[libcamera-devel] [PATCH v4 1/3] Documentation: Guides: Developers Guide to Libcamera
Kieran Bingham
kieran.bingham at ideasonboard.com
Thu Aug 20 18:01:11 CEST 2020
Hi Laurent,
On 20/08/2020 16:42, Laurent Pinchart wrote:
> Hi Kieran,
>
> Thank you for the patch.
>
> s/Libcamera/libcamera/ in the subject line. This is the only issue that
> really can't be fixed on top :-)
>
> On Thu, Aug 20, 2020 at 02:47:49PM +0100, Kieran Bingham wrote:
>> From: Chris Chinchilla <chris at gregariousmammal.com>
>>
>> Create an introduction and overview for new developers to libcamera.
>>
>> Provide an overview of the Camera Stack, and Architecture of libcamera
>> and introduce the main concepts of libcamera.
>>
>> Signed-off-by: Chris Chinchilla <chris at gregariousmammal.com>
>> [Kieran: Rework/Reflow, add diagrams, licensing]
>> Signed-off-by: Kieran Bingham <kieran.bingham at ideasonboard.com>
>>
>> ---
>> Documentation/guides/introduction.rst | 319 ++++++++++++++++++++++++++
>> Documentation/index.rst | 2 +
>> Documentation/meson.build | 1 +
>> 3 files changed, 322 insertions(+)
>> create mode 100644 Documentation/guides/introduction.rst
>>
>> diff --git a/Documentation/guides/introduction.rst b/Documentation/guides/introduction.rst
>> new file mode 100644
>> index 000000000000..f34d2cf7cbdc
>> --- /dev/null
>> +++ b/Documentation/guides/introduction.rst
>> @@ -0,0 +1,319 @@
>> +.. 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) recievers, 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 requried to obtain
>> +desireable 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 |
>> + | | Compatability| | 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 abstractractions 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: http://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 it's
>> + 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: http://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: http://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 (raspberrypi)
>> +
>> +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 last sentence can be scary as it could be interpreted as forbidding
> #include of the libcamera headers. How about reusing the wording of
> COPYING.rst ?
>
> "..., provided they comply with the licensing requirements of any
> software they include or link to."
Do we have different versions of COPYING.rst?
This *was* taken from COPYING.rst....
https://git.linuxtv.org/libcamera.git/tree/COPYING.rst#n10
>
>> +
>> +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
>
> Once we integrate this documentation with the website (which I assume
> will require some refactoring), I'd prefer linking to COPYING.rst
> instead of duplicating part of the information here. Nothing that needs
> to be changed now, let's just keep this in mind for future work.
>
> I'll propose in the future small changes that could go on top. For now,
>
> Acked-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
>
>> \ No newline at end of file
>> diff --git a/Documentation/index.rst b/Documentation/index.rst
>> index 4e746bb17c4a..cfcfd388699b 100644
>> --- a/Documentation/index.rst
>> +++ b/Documentation/index.rst
>> @@ -12,3 +12,5 @@
>> Home <self>
>> Docs <docs>
>> Contribute <contributing>
>> +
>> + Developer Guide <guides/introduction>
>> diff --git a/Documentation/meson.build b/Documentation/meson.build
>> index 6d9a397cf1a3..dd7ae700af11 100644
>> --- a/Documentation/meson.build
>> +++ b/Documentation/meson.build
>> @@ -52,6 +52,7 @@ if sphinx.found()
>> 'contributing.rst',
>> 'docs.rst',
>> 'index.rst',
>> + 'guides/introduction.rst',
>> ]
>>
>> release = 'release=v' + libcamera_git_version
>
--
Regards
--
Kieran
More information about the libcamera-devel
mailing list