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libaribcaption

A portable caption decoder / renderer for handling ARIB STD-B24 based TV broadcast captions.

Background

While CEA-608/708 closed caption standards are used by the ATSC system in North America, DVB Subtitles / DVB Teletext defined in DVB standard are used in Europe and many parts of the world, Japan established its own TV broadcasting standard ISDB that includes a kind of caption service defined in ARIB STD-B24 by the Association of Radio Industries and Businesses (ARIB).

Brazil also adopted ISDB-T International for their broadcasting by establishing Brazilian version SBTVD / ISDB-Tb based on the Japanese standard, which has been widely used in South America countries and around the world. Brazilian version also includes a caption service for Latin languages defined in ABNT NBR 15606-1 which is modified from ARIB STD-B24 specification. Philippines also adopted ISDB-T International based on the Brazilian standards, but uses UTF-8 for caption encoding based on the Japansese specification ARIB STD-B24.

Though ISDB-based TV broadcasting has been operating for about 20 years, ARIB based caption is still lacking support in general players.

Overview

libaribcaption provides decoder and renderer for handling ARIB STD-B24 based broadcast captions, making it possible for general players to render ARIB captions with the same effect (or even better) as Television.

libaribcaption is written in C++17 but also provides C interfaces to make it easier to integrate into video players. It is a lightweight library that only depends on libfreetype and libfontconfig in the worst case.

libaribcaption is a cross-platform library that works on various platforms, including but not limited to:

Windows 7+
Windows XP+ (libfreetype required)
Linux (libfreetype and libfontconfig required)
Android 2.x+ (libfreetype required)
macOS
iOS

Screenshot

Features

Support captions in Japanese (ARIB STD-B24 JIS), Latin languages (ABNT NBR 15606-1) and Philippine (ARIB STD-B24 UTF-8)
Full support for rendering ARIB additional symbols (Gaiji) and DRCS characters
Lightweight and portable implementation that works on various platforms
Performance optimized (SSE2 on x86/x64) graphics rendering
Multiple text rendering backend driven by DirectWrite / CoreText / FreeType
Zero third-party dependencies on Windows (using DirectWrite) and macOS / iOS (using CoreText)
Built-in font fallback mechanism
Built-in DRCS converting table for replacing / rendering known DRCS characters into / by alternative Unicode

Build

CMake 3.11+ and a C++17 compatible compiler will be necessary for building. Usually you just have to:

cd libaribcaption
mkdir build
cd build
cmake .. -DCMAKE_BUILD_TYPE=Release
cmake --build . -j8
cmake --install .      # Optional

By default libaribcaption is compiled as static library, indicate ARIBCC_SHARED_LIBRARY:BOOL=ON to build as a shared library:

cmake .. -DCMAKE_BUILD_TYPE=Release -DARIBCC_SHARED_LIBRARY:BOOL=ON    # or -DBUILD_SHARED_LIBS:BOOL=ON

libaribcaption has several CMake options that can be specified:

ARIBCC_BUILD_TESTS:BOOL            # Compile test codes inside /test. Default to OFF
ARIBCC_SHARED_LIBRARY:BOOL         # Compile as shared library. Default to OFF
ARIBCC_NO_EXCEPTIONS:BOOL          # Disable C++ Exceptions. Default to OFF
ARIBCC_NO_RTTI:BOOL                # Disable C++ RTTI. Default to OFF
ARIBCC_NO_RENDERER:BOOL            # Disable the renderer and leave only the decoder behind. Default to OFF
ARIBCC_IS_ANDROID:BOOL             # Indicate target platform is Android. Detected automatically by default.
ARIBCC_USE_DIRECTWRITE:BOOL        # Enable DirectWrite font provider & renderer. Default to ON on Windows
ARIBCC_USE_GDI_FONT:BOOL           # Enable GDI font provider which is necessary for WinXP support. Default to OFF.
ARIBCC_USE_CORETEXT:BOOL           # Enable CoreText font provider & renderer. Default to ON on macOS / iOS
ARIBCC_USE_FREETYPE:BOOL           # Enable FreeType based renderer. Default to ON on Linux / Android
ARIBCC_USE_EMBEDDED_FREETYPE:BOOL  # Use embedded FreeType instead of searching system library. Default to OFF
ARIBCC_USE_FONTCONFIG:BOOL         # Enable Fontconfig font provider. Default to ON on Linux and other platforms

By default, libaribcaption only enables DirectWrite on Windows and CoreText on macOS / iOS without any third-party dependencies, But you can still enable the FreeType based text renderer by indicating -DARIBCC_USE_FREETYPE:BOOL=ON.

For Windows XP support, you have to turn off DirectWrite (which will result in a crash), enable GDI font provider and FreeType:

cmake .. -DCMAKE_BUILD_TYPE=Release -ARIBCC_USE_DIRECTWRITE:BOOL=OFF -DARIBCC_USE_GDI_FONT:BOOL=ON -DARIBCC_USE_FREETYPE:BOOL=ON

For enabling FreeType text renderer on Windows, consider using vcpkg or msys2 for accessing third-party libraries.

If you are under some kind of environment (like Android NDK or Windows) that is hard to prepare system-wide installed FreeType, consider using embedded FreeType by indicating -DARIBCC_USE_EMBEDDED_FREETYPE:BOOL=ON. This option will automatically fetch and compile a static-linked FreeType library internally.

Usage

libaribcaption could be imported through find_package() if you have installed it into system:

cmake_minimum_required(VERSION 3.11)
project(testarib LANGUAGES C CXX)

find_package(aribcaption REQUIRED)

add_executable(testarib main.cpp)

target_compile_features(testarib
    PRIVATE
        cxx_std_17
)

target_include_directories(testarib
    PRIVATE
        ${ARIBCAPTION_INCLUDE_DIR}
)

target_link_libraries(testarib
    PRIVATE
        aribcaption::aribcaption
)

Or using add_subdirectory() to import source folder directly:

cmake_minimum_required(VERSION 3.11)
project(testarib2 LANGUAGES C CXX)

set(ARIBCC_USE_FREETYPE ON CACHE BOOL "Enable FreeType")    # Indicate options here (optional)
add_subdirectory(thirdparty/libaribcaption)

add_executable(testarib2 main.cpp)

target_compile_features(testarib2
    PRIVATE
        cxx_std_17
)

target_link_libraries(testarib2
    PRIVATE
        aribcaption::aribcaption
)

Or using pkg-config if you have installed it into system:

# Link to libaribcaption static library
gcc main.c -o main `pkg-config --cflags --libs --static libaribcaption`

# Link to libaribcaption shared library
gcc main.c -o main `pkg-config --cflags --libs libaribcaption`

Documents

See the comments in public headers, and sample code with ffmpeg

Hints

libaribcaption's C++ headers are also written in C++17. If your environment doesn't support C++17, consider using the C API or switch to a newer compiler.

The C API (public headers with ".h" extensions) could be useful for calling from Pure C or other languages, see capi sample for usage.