Android Camera Architecture

Android's camera HAL connects the higher level camera framework APIs in android.hardware to your underlying camera driver and hardware. The figure and list describe the components involved and where to find the source for each: 

Application framework

At the application framework level is the app's code, which utilizes the android.hardware.Camera API to interact with the camera hardware. Internally, this code calls a corresponding JNI glue class to access the native code that interacts with the camera.

JNI

The JNI code associated with android.hardware.Camera is located in frameworks/base/core/jni/android_hardware_Camera.cpp. This code calls the lower level native code to obtain access to the physical camera and returns data that is used to create the android.hardware.Camera object at the framework level.

Native framework

The native framework defined in frameworks/av/camera/Camera.cpp provides a native equivalent to the android.hardware.Camera class. This class calls the IPC binder proxies to obtain access to the camera service.

Binder IPC proxies

The IPC binder proxies facilitate communication over process boundaries. There are three camera binder classes that are located in the frameworks/av/camera directory that calls into camera service. ICameraService is the interface to the camera service, ICamera is the interface to a specific opened camera device, and ICameraClient is the device's interface back to the application framework.

Camera service

The camera service, located in frameworks/av/services/camera/libcameraservice/CameraService.cpp, is the actual code that interacts with the HAL.

HAL

The hardware abstraction layer defines the standard interface that the camera service calls into and that you must implement to have your camera hardware function correctly.

Kernel driver

The camera's driver interacts with the actual camera hardware and your implementation of the HAL. The camera and driver must support YV12 and NV21 image formats to provide support for previewing the camera image on the display and video recording.

Implementing the HAL

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The HAL sits between the camera driver and the higher level Android framework and defines an interface that you must implement so that apps can correctly operate the camera hardware. The HAL interface is defined in the hardware/libhardware/include/hardware/camera.h and hardware/libhardware/include/hardware/camera_common.h header files.
camera_common.h defines an important struct, camera_module, which defines a standard structure to obtain general information about the camera, such as its ID and properties that are common to all cameras such as whether or not it is a front or back-facing camera.
camera.h contains the code that corresponds mainly with android.hardware.Camera. This header file declares a camera_device struct that contains a camera_device_ops struct with function pointers that point to functions that implement the HAL interface. For documentation on the different types of camera parameters that a developer can set, see the frameworks/av/include/camera/CameraParameters.h file. These parameters are set with the function pointed to by int (*set_parameters)(struct camera_device *, const char *parms) in the HAL.
For an example of a HAL implementation, see the implementation for the Galaxy Nexus HAL in hardware/ti/omap4xxx/camera.

Configuring the Shared Library

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You need to set up the Android build system to correctly package the HAL implementation into a shared library and copy it to the appropriate location by creating an Android.mk file:

1. Create a device/<company_name>/<device_name>/camera directory to contain your library's source files.
2. Create an Android.mk file to build the shared library. Ensure that the Makefile contains the following lines: 

   LOCAL_MODULE := camera.<device_name>
   LOCAL_MODULE_RELATIVE_PATH := hw
   

   Notice that your library must be named camera.<device_name> (.so is appended automatically), so that Android can correctly load the library. For an example, see the Makefile for the Galaxy Nexus camera located in hardware/ti/omap4xxx/Android.mk.
   
3. Specify that your device has camera features by copying the necessary feature XML files in the frameworks/native/data/etc directory with your device's Makefile. For example, to specify that your device has a camera flash and can autofocus, add the following lines in your device's <device>/<company_name>/<device_name>/device.mk Makefile:

   PRODUCT_COPY_FILES := \ ...
   
   PRODUCT_COPY_FILES += \
   frameworks/native/data/etc/android.hardware.camera.flash-autofocus.xml:system/etc/permissions/android.hardware.camera.flash-autofocus.xml \    
   

   For an example of a device Makefile, see device/samsung/tuna/device.mk.
   
4. Declare your camera’s media codec, format, and resolution capabilities in device/<company_name>/<device_name>/media_profiles.xml and device/<company_name>/<device_name>/media_codecs.xml XML files. For more information, see Exposing Codecs and Profiles to the Framework for information on how to do this.
5. Add the following lines in your device's device/<company_name>/<device_name>/device.mk Makefile to copy the media_profiles.xml and media_codecs.xml files to the appropriate location:

   # media config xml file
   PRODUCT_COPY_FILES += \
       <device>/<company_name>/<device_name>/media_profiles.xml:system/etc/media_profiles.xml
   # media codec config xml file
   PRODUCT_COPY_FILES += \
       <device>/<company_name>/<device_name>/media_codecs.xml:system/etc/media_codecs.xml
   

6. Declare that you want to include the Camera app in your device's system image by specifying it in the PRODUCT_PACKAGES variable in your device's device/<company_name>/<device_name>/device.mk Makefile:
   

   PRODUCT_PACKAGES := \
   Gallery2 \
   ...
   

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