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SPDX-License-Identifier: CC-BY-SA-4.0 - -Developers guide to libcamera -============================= - -The Linux kernel handles multimedia devices through the 'Linux media' subsystem -and provides a set of APIs (application programming interfaces) known -collectively as V4L2 (`Video for Linux 2`_) and the `Media Controller`_ API -which provide an interface to interact and control media devices. - -Included in this subsystem are drivers for camera sensors, CSI2 (Camera -Serial Interface) receivers, and ISPs (Image Signal Processors) - -The usage of these drivers to provide a functioning camera stack is a -responsibility that lies in userspace which is commonly implemented separately -by vendors without a common architecture or API for application developers. - -libcamera provides a complete camera stack for Linux based systems to abstract -functionality desired by camera application developers and process the -configuration of hardware and image control algorithms required to obtain -desirable results from the camera. - -.. _Video for Linux 2: https://www.linuxtv.org/downloads/v4l-dvb-apis-new/userspace-api/v4l/v4l2.html -.. _Media Controller: https://www.linuxtv.org/downloads/v4l-dvb-apis-new/userspace-api/mediactl/media-controller.html - - -In this developers guide, we will explore the `Camera Stack`_ and how it is -can be visualised at a high level, and explore the internal `Architecture`_ of -the libcamera library with its components. The current `Platform Support`_ is -detailed, as well as an overview of the `Licensing`_ requirements of the -project. - -This introduction is followed by a walkthrough tutorial to newcomers wishing to -support a new platform with the `Pipeline Handler Writers Guide`_ and for those -looking to make use of the libcamera native API an `Application Writers Guide`_ -provides a tutorial of the key APIs exposed by libcamera. - -.. _Pipeline Handler Writers Guide: pipeline-handler.html -.. _Application Writers Guide: application-developer.html - -.. TODO: Correctly link to the other articles of the guide - -Camera Stack ------------- - -The libcamera library is implemented in userspace, and makes use of underlying -kernel drivers that directly interact with hardware. - -Applications can make use of libcamera through the native `libcamera API`_'s or -through an adaptation layer integrating libcamera into a larger framework. - -.. _libcamera API: https://www.libcamera.org/api-html/index.html - -:: - - Application Layer - / +--------------+ +--------------+ +--------------+ +--------------+ - | | Native | | Framework | | Native | | Android | - | | V4L2 | | Application | | libcamera | | Camera | - | | Application | | (gstreamer) | | Application | | Framework | - \ +--------------+ +--------------+ +--------------+ +--------------+ - - ^ ^ ^ ^ - | | | | - | | | | - v v | v - Adaptation Layer | - / +--------------+ +--------------+ | +--------------+ - | | V4L2 | | gstreamer | | | Android | - | | Compatibility| | element | | | Camera | - | | (preload) | |(libcamerasrc)| | | HAL | - \ +--------------+ +--------------+ | +--------------+ - | - ^ ^ | ^ - | | | | - | | | | - v v v v - libcamera Framework - / +--------------------------------------------------------------------+ - | | | - | | libcamera | - | | | - \ +--------------------------------------------------------------------+ - - ^ ^ ^ - Userspace | | | - --------------------- | ---------------- | ---------------- | --------------- - Kernel | | | - v v v - - +-----------+ +-----------+ +-----------+ - | Media | <--> | Video | <--> | V4L2 | - | Device | | Device | | Subdev | - +-----------+ +-----------+ +-----------+ - -The camera stack comprises of four software layers. From bottom to top: - -* The kernel drivers control the camera hardware and expose a low-level - interface to userspace through the Linux kernel V4L2 family of APIs - (Media Controller API, V4L2 Video Device API and V4L2 Subdev API). - -* The libcamera framework is the core part of the stack. It handles all control - of the camera devices in its core component, libcamera, and exposes a native - C++ API to upper layers. - -* The libcamera adaptation layer is an umbrella term designating the components - that interface to libcamera in other frameworks. Notable examples are the V4L2 - compatibility layer, the gstreamer libcamera element, and the Android camera - HAL implementation based on libcamera which are provided as a part of the - libcamera project. - -* The applications and upper level frameworks are based on the libcamera - framework or libcamera adaptation, and are outside of the scope of the - libcamera project, however example native applications (cam, qcam) are - provided for testing. - - -V4L2 Compatibility Layer - V4L2 compatibility is achieved through a shared library that traps all - accesses to camera devices and routes them to libcamera to emulate high-level - V4L2 camera devices. It is injected in a process address space through - ``LD_PRELOAD`` and is completely transparent for applications. - - The compatibility layer exposes camera device features on a best-effort basis, - and aims for the level of features traditionally available from a UVC camera - designed for video conferencing. - -Android Camera HAL - Camera support for Android is achieved through a generic Android camera HAL - implementation on top of libcamera. The HAL implements features required by - Android and out of scope from libcamera, such as JPEG encoding support. - - This component is used to provide support for ChromeOS platforms - -GStreamer element (gstlibcamerasrc) - A `GStreamer element`_ is provided to allow capture from libcamera supported - devices through GStreamer pipelines, and connect to other elements for further - processing. - - Development of this element is ongoing and is limited to a single stream. - -Native libcamera API - Applications can make use of the libcamera API directly using the C++ - API. An example application and walkthrough using the libcamera API can be - followed in the `Application Writers Guide`_ - -.. _GStreamer element: https://gstreamer.freedesktop.org/documentation/application-development/basics/elements.html - -Architecture ------------- - -While offering a unified API towards upper layers, and presenting itself as a -single library, libcamera isn't monolithic. It exposes multiple components -through its public API and is built around a set of separate helpers internally. -Hardware abstractions are handled through the use of device-specific components -where required and dynamically loadable plugins are used to separate image -processing algorithms from the core libcamera codebase. - -:: - - --------------------------< libcamera Public API >--------------------------- - ^ ^ - | | - v v - +-------------+ +---------------------------------------------------+ - | Camera | | Camera Device | - | Manager | | +-----------------------------------------------+ | - +-------------+ | | Device-Agnostic | | - ^ | | | | - | | | +--------------------------+ | - | | | | ~~~~~~~~~~~~~~~~~~~~~~~ | - | | | | { +-----------------+ } | - | | | | } | //// Image //// | { | - | | | | <-> | / Processing // | } | - | | | | } | / Algorithms // | { | - | | | | { +-----------------+ } | - | | | | ~~~~~~~~~~~~~~~~~~~~~~~ | - | | | | ========================== | - | | | | +-----------------+ | - | | | | | // Pipeline /// | | - | | | | <-> | /// Handler /// | | - | | | | | /////////////// | | - | | +--------------------+ +-----------------+ | - | | Device-Specific | - | +---------------------------------------------------+ - | ^ ^ - | | | - v v v - +--------------------------------------------------------------------+ - | Helpers and Support Classes | - | +-------------+ +-------------+ +-------------+ +-------------+ | - | | MC & V4L2 | | Buffers | | Sandboxing | | Plugins | | - | | Support | | Allocator | | IPC | | Manager | | - | +-------------+ +-------------+ +-------------+ +-------------+ | - | +-------------+ +-------------+ | - | | Pipeline | | ... | | - | | Runner | | | | - | +-------------+ +-------------+ | - +--------------------------------------------------------------------+ - - /// Device-Specific Components - ~~~ Sandboxing - - -Camera Manager - The Camera Manager enumerates cameras and instantiates Pipeline Handlers to - manage each Camera that libcamera supports. The Camera Manager supports - hotplug detection and notification events when supported by the underlying - kernel devices. - - There is only ever one instance of the Camera Manager running per application. - Each application's instance of the Camera Manager ensures that only a single - application can take control of a camera device at once. - - Read the `Camera Manager API`_ documentation for more details. - -.. _Camera Manager API: https://libcamera.org/api-html/classlibcamera_1_1CameraManager.html - -Camera Device - The Camera class represents a single item of camera hardware that is capable - of producing one or more image streams, and provides the API to interact with - the underlying device. - - If a system has multiple instances of the same hardware attached, each has its - own instance of the camera class. - - The API exposes full control of the device to upper layers of libcamera through - the public API, making it the highest level object libcamera exposes, and the - object that all other API operations interact with from configuration to - capture. - - Read the `Camera API`_ documentation for more details. - -.. _Camera API: https://libcamera.org/api-html/classlibcamera_1_1Camera.html - -Pipeline Handler - The Pipeline Handler manages the complex pipelines exposed by the kernel - drivers through the Media Controller and V4L2 APIs. It abstracts pipeline - handling to hide device-specific details from the rest of the library, and - implements both pipeline configuration based on stream configuration, and - pipeline runtime execution and scheduling when needed by the device. - - The Pipeline Handler lives in the same process as the rest of the library, and - has access to all helpers and kernel camera-related devices. - - Hardware abstraction is handled by device specific Pipeline Handlers which are - derived from the Pipeline Handler base class allowing commonality to be shared - among the implementations. - - Derived pipeline handlers create Camera device instances based on the devices - they detect and support on the running system, and are responsible for - managing the interactions with a camera device. - - More details can be found in the `PipelineHandler API`_ documentation, and the - `Pipeline Handler Writers Guide`_. - -.. _PipelineHandler API: https://libcamera.org/api-html/classlibcamera_1_1PipelineHandler.html - -Image Processing Algorithms - An image processing algorithm (IPA) component is a loadable plugin that - implements 3A (Auto-Exposure, Auto-White Balance, and Auto-Focus) and other - algorithms. - - The algorithms run on the CPU and interact with the camera devices through the - Pipeline Handler to control hardware image processing based on the parameters - supplied by upper layers, maintaining state and closing the control loop - of the ISP. - - The component is sandboxed and can only interact with libcamera through the - API provided by the Pipeline Handler and an IPA has no direct access to kernel - camera devices. - - Open source IPA modules built with libcamera can be run in the same process - space as libcamera, however external IPA modules are run in a separate process - from the main libcamera process. IPA modules have a restricted view of the - system, including no access to networking APIs and limited access to file - systems. - - IPA modules are only required for platforms and devices with an ISP controlled - by the host CPU. Camera sensors which have an integrated ISP are not - controlled through the IPA module. - -Platform Support ----------------- - -The library currently supports the following hardware platforms specifically -with dedicated pipeline handlers: - - - Intel IPU3 (ipu3) - - Rockchip RK3399 (rkisp1) - - RaspberryPi 3 and 4 (rpi/vc4) - -Furthermore, generic platform support is provided for the following: - - - USB video device class cameras (uvcvideo) - - iMX7, Allwinner Sun6i (simple) - - Virtual media controller driver for test use cases (vimc) - -Licensing ---------- - -The libcamera core, is covered by the `LGPL-2.1-or-later`_ license. Pipeline -Handlers are a part of the libcamera code base and need to be contributed -upstream by device vendors. IPA modules included in libcamera are covered by a -free software license, however third-parties may develop IPA modules outside of -libcamera and distribute them under a closed-source license, provided they do -not include source code from the libcamera project. - -The libcamera project itself contains multiple libraries, applications and -utilities. Licenses are expressed through SPDX tags in text-based files that -support comments, and through the .reuse/dep5 file otherwise. A copy of all -licenses are stored in the LICENSES directory, and a full summary of the -licensing used throughout the project can be found in the COPYING.rst document. - -Applications which link dynamically against libcamera and use only the public -API are an independent work of the authors and have no license restrictions -imposed upon them from libcamera. - -.. _LGPL-2.1-or-later: https://spdx.org/licenses/LGPL-2.1-or-later.html |