libcamera Architecture — libcamera v0.7.2 documentation

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# libcamera 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 CameraManager API documentation for more details.

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.

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.

Image Processing Algorithms

  Together with the hardware image processing and hardware statistics collection, the Image Processing Algorithms (IPA) implement 3A (Auto-Exposure, Auto-White Balance and Auto-Focus) and other algorithms. They run on the CPU and control hardware image processing based on the parameters supplied by upper layers, closing the control loop of the ISP.

  IPAs are loaded as external plugins named IPA Modules. IPA Modules can be part of the libcamera code base or provided externally by camera vendors. In-tree IPA modules are covered by the same license as the rest of libcamera, external IPA modules can be open-source or closed-source.

  IPA Modules built with libcamera are run in the same process space as the library. External IPA Modules are run in a separate sandboxed process. In either case, they can only interact with libcamera through the API provided by the Pipeline Handler. They have a restricted view of the system, with no direct access to kernel camera devices, no access to networking APIs, and limited access to file systems. All their accesses to image and metadata are mediated by dmabuf instances explicitly passed by the Pipeline Handler to the IPA Module.

  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.

Helpers and Support Classes

  While Pipeline Handlers are device-specific, implementations are expected to share code due to usage of identical APIs towards the kernel camera drivers and the Image Processing Algorithms. This includes without limitation handling of the MC and V4L2 APIs, buffer management through dmabuf, and pipeline discovery, configuration and scheduling. Such code will be factored out to helpers when applicable.

  Other parts of libcamera will also benefit from factoring code out to 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.

## Dependencies

As a system component, libcamera tries to minimize its dependency on third-party libraries. New dependencies are evaluated on a case-by-case basis, to balance the value they bring with the impact on distributions. Dependencies for optional features must be conditioned by a meson feature option, as usage of dependencies based only on auto-detection hinders reproducible builds and dependency management for distributions. All dependencies must be compatible with the libcamera license.

Dependencies may not be used to load closed-source components from pipeline handlers or in-tree IPA modules. This includes GPU shaders or neural network models. Usage of closed-source components is permitted in external IPA Modules only. Pipeline handlers and in-tree IPA Modules are allowed to use neural networks provided that both the model and its training data are available under an open-source license compatible with libcamera.