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authorDaniel Scally <dan.scally@ideasonboard.com>2024-08-20 14:07:37 +0100
committerLaurent Pinchart <laurent.pinchart@ideasonboard.com>2024-08-21 01:35:44 +0300
commit860a3e30319cbdb66700ceab1071dcec594b3313 (patch)
tree47e80b42a85746d0adb805aa88e83165ac3f46b1 /Documentation/introduction.rst
parent3fe819eecfc6fce45aed35708affb3aa8fa1095e (diff)
Documentation: Rework docs.rst into introduction.rst
docs.rst is the landing page for the documentation from the libcamera website, but isn't particularly introductory. Move much of the content from guides/introduction.rst to docs.rst, which will serve as the new introductory page. Remove guides/introduction.rst. Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
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+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. contents::
+ :local:
+
+.. include:: documentation-contents.rst
+
+*************
+Documentation
+*************
+
+.. toctree::
+ :hidden:
+
+ API <api-html/index>
+
+What is libcamera?
+==================
+
+libcamera is an open source complex camera support library for Linux, Android
+and ChromeOS. The library interfaces with Linux kernel device drivers and
+provides an intuitive API to developers in order to simplify the complexity
+involved in capturing images from complex cameras on Linux systems.
+
+What is a "complex camera"?
+===========================
+
+A modern "camera" tends to infact be several different pieces of hardware which
+must all be controlled together in order to produce and capture images of
+appropriate quality. A hardware pipeline typically consists of a camera sensor
+that captures raw frames and transmits them on a bus, a receiver that decodes
+the bus signals, and an image signal processor that processes raw frames to
+produce usable images in a standard format. The Linux kernel handles these
+multimedia devices through the 'Linux media' subsystem and provides a set of
+application programming interfaces known collectively as the
+V4L2 (`Video for Linux 2`_) and the `Media Controller`_ APIs, which provide an
+interface to interact and control media devices.
+
+.. _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
+
+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, and is commonly implemented separately
+by vendors without a common architecture or API for application developers. This
+adds a lot of complexity to the task, particularly when considering that the
+differences in hardware pipelines and their representation in the kernel's APIs
+often necessitate bespoke handling.
+
+What is libcamera for?
+======================
+
+libcamera provides a complete camera stack for Linux-based systems to abstract
+the configuration of hardware and image control algorithms required to obtain
+desirable results from the camera through the kernel's APIs, reducing those
+operations to a simple and consistent method for developers. In short instead of
+having to deal with this:
+
+.. graphviz:: mali-c55.dot
+
+you can instead simply deal with:
+
+.. code-block:: python
+
+ >>> import libcamera as lc
+ >>> camera_manager = lc.CameraManager.singleton()
+ [0:15:59.582029920] [504] INFO Camera camera_manager.cpp:313 libcamera v0.3.0+182-01e57380
+ >>> for camera in camera_manager.cameras:
+ ... print(f' - {camera.id}')
+ ...
+ - mali-c55 tpg
+ - imx415 1-001a
+
+The library handles the rest for you. These documentary pages give more
+information on the internal workings of libcamera (and the kernel camera stack
+that lies behind it) as well as guidance on using libcamera in an application or
+extending the library with support for your hardware (through the pipeline
+handler and IPA module writer's guides).
+
+How should I use it?
+====================
+
+There are a few ways you might want to use libcamera, depending on your
+application. It's always possible to use the library directly, and you can find
+detailed information on how to do so in the
+:doc:`application writer's guide <guides/application-developer>`.
+
+It is often more appropriate to use one of the frameworks with libcamera
+support. For example an application powering an embedded media device
+incorporating capture, encoding and streaming of both video and audio would
+benefit from using `GStreamer`_, for which libcamera provides a plugin.
+Similarly an application for user-facing devices like a laptop would likely
+benefit accessing cameras through the XDG camera portal and `pipewire`_, which
+brings the advantages of resource sharing (multiple applications accessing the
+stream at the same time) and access control.
+
+.. _GStreamer: https://gstreamer.freedesktop.org/
+.. _pipewire: https://pipewire.org/
+
+Camera Stack
+============
+
+::
+
+ a c / +-------------+ +-------------+ +-------------+ +-------------+
+ p a | | Native | | Framework | | Native | | Android |
+ p t | | V4L2 | | Application | | libcamera | | Camera |
+ l i | | Application | | (gstreamer) | | Application | | Framework |
+ i o \ +-------------+ +-------------+ +-------------+ +-------------+
+ n ^ ^ ^ ^
+ | | | |
+ l a | | | |
+ i d v v | v
+ b a / +-------------+ +-------------+ | +-------------+
+ c p | | V4L2 | | Camera | | | Android |
+ a t | | Compat. | | Framework | | | Camera |
+ m a | | | | (gstreamer) | | | HAL |
+ e t \ +-------------+ +-------------+ | +-------------+
+ r i ^ ^ | ^
+ a o | | | |
+ n | | | |
+ / | ,................................................
+ | | ! : Language : !
+ l f | | ! : Bindings : !
+ i r | | ! : (optional) : !
+ b a | | \...............................................'
+ c m | | | | |
+ a e | | | | |
+ m w | v v v v
+ e o | +----------------------------------------------------------------+
+ r r | | |
+ a k | | libcamera |
+ | | |
+ \ +----------------------------------------------------------------+
+ ^ ^ ^
+ Userspace | | |
+ ------------------------ | ---------------- | ---------------- | ---------------
+ Kernel | | |
+ v v v
+ +-----------+ +-----------+ +-----------+
+ | Media | <--> | Video | <--> | V4L2 |
+ | Device | | Device | | Subdev |
+ +-----------+ +-----------+ +-----------+
+
+The camera stack comprises 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. Optional
+ language bindings allow interfacing to libcamera from other
+ programming languages.
+
+ Those components live in the same source code repository and
+ all together constitute the libcamera framework.
+
+* The libcamera adaptation is an umbrella term designating the
+ components that interface to libcamera in other frameworks.
+ Notable examples are a V4L2 compatibility layer, a gstreamer
+ libcamera element, and an Android camera HAL implementation based
+ on libcamera.
+
+ Those components can live in the libcamera project source code
+ in separate repositories, or move to their respective project's
+ repository (for instance the gstreamer libcamera element).
+
+* 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.
+
+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.
+
+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 :doc:`Application writer's guide </guides/application-developer>`
+
+.. _GStreamer element: https://gstreamer.freedesktop.org/documentation/application-development/basics/elements.html
+
+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