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Okay, good morning everyone.

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I'm going to talk about the
Linux Font Rendering stack,

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and that is what I learned the last
four and a half years.

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I worked at the
city administration of Munich.

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For introduction to font rendering,

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I can say writing is
the most used interface,

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or probably the most used interface
between humans and the machines.

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So all of you probably use it everyday
on your computer.

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Screen text is replacing text on paper
more and more

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and this is still an ongoing process.

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The way from a string in a computer to
display text is surprisingly complex,

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And writing in itself is quite profound.

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For example, it's influenced by history
so probably all, or at least most,

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writing systems are derived from some
pictograph writing systems.

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This is also true for Roman letters.

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And also the Roman letters themselves,
they changed a lot since 2000 years ago.

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In the Roman Empire they used some
different letters than we use now.

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So the goal of this talk is to raise
awareness of font rendering in computing,

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with the focus on the Linux desktop.

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Now to the history,
and then I talk about typography.

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Text display in early computing.

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In early computing it was just
a way to display results.

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Before that there were
only flashing lights

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where you can read the results
and...

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[audience member] ??... BT100
?? raise your hand

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[Max] And the output device receives
a page or a character stream,

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and then the presentation is the job
of the output device.

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So for example, a typewriter
or a printer or a terminal.

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And this is a BT100
you can see on the right side.

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So this is a real terminal.

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But going on in history
there's the home computer era.

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In the home computer era you have
fixed character sets

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and most of the time due to system
limiatations, memory limitations,

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you have graphics mode and text mode.

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In the graphics mode you can basically
set the pixels more or less as you want,

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and in the text mode you can use fonts.

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The screen configuration at that time
is more or less defined

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by the state of technology.

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So every screen at that time has more or
less the same resolution,

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and that's something that the engineers
of that time relied on,

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and also software engineers relied on.

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The text rendering was just
using bitmap fonts.

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That means you have a fixed size character
and you just define it pixelwise

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so it's some kind of hand-crafted
characters.

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You have a given raster and then
you really display

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these hard-defined characters.

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So then I need to introduce a few
typography terms.

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First, there's glyph or a character.

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That's...well you can say a symbol.

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So I think most people can understand what
that means.

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Then a font, this is a set of glyphs.

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And then we have a collection of fonts,
that is called typeface or font family.

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Those are closely related fonts with
the same visual appearance,

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And they differ only in slopes or widths
or weights for example.

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Usually with typesetting, with moving
letters in the printing press,

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you really had, for weight for example,
different pieces/letters/hardware.

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And then the point size,
or typographic unit.

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This is a measure of the size, and
derives directly from the printing press

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with moving letters.

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There are different definitions, and what
we probably all use as point size is the

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DTP point – desktop publishing point,

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where one point is defined as 1/72 inch,
which is 0.3527 period millimeters.

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It is interesting to note that this has
nothing to do with pixels.

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[Audience Member] (unintelligible question)

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[Max] Please use the microphone
if you a question.

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[Audience Member] ...3257, what's the bar
over the 7 mean?

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[Max] It means that the 7 goes
on and on and on forever.

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So this is typography of the
Latin alphabet...terminology.

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There's something called kerning.

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You can see an example of kerning
on the right side.

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So in the printing press you have letters
and they usually have a gap.

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To improve the readability, and to
make all the text appear more uniform

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the typesetter reduces the gap.

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With moving letters they have special
letter pairs...

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if you go on with this
there are ligatures,

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which are replacement glyphs for close
standing characters.

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An example of this is a double f, fi or fl.

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So you can see also in the slides here
that double f is such a ligature.

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It's one connected glyph.

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And then there's a speciality
if you reverse all of the ?? [static].

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The term non-proportional font.

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So you have a font where very character,
every glyph, has the same width.

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This is also called monospace font.

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Sometimes you distinguish half-width
and full width letters.

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This is also something you would see
on a typewriter for example.

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Early bitmap fonts are usually
monospaced fonts.

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Okay, let's go to
the text rendering stack.

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Text rendering today.

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In all computing nowadays,
typography is introduced.

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So won't find many devices on the market
which don't use accurate typography.

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Glyphs are now represented as
size-independent outlines.

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We can see on the right side there's
a font editor: Font Forge.

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This is the way that such fonts
are created.

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So you can see here in the picture
the outline of the "S".

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It's a size-independent outline and
you can just use that for

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every kind of display.

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And then you do something that is called
rasterization, or sampling, or rendering

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for displaying it at the actual display.

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In the past this allowed for new
applications for computers.

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First it allowed word processing and
computer-based typesetting.

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Before that typesetting was
a very hard job.

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It allowed internationalization.

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It allows universal graphical interface
which goes

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hand-in-hand with internationalization.

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Come to think of it, not so long ago it
was not so common that you could

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buy devices where you can easily switch
the language to

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any language you would like.

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This is just a recent development.

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So now I will give you an overview of
the formats we see on Linux systems.

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First I talk about the bitmap formats.

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Today we have bitmap formats in the
PC Screen Font format,

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PSF or PSFU.

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These are the fonts that are used for the
Linux VT, also known as the console.

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Usually you can store in such a font
256 or 512 glyphs.

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Some of them contain
a Unicode translation table.

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And on the other hand there are
X Window System Bitmap Fonts.

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There are three formats:

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There's a Sever normal Format (SNF).

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Glyph Bitmap Distribution Format (BDF).

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And Portable Compiled Format (PCF).

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Nowadays PCF is the only letter that
you can find on a Linux system if so,

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and the first two are deprecated.

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And also the X Window Bitmap Fonts
themselves are not very common these days.

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To the outline font formats.

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There are Postscript Formats.

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Postscript Formats have different versions
as I would call them. It's called types.

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Type 1 is the one that is still relevant,
to a certain degree.

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Those are the ones you can probably

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find on a standard Debian installation
for example.

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It uses cubic bezier curves.

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The file format is divided into
different files.

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So for every font you have
Printer Font ASCII

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or a Printer Font Binary file,
and you have

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a Print Font Metric or
Adobe Font Metric file.

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Then you have the Truetype file format.

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This is more common. I guess everyone
heard of that already.

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It uses quadratic bezier curves.

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It can contain optional code for
TrueType Hinting Virtual Machine.

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What that means I will explain latter.

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There's a third format,
the OpenType format,

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which has two possible glyph formats:
one is truetype,

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and the other is Compact Font Format,
which is based on Postscript Type 2.

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So you can say what is new in this format,
or what's the difference?

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Well it supports so-called Smartfonts.

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That means you can have language-specific
ligatures or character substitutions.

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For example, kerning classes, which means
you have a class of characters like

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the A and different variations.

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So these are A with different diacritic
symbols,

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and you can just use one kerning class for
all of those characters and don't have to

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invent the wheel for every A with
diacritic symbols anew.

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Now to the font rendering techniques that
I use today.

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This only applies to the outline fonts
because well rendering bitmap fonts is

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obviously quite easy. You just paint the
pixels that are in the bitmap.

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So rasterization is all about using
outline fonts.

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The one library library that is used all
over the free and open source world

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is FreeType.

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It is used on Linux system, BSD desktops,
Android, and ReactOS, and also some others.

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For example it is contained in Ghostscript
and therefore in most of the printers.

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It's also on iOS [exhale].

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So the naive rasterization algorithm would
be just lay a pixel raster over the outline,

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over the character you want to display,
and if the center of the pixel is inside

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the outline then you
set the color to black.

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The problem is the so called
aliasing effect.

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This is what you can see
in the picture below.

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So those are the same words,
rendered at different resolutions.

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The left one has 10x the resolution
of the right one.

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You can see that, for example,
the 'w' is quite deformed.

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Or the curl of the 'g';
there's even a part missing.

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So you have details of the font
which are lost,

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and you have artefacts.

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Somehow, especially in the early computing
days you wanted to reduce those,

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and therefore you used a technique
called hinting.

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This is to avoid such artefacts and
improve the readability

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at lower resolutions.

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Therefore the outline adjusted to fit the
pixel raster at the rasterization.

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Some of the fonts can contain
instructions, the so-called hints

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where the the name comes from.

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You can see in the example here,
in the picture above,

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that it's quite a good result.

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This is much more readable
than the word above.

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However, with the use of hinting, there
are also characteristics of the font lost

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which is obvious because
if you change the outline

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then obviously characteristics get lost.

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And especially important for
word processing,

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what you see is what you get
word processing,

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that the tracking of the font is changed.

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So tracking means the width
of single characters.

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The picture below, this is a picture I
took from LibreOffice.

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If you have a close look, you can see
that the gap between those 'i' characters

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is not always the same.

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But if do word processing, you actually
want to have a result which looks the same

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as on the printout.

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So I would recommend no
to hint fonts in

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a what you see is what you get
word processor.

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If you have a look beyond the backyard,
on macOS, there exists no hinting,

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and in the Windows world, hint can't
be turned off,

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so this is hardwired in the
font rendering of the Windows system.

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Well anther approach to improve
readability is anti-aliasing

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which applies multisampling.

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So for every pixel you take samples at
different spots

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and then you compute from
that a gray value,

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which is a measure of how much of the
area of the pixel is covered by the glyph.

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On the picture you can see again
the simple raster word,

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and then in the middle you see the word
with anti-alising,

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and below you can see a combination of
hinting and antialiasing.

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Suddenly the text becomes very readable
also on low resolutions.

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This is what is usually used in
computing in the 90s.

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So there is another approach to improve
and therefore I have to

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talk a bit about LCD displays.

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So in the picture on the left, above you
can see tube monitor pixels and

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on the bottom you can see
LCD monitor pixels.

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All of the pixels consist of so-called
subpixels with different colors.

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The mixture of the three different colors

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gives them a wide range
of different colors.

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With the tube monitors this is not used,
but with LCD display

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depending on the
configuration of the subpixels

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it can be used to improve improve the
resolution in one direction.

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Therefore you have to know how the
configuration of the display is.

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So usually, one pixel, which you can see
in the picture below right?

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These are the usual computer monitors.

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One pixel consists of a red, green and a
blue subpixel in this order.

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But you have to keep in mind, especially
with tablets, or smartphones,

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you can rotate the screen, so you have to
keep that in mind.

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Also there are other subpixel
configurations.

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For example, depending on the technology
there can be additional red or green

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or even white subpixels.

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So there are also multiple possibilities
of the configuration.

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We can use this configuration as I said
to improve the resolution

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and in the usual case
this is horizontally.

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You can see on the picture
on the left side,

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first there's the naive,
simple rendered character,

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then the character
just using anti-aliasing,

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99:59:59,999 --> 99:59:59,999
and then there's the
subpixel rendered character.

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This one is the most readable
or most sharp character.

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Not sharp!... but it's most correctly
rendered according to the outline font.

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Depending on the technique of the display,

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you can see a color haze
around the characters.

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So this happens when the software
and the display technology

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don't match each other.

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So this is what you can see
on the picture on the right.

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Okay, let's talk about the font rendering
software parts in the Linux desktop.

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There's the so-called server-side
text rendering.

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So in the X server there's the
Core Font subsystem.

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With that, X11 clients can request the
server to display a text

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by sending a string via libXfont

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and using the so-called
X Logical Font Description (XLFD).

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Here's an example for Adobe Career font,

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then the X server has to then
render the text using the font.

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If you imagine a terminal server setup
with thin clients,

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then the thin client runs the X server
and the terminal server runs the X client

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and every thin client then has to
handle the font rendering.

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So I don't know if
this is the only reason,

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but nowadays server-side font
rendering is not so common.

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Probably not used anymore.

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Now I have to talk first
about font management.

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In Linux systems there is
a software called fontconfig,

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which manages installed fonts
on the system

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and it configures for example how to
substitute fonts.

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For example if in a document there's a
font to render a text

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and the font is not
available on the system

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then in the font config system there can
be rules to replace the font

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with a similar font.

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This is heavily used in Linux systems.

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There are also rules for what font to
use if the current selected font

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doesn't contain a character
you want to display.

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You can see on the picture on the right
that this is also used...

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although I have to admit,

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I had to take this picture on a Debian 5.0
system in LibreOffice

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because nowadays
it works a bit different,

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but this is a good example where
you can see that those characters

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come from different fonts actually.

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There's also a command line tool
which is quite nice.

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fc-match command line tool.

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So for example if you want to see what is
the replacement font for, let's say, Arial

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then you get the output:
okay, it's Nimbus Sans.

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And you can also set rendering options,

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which means you can set anti-aliasing
or you can turn off hinting.

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Usually these configurations are stored in
/etc/fonts

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and there is also a per-user configuration
in .config/fontconfig

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and it's in XML file format snippets.

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In the picture you can see an example.

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This is the replacement rule for
Carlito and Calibri.

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Carlito is a replacement font for the
nowadays often used

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Calibri font from Microsoft.

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So this configuration says the one way,
Carlito is the same as Calibri,

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and the other way around.

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This is just an example,

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you can have a look at /etc/fonts
and a lot of such snippets.

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It defines how fonts are
displayed on the system.

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Another thing, and this is the reason
why I had to take the screenshot

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on an older system, there's a piece of
software called HarfBuzz.

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HarfBuzz is Persian for OpenType,

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so this software relies on the OpenType
font format.

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You can see on the right side
the HarfBuzz logo

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that actually says
HarfBuzz in Persian.

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Before I talked about ligatures,

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In some languages ligatures are required
to render fonts correctly

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and this is an example on the left of
Devanagari, which is an Indic script.

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So if you have the first two characters
combined then this is the rule

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of how to replace those characters
with a third glyph.

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There were early implementations
by Qt for Pangul

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and those were integrated in HarfBuzz.

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Well those parts are now known as
Harfbuzz Old,

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and the current HarfBuzz is a rewrite.

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Nowaday's it's also used for
the so-called simple script,

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meaning especially Latin script,

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and is integrated into Qt, GTK,
LibreOffice, Firefox, Android, and XETEX.

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This is what I use for the slides here,

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so all the slides here are
also rendered using HarfBuzz.

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And of course the whole
font rendering stack.

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HarfBuzz also has fancy features like
variable widths or weight

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with only one font or you can define
in the font characters with

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99:59:59,999 --> 99:59:59,999
variable widths and without stepping you
can change it using HarfBuzz.

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Those techniques are used in
client-side rendering.

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One library often was
until recently often used, Xft.