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libretro.lyx
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#LyX 2.0 created this file. For more info see http://www.lyx.org/
\lyxformat 413
\begin_document
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\pdf_title "Libretro - Implementing the core"
\pdf_author "Hans-Kristian Arntzen, Daniel De Matteis"
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\begin_body
\begin_layout Title
Libretro - Implementing the core
\end_layout
\begin_layout Author
Hans-Kristian Arntzen, Daniel De Matteis
\end_layout
\begin_layout Standard
\begin_inset CommandInset toc
LatexCommand tableofcontents
\end_inset
\end_layout
\begin_layout Abstract
This document explains how to successfully implement a library based on
the
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro API.
Several reasons for why more emulators and game engines should run in
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro are outlined.
\end_layout
\begin_layout Section
Background
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch is a project that aims to be a center for “retro” gaming experiences.
For your quick fix of 2D gaming goodness,
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch aims to provide a simple, featureful uniform interface for many
different gaming systems.
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
Libretro is an API that abstracts the inner functionality of a gaming system.
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch can load a library that implements this API, give it a ROM, and
play.
A key design is that libretro implementations are loaded as libraries.
This ensures great modularity, and flexibility for a developer of a core.
\end_layout
\begin_layout Standard
Due to its simple and open API, other frontends can just as easily utilize
libretro implementations.
\end_layout
\begin_layout Standard
While “retro” would often imply an emulator of a classic system,
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro can also abstract a game engine.
Classics such as
\begin_inset Index idx
status open
\begin_layout Plain Layout
Cave Story
\end_layout
\end_inset
Cave Story (NXEngine) and
\begin_inset Index idx
status open
\begin_layout Plain Layout
Doom
\end_layout
\end_inset
DOOM (PrBoom) have already been ported over.
\end_layout
\begin_layout Section
Portability
\end_layout
\begin_layout Standard
Most emulator authors write both the backend and frontend to their project.
The question of portability inevitably rises when the frontend is developed.
Should one target a single platform with high level of integration or take
a multi-platform approach with libraries like
\begin_inset Index idx
status open
\begin_layout Plain Layout
Qt
\end_layout
\end_inset
Qt? Backends for essentials like video, audio and input take a lot of time
and effort to get right, especially on multiple platforms.
\end_layout
\begin_layout Standard
By implementing for
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro, one can target standard C/C++ (or any language that can export
a C API), and achieve instant portability across tons of platforms.
\end_layout
\begin_layout Standard
In 2012, we would argue that portability has never been more relevant.
There are three major operating systems on the desktop, two major smart
phone platforms, and three gaming consoles.
If your system is implemented with software rendering, your implementation
can run on all systems supported by the frontend, without writing any platform
specific code.
In the case of
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch these are currently:
\end_layout
\begin_layout Itemize
Windows
\end_layout
\begin_layout Itemize
Linux
\end_layout
\begin_layout Itemize
OSX
\end_layout
\begin_layout Itemize
GameCube
\end_layout
\begin_layout Itemize
PlayStation 3
\end_layout
\begin_layout Itemize
XBox 1
\end_layout
\begin_layout Itemize
XBox 360
\end_layout
\begin_layout Itemize
Wii
\end_layout
\begin_layout Itemize
Android
\end_layout
\begin_layout Section
Features
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch has been in development for about two years.
During this time, some features have shown to be quinessential to retro
gaming systems.
Implementing the
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro API correctly, these features can be utilized without any additional
work.
\end_layout
\begin_layout Subsection
Frame-by-frame rewind
\end_layout
\begin_layout Standard
A libretro implementation that implements serialization and unserialization
of internal state is able to transparently support rewind mechanics.
While many emulators support coarse grained rewind,
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch supports rewind at the frame level, i.e., frames can be rewound
one frame at a time, similar to the indie-title
\begin_inset Index idx
status open
\begin_layout Plain Layout
Braid
\end_layout
\end_inset
Braid.
\end_layout
\begin_layout Subsection
\begin_inset Index idx
status open
\begin_layout Plain Layout
FFmpeg
\end_layout
\end_inset
FFmpeg lossless recording
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch can utilize the
\begin_inset Index idx
status open
\begin_layout Plain Layout
libavcodec
\end_layout
\end_inset
libavcodec library to encode video and audio output from a libretro implementati
on.
The data is encoded losslessly, with
\begin_inset Index idx
status open
\begin_layout Plain Layout
FLAC
\end_layout
\end_inset
FLAC as audio codec, and cutting-edge
\begin_inset Index idx
status open
\begin_layout Plain Layout
H.264
\end_layout
\end_inset
H.264/RGB (libx264) encoding, with a fallback to FFV1 for older playback
systems that don't support the modern
\begin_inset Index idx
status open
\begin_layout Plain Layout
H.264
\end_layout
\end_inset
H.264/RGB variant.
The recorder is multithreaded, and easily performs real-time.
\end_layout
\begin_layout Subsection
Advanced GPU shader support
\end_layout
\begin_layout Standard
Classic 2D games have the advantage that their video output is very flexible,
that is, it can be post- processed easily.
Before the advent of programmable GPUs, video filters had to be performed
on the main CPU, which cut directly into performance, and severely limited
the types of filters possible to acheive in real-time.
RetroArch aims to move this processing to the GPU by using shaders.
A vast amount of shader effects are written already, and the shader format
used is documented and implemented independently by other frontends as
well.
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch's shader support is more advanced than most emulators.
It supports an arbitrary amount of shader passes, look-up textures (borders),
scriptable shaders that can react dynamically to input or game content.
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch still supports use of traditional CPU filters, however, it should
be considered a fallback if GPU support is broken.
\end_layout
\begin_layout Subsection
Peer-to-peer netplay
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch supports two-player action over the network.
It employs a rollback technique that aims to hide latency as much as possible,
similar to the method employed by
\begin_inset Index idx
status open
\begin_layout Plain Layout
GGPO
\end_layout
\end_inset
GGPO.
\end_layout
\begin_layout Standard
In addition to head-to-head multiplayer, a spectator mode is implemented.
This mode allows a host to live stream playback to several watchers in
real-time.
The bandwidth required for this mode is near- zero as only raw input data
is transferred over the network.
\end_layout
\begin_layout Subsection
Audio DSP plugins
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch supports plugins that allows post-processing of audio data, similar
to post-processing of video data.
It supports use of on-the-fly configurable plugins with aid of a GUI.
\end_layout
\begin_layout Section
Implementing the API
\end_layout
\begin_layout Standard
The
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro API consists of several functions outlined in libretro.h, found
in the
\begin_inset Index idx
status open
\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch source package.
A libretro implementation should be compiled into a dynamically loadable
executable (.dll/.so/.dylib) or a static library (.a/.lib) that exports all
the functions outlined in libretro.h.
These will be called by the frontend.
Implementations are designed to be single-instance, so global state is
allowed.
Should the frontend call these functions in wrong order, undefined behavior
occurs.
\end_layout
\begin_layout Standard
The API header is compatible with
\begin_inset Index idx
status open
\begin_layout Plain Layout
C99
\end_layout
\end_inset
C99 and
\begin_inset Index idx
status open
\begin_layout Plain Layout
C++
\end_layout
\end_inset
C++.
From
\begin_inset Index idx
status open
\begin_layout Plain Layout
C99
\end_layout
\end_inset
C99, the bool type and <stdint.h> are used.
\end_layout
\begin_layout Standard
The program flow of a frontend using the
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro API can be expressed as follows:
\end_layout
\begin_layout Subsection
Startup
\end_layout
\begin_layout Subsubsection
retro_api_version()
\end_layout
\begin_layout Standard
This function should return RETRO_API_VERSION, defined in libretro.h.
It is used by the frontend to determine if ABI/API are mismatched.
The version will be bumped should there be any non- compatible changes
to the API.
Changes to retro_* structures, as well as changes in publically visible
functions and/or their arguments will warrant a bump in API version.
\end_layout
\begin_layout Subsubsection
retro_init()
\end_layout
\begin_layout Standard
This function is called once, and gives the implementation a chance to initializ
e data structures.
This is sometimes implemented as a no-op.
\end_layout
\begin_layout Subsubsection
retro_set_*()
\end_layout
\begin_layout Standard
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
Libretro is callback based.
The frontend will set all callbacks at this stage, and the implementation
must store these function pointers somewhere.
The frontend can, at a later stage, call these.
\end_layout
\begin_layout Subsubsection
Environment callback
\end_layout
\begin_layout Standard
While
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro has callbacks for video, audio and input, there's a callback type
dubbed the environment callback.
This callback (retro_environment_t) is a generic way for the
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro implementation to access features of the API that are considered
too obscure to deserve its own symbols.
It can be extended without breaking
\begin_inset Index idx
status open
\begin_layout Plain Layout
ABI
\end_layout
\end_inset
ABI.
The callback has a return type of bool which tells if the frontend recognized
the request given to it.
\end_layout
\begin_layout Standard
Most implementations of
\begin_inset Index idx
status open
\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro will not use this callback at all.
\end_layout
\begin_layout Subsubsection
retro_set_controller_port_device()
\end_layout
\begin_layout Standard
By default, joypads will be assumed to be inserted into the implementation.
If the engine is sensitive to which type of input device is plugged in,
the frontend may call this function to set the device to be used for a
certain player.
The implementation should try to auto-detect this if possible.
\end_layout
\begin_layout Subsubsection
retro_get_system_info()
\end_layout
\begin_layout Standard
The frontend will typically request statically known information about the
core such as the name of the implementation, version number, etc.
The information returned should be stored statically.
If dynamic allocation must take place, the implementation must make sure
to free this storage in retro_deinit() later.
\end_layout
\begin_layout Subsubsection
retro_load_game()
\end_layout
\begin_layout Standard
This function will load a ROM that the implementation will use to play the
game.
If the implementation is an emulator, this would be a game ROM image, if
it is a game engine, this could be packaged upassets for the game, etc.
The function takes a structure that points to the path where the ROM was
loaded from, as well as a memory chunk of the already loaded file.
\end_layout
\begin_layout Standard
There are two modes of loading files with libretro.
If the game engine requires to know the path of where the ROM image was
loaded from, the need_fullpath field in retro_system_info must be set to
true.
If the path is required, the frontend will not load the file into the data/size
fields, and it is up to the implementation to load the file from disk.
The path might be both relative and absolute, and the implementation must
check for both cases.
\end_layout
\begin_layout Standard
This is useful if the ROM image is too large to load into memory at once.
It is also useful if the assests consist of many smaller files, where it
is necessary to know the path of a master file to infer the paths of the
others.
If need_fullpath is set to false, the frontend will load the ROM image
into memory beforehand.
In this mode, the path field is not guaranteed to be non-NULL.
It should point to a valid path if the file was indeed, loaded from disk,
however, it is possible that the file was loaded from stdin, or similar,
which has no well-defined path.
\end_layout
\begin_layout Standard
It is recommended that need_fullpath is set to false if possible, as it
allows more features, such as soft- patching to work correctly.
\end_layout
\begin_layout Standard
retro_load_game_special() is a special case of retro_load_game().
It is designed to allow the loading of several ROMs together.
This is needed for certain odd cases like Super Nintendo with e.g.
Super GameBoy, Sufami Turbo, etc that consist of a "BIOS" + Game(s).
The function takes the type of game as an argument, and if a new game type
is to be added, it needs to be reserved in the libretro.h header.
Almost any libretro implementations should simply implement this as return
false;.
If a game consist of many smaller files it is encouraged to load a single
zipped file, or something similar.
\end_layout
\begin_layout Standard
Each ROM image can take an optional meta-argument, a string that gives extra
metadeta to the implementation.
The metadata is implementation specific, and can be ignored completely
in almost any implementation.
\end_layout
\begin_layout Subsubsection
retro_get_system_av_info()
\end_layout
\begin_layout Standard
This function lets the frontend know essential audio/video properties of
the game.
As this information can depend on the game being loaded, this info will
only be queried after a valid ROM image has been loaded.
It is important to accuractely report FPS and audio sampling rates, as
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\begin_layout Plain Layout
FFmpeg
\end_layout
\end_inset
FFmpeg recording relies on exact information to be able to run in sync for
several hours.
\end_layout
\begin_layout Subsection
Running
\end_layout
\begin_layout Subsubsection
retro_run()
\end_layout
\begin_layout Standard
After a game has been loaded successfully, retro_run() will be called repeatedly
as long as the user desires.
When called, the implementation will perform its inner functionality for
one video frame.
During this time, the implementation is free to call callbacks for video
frames, audio samples, as well as polling input, and querying current input
state.
The requirements for the callbacks are that video callback is called exactly
once, i.e.
it does not have to come last.
Also, input polling must be called at least once.
\end_layout
\begin_layout Subsubsection
Video/Audio synchronization considerations
\end_layout
\begin_layout Standard
Libretro is based on fixed rates.
Video FPS and audio sampling rates are always assumed to be constant.
Frontends will have control of the speed of playing, typically using VSync
to obtain correctspeed.
The frontend is free to "fast-forward", i.e.
play as fast as possible without waiting, or slow- motion.
For this reason, the engine should not rely on system timers to perform
arbitrary synchronization.
This is common and often needed in 3D games to account for varying frame
rates while still maintaining a playable game.
However, libretro targets classic systems where one can assume that 100
% real-time performance will always be met, thus avoiding the need for
careful timing code.
\end_layout
\begin_layout Standard
By default, the libretro implementation should replace any arbitrary sleep()/tim
e() patterns with simply calling video/audio callbacks.
The frontend will make sure to apply the proper synchronization.
\end_layout
\begin_layout Standard
This is mostly a problem with game ports, such as PrBoom.
For the libretro port of PrBoom, which heavily relied on timers and sleeping
patterns, sleeping was replaced with simply running for one frame, and
calling the video callback.
After that, enough audio was rendered to correspond to one frames worth
of time, 1 / fps seconds.
All sleeping and timing patterns could be removed, and synchronization
was correct.
\end_layout
\begin_layout Subsubsection
Audio callback considerations
\end_layout
\begin_layout Standard
The
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\begin_layout Plain Layout
libretro
\end_layout
\end_inset
libretro API has two different audio callbacks.
Only one of these should be used; the implementation must choose which
callback is best suited.
\end_layout
\begin_layout Standard
The first audio callback is per-sample, and has the type void (*)(int16_t,
int16_t).
This should be used if the implementation outputs audio on a per-sample
basis.
The frontend will make sure to partition the audio data into suitable chunks
to avoid incurring too much syscall overhead.
\end_layout
\begin_layout Standard
If audio is output in a "batch" fashion, i.e.
1 / fps seconds worth of audio data at a time, the batch approach should
be considered.
Rather than looping over all samples and calling per-sample callback every
time, the batch callback should be used instead, size_t (*)(const int16_t
*, size_t).
\end_layout
\begin_layout Standard
Using the batch callback, audio will not be copied in a temporary buffer,
which can buy a slight performance gain.
Also, all data will be pushed to audio driver in one go, saving some slight
overhead.
It is not recommended to use the batch callback for very small (< 32 frames)
amounts of data.
\end_layout
\begin_layout Standard
The data passed to the batch callback should, if possible, be aligned to
16 bytes (depends on platform), to allow accelerated
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\begin_layout Plain Layout
SIMD
\end_layout
\end_inset
SIMD operations on audio.
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\begin_layout Plain Layout
RetroArch
\end_layout
\end_inset
RetroArch implements
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\begin_layout Plain Layout
SSE
\end_layout
\end_inset
SSE/
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\begin_layout Plain Layout
AltiVec
\end_layout
\end_inset
AltiVec optimized audio processing for conversions and resampling.
\end_layout
\begin_layout Subsubsection
Input device abstraction
\end_layout
\begin_layout Standard
Abstracting input devices is the hardest part of defining a multi-system
API as it differs across every system.
The common input devices are:
\end_layout