android framework之Camera

Camera

Android's camera Hardware Abstraction Layer (HAL) connects the higher level camera framework APIs in android.hardware to your underlying camera driver and hardware. The camera subsystem includes implementations for camera pipeline components while the camera HAL provides interfaces for use in implementing your version of these components.

For the most up-to-date information, refer to the following resources:

• camera.h source file
• camera3.h source file
• camera_common.h source file
• CameraMetadata developer reference

Architecture

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The following figure and list describe the HAL components:

 

Figure 1. Camera architecture

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 inframeworks/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.Cameraobject at the framework level.

Native framework

The native framework defined in frameworks/av/camera/Camera.cpp provides a native equivalent to theandroid.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 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 you must implement so apps can correctly operate the camera hardware. The HAL interface is defined in thehardware/libhardware/include/hardware/camera.h and hardware/libhardware/include/hardware/camera_common.hheader files.

camera_common.h defines camera_module, a standard structure to obtain general information about the camera, such as the camera ID and properties common to all cameras (i.e., whether it is a front- or back-facing camera).

camera.h contains code that corresponds to android.hardware.Camera. This header file declares a camera_devicestruct that in turn contains a camera_device_ops struct with pointers to functions that implement the HAL interface. For documentation on the camera parameters developers can set, refer toframeworks/av/include/camera/CameraParameters.h. 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, refer to the implementation for the Galaxy Nexus HAL inhardware/ti/omap4xxx/camera.

Configuring the shared library

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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 the Makefile contains the following lines:

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

   Your library must be named camera.<device_name> (.so is appended automatically), so Android can correctly load the library. For an example, see the Makefile for the Galaxy Nexus camera located inhardware/ti/omap4xxx/Android.mk.

3. Specify your device has camera features by copying the necessary feature XML files in theframeworks/native/data/etc directory with your device's Makefile. For example, to specify 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 indevice/<company_name>/<device_name>/media_profiles.xml anddevice/<company_name>/<device_name>/media_codecs.xml XML files. For details, see Exposing codecs to the framework.
5. Add the following lines in your device's device/<company_name>/<device_name>/device.mk Makefile to copy themedia_profiles.xml and media_codecs.xml files to the appropriate location:

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

6. To include the Camera app in your device's system image, specify it in the PRODUCT_PACKAGES variable in your device's device/<company>/<device>/device.mk Makefile:

   PRODUCT_PACKAGES :=\
   Gallery2\
   ...