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<title>The new v40 TrueType interpreter mode</title>
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<h1><a href="http://freetype.org/index.html">FreeType</a> Subpixel Hinting</h1>
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<h2>The new v40 TrueType interpreter mode</h2>
<div class="date">2016-09-08, contributed by Nikolaus
Waxweiler</div>
<p>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.</p>
<p><img src="image/freetype-27-hinting-comparison.png"
alt="Demonstration and comparison of the various
hinting modes."></p>
<p>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.</p>
<p>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
<a href="http://www.beatstamm.com/typography/RTRCh4.htm#Sec1">on
his site</a>. 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.</p>
<p>FreeType didn't implement a similar compatibility mode.
This had two side effects.</p>
<ol>
<li>
<p>It lead to glitches when rendering fonts that assumed
a compatibility mode with supersampler and interpreter
rigs is present, i.e., most modern fonts.</p>
<p><img src="image/freetype-27-consolas-v35-v40.png"
alt="Hinting comparison of the Consolas font."></p>
<p>The same can happen on Windows if you disable
ClearType in the system settings and look at random
web pages with custom fonts.</p>
</li>
<li>
<p>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.</p>
<p><img src="image/freetype-27-v35-v40-different-hinting.png"
alt="Hinting comparison between Georgia and NotoSerif."></p>
<p>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.</p>
</li>
</ol>
<p>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.</p>
<p>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.</p>
<p>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 <em>all</em> horizontal hinting
instructions. That's less work than supersampling and
gets us almost identical results <em>and</em> 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.</p>
<p>No upside without a downside, though. What made v38 so
slow was its attempt to implement the hacks Microsoft
describes in
a <a href="https://www.microsoft.com/typography/cleartype/truetypecleartype.aspx">whitepaper</a>
for their interpreter and rasterizer <em>and</em> 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.</p>
<p>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.</p>
<p>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
<a href="https://fedorahosted.org/liberation-fonts">here</a>.</p>
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<div class="updated">
<p>Last update: 8-Sep-2016</p>
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