[PATCH v3 3/7] Documentation: Synchronise camera stack details
Laurent Pinchart
laurent.pinchart at ideasonboard.com
Mon Aug 19 23:15:34 CEST 2024
Hi Dan,
Thank you for the patch.
On Mon, Aug 19, 2024 at 05:09:17PM +0100, Daniel Scally wrote:
> There are two near-duplicate instances of the camera stack details
> in the Documentation, in docs.rst and guides/introduction.rst. Remove
> them from guides/introduction.rst, with the exception of the
> explanations of the V4L2 Compatibility Layer and the Android HAL
> which are moved to the Camera Stack section in docs.rst. The Docs
> page already had its own separate version of those details but they
> are distinct and seemingly out of date - remove them.
>
> Signed-off-by: Daniel Scally <dan.scally at ideasonboard.com>
> ---
> Changes since v2:
>
> - Refocused this patch to focus on camera stack section instead of the
> libcamera architecture section. Dropped the R-b.
>
> Changes since v1:
>
> - None
>
>
> Documentation/docs.rst | 56 +++++++------
> Documentation/guides/introduction.rst | 109 +-------------------------
> 2 files changed, 31 insertions(+), 134 deletions(-)
>
> diff --git a/Documentation/docs.rst b/Documentation/docs.rst
> index 5871961c..0eacc924 100644
> --- a/Documentation/docs.rst
> +++ b/Documentation/docs.rst
> @@ -239,6 +239,36 @@ The camera stack comprises four software layers. From bottom to top:
> 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
s/$/./
> +
> +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.
This isn't true anymore, we now support multiple streams. This can be
addressed in a separate patch, it's easier to review refactorings
separately from documentation updates.
Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> +
> +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
>
> libcamera Architecture
> ======================
> @@ -374,29 +404,3 @@ Helpers and Support Classes
> self-contained support classes, even if such code is present only once in the
> code base, in order to keep the source code clean and easy to read. This
> should be the case for instance for plugin management.
> -
> -
> -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 will implement internally
> -features required by Android and missing from libcamera, such as JPEG encoding
> -support.
> -
> -The Android camera HAL implementation will initially target the
> -LIMITED hardware level, with support for the FULL level then being gradually
> -implemented.
> diff --git a/Documentation/guides/introduction.rst b/Documentation/guides/introduction.rst
> index 8368bd4a..d1e67654 100644
> --- a/Documentation/guides/introduction.rst
> +++ b/Documentation/guides/introduction.rst
> @@ -26,8 +26,7 @@ desirable results from the camera.
> .. _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
> +In this developers guide, we will 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.
> @@ -42,112 +41,6 @@ provides a tutorial of the key APIs exposed by libcamera.
>
> .. 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
> ------------
>
--
Regards,
Laurent Pinchart
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