dent more than they help. Modern fonts like Calibri, + Cambria, Consolas, etc., render well with this approach, + older fonts like Arial, Times New Roman, Georgia and + Verdana display mostly fine with smaller details off. And + that's okay. Basically, the harder a font tries to create + pixel-perfect black and white bitmaps for old CRT + monitors, the worse the results in the new mode. If + someone finds ways to make older fonts render better + without introducing lists or overly complex hacks, I'm + interested.
+ +PS: I recommend using the Liberation family of fonts + (version 2 and up, important!) instead of Arial, + Times New Roman, and Courier. The family harmonizes much + better internally and is equipped with much better + ClearType-ready hinting. Fedora still ships some 1.x + version; you can get the newer 2.x with improved hinting + and glyph coverage from + here.
+Last update: 8-Sep-2016
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