Vadim Pisarevsky
a3a4d4adac
Added harfbuzz; use it instead of STB to render text #29300 Merge with https://github.com/opencv/opencv_extra/pull/1378. This is the next big step to improve font rendering in OpenCV 5.x. See #18760, #26301. See also OpenCV 5.0 release notes, where it was pointed out that complex scripts are not rendered correctly, and HarfBuzz integration is needed to fix it. So, here it is — HarfBuzz integration: * Removed tweaked STB engine. STB truetype rendering engine was included into OpenCV 5.0-pre/5.0 and it was extended to instantiate and render variable fonts, but the support was incomplete and immature. * Instead, we now use [HarfBuzz](https://github.com/harfbuzz/harfbuzz) — famous font shaping library and now also font rendering library, used by many big companies and organizations. * As a result, we now correctly render complex scripts, such as arabic or devanagari, correctly handle font ligatures (ff, fi, ft etc.) <img width="1300" height="600" alt="text_test" src="https://github.com/user-attachments/assets/a7d2c754-0fcf-40f8-8104-578f3a14850b" /> * Potentially, we could also support color emoji, but that would be a next step. * As with STB-based engine, glyphs are cached (the same glyph from the same font is not rendered twice), so the performance should be on par with the previous engine. * When OpenCV is built with `-DWITH_HARFBUZZ=ON`, which is set by default, it tries to detect HarfBuzz in the system and use it. If it's not detected, OpenCV builds our own small subset of HarfBuzz. The following table compares size of libopencv_imgproc.dylib.5.0.0 (Release mode) in different configurations: | Configuration | libopencv_imgproc (stripped) | delta vs 5.0 | |----------------------------------------|------------------------------|----------| | imgproc without text rendering (`-DWITH_HARFBUZZ=OFF`) | 4.27 MB | −2.35 MB | | imgproc with stb-based engine (5.0) | 6.62 MB | 0.0 Mb | | imgproc with HarfBuzz-based engine (this PR) | 7.07 MB | +0.45 MB | The biggest source of the size increase when OpenCV is built with text rendering support (STB- or Harfbuzz-based) is that imgproc in this case includes .ttf fonts (Rubik and 'Wen Quan Yi Micro Hei'), which are gzip-compressed, but still have noticeable size, especially 'Wen Quan Yi' (~2Mb). HarfBuzz itself, compared to STB engine, adds just 0.45Mb, or ~7% to imgproc size. On other platforms (Linux x64, Windows), where IPP or other acceleration libraries are linked into libopencv_imgproc, the harfbuzz footprint is even less noticeable. ### Pull Request Readiness Checklist See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request - [x] I agree to contribute to the project under Apache 2 License. - [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV - [x] The PR is proposed to the proper branch - [x] There is a reference to the original bug report and related work - [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable Patch to opencv_extra has the same branch name. - [x] The feature is well documented and sample code can be built with the project CMake
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HarfBuzz
HarfBuzz started as a text shaping engine but has grown into a
full font platform — the ffmpeg of text shaping. It primarily
supports OpenType, but also Apple Advanced Typography.
HarfBuzz shapes the majority of text on modern screens.
HarfBuzz is optimized for robustness, correctness, and performance — in that order. Achieve all.
Try it live at harfbuzz-world.cc — an interactive playground for shaping, subsetting, rasterization, vector output, and GPU rendering, all running in your browser.
Here is a quick map of its components:
Core libraries
| Library | Description |
|---|---|
| libharfbuzz | Text shaping, draw API, paint API. Highly configurable (see CONFIG.md). Optional integration backends compiled in: hb-ft (FreeType), hb-coretext (macOS), hb-uniscribe (Windows), hb-directwrite (Windows), hb-gdi (Windows), hb-glib, hb-graphite2. |
| libharfbuzz-subset | Font subsetting and variable-font instancing. |
Auxiliary libraries
| Library | Description |
|---|---|
| libharfbuzz-icu | ICU Unicode integration. |
| libharfbuzz-cairo | Cairo rendering integration. |
| libharfbuzz-gobject | GObject/GI bindings. |
Experimental libraries
| Library | Description |
|---|---|
| libharfbuzz-raster | Glyph rasterization to bitmaps, including color fonts. Uses hb-draw and hb-paint. |
| libharfbuzz-vector | Glyph output to vector formats (currently SVG), including color fonts. Uses hb-draw and hb-paint. |
| libharfbuzz-gpu | Encodes glyph outlines for GPU rasterization (Slug algorithm). Provides shader sources in GLSL, WGSL, MSL, and HLSL. Live demo. |
Notable missing feature: font hinting (including autohinting) is not implemented. For hinted rasterization, use FreeType or Skrifa.
For simplified builds, amalgamated sources are available:
harfbuzz.cc (just libharfbuzz), harfbuzz-subset.cc (just
libharfbuzz-subset), or harfbuzz-world.cc (everything, driven
by a custom hb-features.h). For a live in-browser playground
plus a worked example of the world.cc single-file build, see
harfbuzz-world.cc.
Command-line tools
| Tool | Description |
|---|---|
| hb-shape | Shape text and display glyph output. |
| hb-view | Render shaped text to an image. |
| hb-subset | Subset and optimize fonts. |
| hb-info | Display font metadata. |
| hb-raster | Render glyphs to bitmap images. |
| hb-vector | Render glyphs to vector formats (SVG). |
| hb-gpu | Interactive GPU text rendering. |
The canonical source tree and bug trackers are available on github. Both development and user support discussion around HarfBuzz happen on github as well.
For license information, see COPYING.
API stability
The API that comes with hb.h will not change incompatibly. Other, peripheral,
headers are more likely to go through minor modifications, but again, we do our
best to never change API in an incompatible way. We will never break the ABI.
The API and ABI are stable even across major version number jumps. In fact, current HarfBuzz is API/ABI compatible all the way back to the 0.9.x series. If one day we need to break the API/ABI, that would be called a new library.
As such, we bump the major version number only when we add major new features, the minor version when there is new API, and the micro version when there are bug fixes.
Documentation
For user manual as well as API documentation, check: https://harfbuzz.github.io
Download
Tarball releases and Win32/Win64 binary bundles are available on the github releases page.
Development
For build information, see BUILD.md.
For custom configurations, see CONFIG.md.
For testing and profiling, see TESTING.md.
For using with Python, see README.python.md. There is also uharfbuzz.
For cross-compiling to Windows from Linux or macOS, see README.mingw.md.
To report bugs or submit patches please use github issues and pull-requests.
Developer documents
To get a better idea of where HarfBuzz stands in the text rendering stack you may want to read State of Text Rendering 2024. Here are a few presentation slides about HarfBuzz over the years:
- 2026 – HarfBuzz at 20!
- 2016 – Ten Years of HarfBuzz
- 2014 – Unicode, OpenType, and HarfBuzz: Closing the Circle
- 2012 – HarfBuzz, The Free and Open Text Shaping Engine
- 2009 – HarfBuzz: the Free and Open Shaping Engine
More presentations and papers are available on behdad's website. In particular, the following studies are relevant to HarfBuzz development:
- 2025 – AAT layout caches
- 2025 – OpenType Layout lookup caches
- 2025 – Introducing HarfRust
- 2025 – Subsetting
- 2025 – Caching
- 2025 –
hb-decycler - 2022 –
hb-iter - 2022 – A C library written in C++
- 2022 – The case of the slow
hb-ft>h_advancefunction - 2022 – PackTab: A static integer table packer
- 2020 – HarfBuzz OT+AAT "Unishaper"
- 2014 – Building the Indic Shaper
- 2012 – Memory Consumption
Name
HarfBuzz /hærfˈbɒːz/
From Persian حرف (Harf: letter) and باز (Buzz: open). Transliteration of the Persian calque for OpenType.
As a noun: The Open Source text shaping engine.
As an adjective: Insincerely talkative; glib. A nod to the GNOME project where HarfBuzz originates from.
The logo shows حرفباز in the IranNastaliq font, on a Damascus steel background.
Background: Originally there was this font format called TrueType. People and companies started calling their type engines all things ending in Type: FreeType, CoolType, ClearType, etc. And then came OpenType, which is the successor of TrueType. So, for my OpenType implementation, I decided to stick with the concept but use the Persian translation. Which is fitting given that Persian is written in the Arabic script, and OpenType is an extension of TrueType that adds support for complex script rendering, and HarfBuzz is an implementation of OpenType text shaping.
Users
HarfBuzz is used in Android, Chrome, ChromeOS, Firefox, Flutter, GNOME, GTK+, KDE, Qt, LibreOffice, OpenJDK, XeTeX, Adobe Photoshop, Illustrator, InDesign, Microsoft Edge, Amazon Kindle, PlayStation, Godot Engine, Unreal Engine, Figma, Canva, QuarkXPress, Scribus, smart TVs, car displays, and many other places.
Distribution
Packaging status of HarfBuzz
