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// Copyright (C) 2018 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
/*!
\page qtdistancefieldgenerator-index.html
\title Qt Distance Field Generator Manual
\ingroup qttools
If the user interface of an application has a lot of text, it
may cause a small, but noticeable, performance impact the first
time it is displayed to the user. This is especially true if the
text is rendered in multiple different fonts or use a large amount
of distinct characters (common for instance in writing systems such
as Hanzi, written Chinese).
The reason is that in order to render the text efficiently later,
Qt will spend some time creating graphical assets for each of the
glyphs that will later be reused. This happens the first time a
glyph is displayed in the scene.
For advanced users who want to optimize startup performance, it is
possible to pregenerate this font cache, as long as Text.QtRendering
is the rendering type in use. The Qt Distance Field Generator tool can
be used to pregenerate the cache, either for all glyphs in the fonts,
or just a selection that are known to be displayed during a critical
phase.
\note This is a tool that may be used by advanced users
to streamline their application in ways that cannot be done
automatically by Qt. For most common use cases, the default behavior
in Qt will be sufficient.
\image distancefieldgenerator.png "Qt Distance Field Generator UI"
\section1 General Usage
Use the Qt Distance Field Generator in the following way:
\list
\li Load a font file with \gui File > \gui Open font. If the font
file is large, there will be some waiting as it reads the fonts
and generates the distance fields.
\li Once this is done, select the glyphs that you want to save in
the pregenerated cache. This should ideally be the glyphs that
are used by your application during a performance-critical phase,
so that doing the generation ahead of time will give a visible
impact on performance.
\li Finally, click \gui Save to save the new font file.
\endlist
\note In order to modify a font in this way, you will need to make sure
its license does not prohibit it.
\section1 Selecting Glyphs
Glyphs can be selected in multiple ways. The simplest way is to click
the grid of glyphs to select a particular glyph. You can cancel the
selection by clicking on the glyph again.
In addition, you can use the list of Unicode ranges to select all glyphs
matching the characters in a certain range.
If you want to make sure you pregenerate the glyphs for a specific string
from your user interface, you can use the \gui Select > \gui Select string
function.
\note Both of the two latter selection methods base the results
on the CMAP table in the font and will not do any shaping.
\section1 Using the File
Once you have prepared a file, the next step is to load it in your application.
The saved file is a copy of the original font file, and can thus be used in
the same ways as any other font file. In addition, it has a special font table
which is recognized by Qt and used to prepopulate the glyph cache when the
font is used in Qt Quick.
You can, for instance, load the font using a \c FontLoader in your application
code. When it is used to display text in a \c Text element with \c renderType
set to \c Text.QtRendering (the default), then the pregenerated cache will be
loaded and used.
\section1 Measuring performance
For analyzing the impact of distance field generation on your application, you
can set the \c QT_LOGGING_RULES environment variable to
\c "qt.scenegraph.time.glyph=true".
When using normal fonts with no built-in cache, you will give output similar to
this:
\code
qt.scenegraph.time.glyph: distancefield: 50 glyphs prepared in 16ms, rendering=15, upload=1
\endcode
If you have pregenerated all the glyphs in use, the output will instead read something like
this:
\code
qt.scenegraph.time.glyph: distancefield: 50 pre-generated glyphs loaded in 2ms
\endcode
In this case, the time used to prepare the distance fields used to render in the application
has been reduced from one full frame (16 ms) to 2 ms. You can also use the output to verify
that all the glyphs in use are being loaded from the cache and to identify problematic phases
in your application's life cycle, performance-wise.
*/
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