你的位置:首页 > 操作系统

[操作系统]Android源码笔记Camera系统架构


Camera的架构与Android系统的整体架构保持一致,如下图所示,本文主要从以下四个方面对其进行说明。

  1. Framework:Camera.java
  2. Android Runtime:android_hardware_Camera.cpp
  3. Library:Camera Client和Camera Service
  4. HAL:CameraHardwareInterface

14527792381330663_thumb9


 

一、Framework:Camera.java

Camera是应用层软件直接使用的类,涵盖了启动、预览、拍摄及关闭等操作摄像头的全部接口。Camera.java在Android源码中的路径为:framework/base/core/java/android/hardware。为了说明整个Camera系统的架构,这里暂不横向分析Camera.java的功能,下面从open()方法着手:

public static Camera open() {  int numberOfCameras = getNumberOfCameras();  CameraInfo cameraInfo = new CameraInfo();  for (int i = 0; i < numberOfCameras; i++) {    getCameraInfo(i, cameraInfo);    if (cameraInfo.facing == CameraInfo.CAMERA_FACING_BACK) {      return new Camera(i);    }  }  return null;}

open()方法需要注意以下几点:

  • getNumberOfCameras为native方法,实现在android_hardware_Camera.cpp中;
  • CameraInfo是Camera定义的静态内部类,包含facing、orientation、canDisableShutterSound;
  • getCameraInfo内部调用native方法_getCameraInfo获取摄像头信息;
  • open()默认启动的是后置摄像头(CAMERA_FACING_BACK)。
/** used by Camera#open, Camera#open(int) */Camera(int cameraId) {  int err = cameraInitNormal(cameraId);  if (checkInitErrors(err)) {    switch(err) {      case EACCESS:        throw new RuntimeException("Fail to connect to camera service");      case ENODEV:        throw new RuntimeException("Camera initialization failed");      default:        // Should never hit this.        throw new RuntimeException("Unknown camera error");    }  }}

Camera构造器的核心实现在cameraInitNormal中,cameraInitNormal调用cameraInitVersion,并传入参数cameraId和CAMERA_HAL_API_VERSION_NORMAL_CONNECT,后者代表HAL的版本。

private int cameraInitVersion(int cameraId, int halVersion) {
  ……
  String packageName = ActivityThread.currentPackageName();  return native_setup(new WeakReference<Camera>(this), cameraId, halVersion, packageName);}

cameraInitNormal调用本地方法native_setup(),由此进入到android_hardware_Camera.cpp中,native_setup()的签名如下:

private native final int native_setup(Object camera_this, int cameraId, int halVersion, String packageName);

 

二、Android Runtime:android_hardware_Camera.cpp

native_setup()被动态注册到JNI,通过JNI调用android_hardware_Camera_native_setup()方法。

static JNINativeMethod camMethods[] = {  ……  { "native_setup",  "(Ljava/lang/Object;ILjava/lang/String;)V",  (void*)android_hardware_Camera_native_setup }  ……};

JNI的重点是android_hardware_Camera_native_setup()方法的实现:

// connect to camera servicestatic jint android_hardware_Camera_native_setup(JNIEnv *env, jobject thiz,  jobject weak_this, jint cameraId, jint halVersion, jstring clientPackageName){  // Convert jstring to String16  const char16_t *rawClientName = env->GetStringChars(clientPackageName, NULL);  jsize rawClientNameLen = env->GetStringLength(clientPackageName);  String16 clientName(rawClientName, rawClientNameLen);  env->ReleaseStringChars(clientPackageName, rawClientName);  sp<Camera> camera;  if (halVersion == CAMERA_HAL_API_VERSION_NORMAL_CONNECT) {    // Default path: hal version is don't care, do normal camera connect.    camera = Camera::connect(cameraId, clientName,        Camera::USE_CALLING_UID);  } else {    jint status = Camera::connectLegacy(cameraId, halVersion, clientName,        Camera::USE_CALLING_UID, camera);    if (status != NO_ERROR) {      return status;    }  }  if (camera == NULL) {    return -EACCES;  }  // make sure camera hardware is alive  if (camera->getStatus() != NO_ERROR) {    return NO_INIT;  }  jclass clazz = env->GetObjectClass(thiz);  if (clazz == NULL) {    // This should never happen    jniThrowRuntimeException(env, "Can't find android/hardware/Camera");    return INVALID_OPERATION;  }  // We use a weak reference so the Camera object can be garbage collected.  // The reference is only used as a proxy for callbacks.  sp<JNICameraContext> context = new JNICameraContext(env, weak_this, clazz, camera);  context->incStrong((void*)android_hardware_Camera_native_setup);  camera->setListener(context);  // save context in opaque field  env->SetLongField(thiz, fields.context, (jlong)context.get());  return NO_ERROR;}

android_hardware_Camera_native_setup()方法通过调用Camera::connect()方法请求连接CameraService服务。入参中:

  • clientName是通过将clientPackageName从jstring转换为String16格式得到;
  • Camera::USE_CALLING_UID是定义在Camera.h中的枚举类型,其值为ICameraService::USE_CALLING_UID(同样为枚举类型,值为-1)。

Camera::connect()位于Camera.cpp中,由此进入到Library层。

 

三、Library:Camera Client和Camera Service

如上述架构图中所示,ICameraService.h、ICameraClient.h和ICamera.h三个类定义了Camera的接口和架构,ICameraService.cpp和Camera.cpp两个文件用于Camera架构的实现,Camera的具体功能在下层调用硬件相关的接口来实现。Camera.h是Camera系统对上层的接口。

具体的,Camera类继承模板类CameraBase,Camera::connect()调用了CameraBase.cpp中的connect()方法。

sp<Camera> Camera::connect(int cameraId, const String16& clientPackageName,    int clientUid){  return CameraBaseT::connect(cameraId, clientPackageName, clientUid);}

CameraBase实际上又继承了IBinder的DeathRecipient内部类,DeathRecipient虚拟继承自RefBase。RefBase是Android中的引用计数基础类,其中定义了incStrong、decStrong、incWeak和decWeak等涉及sp/wp的指针操作函数,当然这扯远了。

template <typename TCam>struct CameraTraits {};template <typename TCam, typename TCamTraits = CameraTraits<TCam> >class CameraBase : public IBinder::DeathRecipient{public:    static sp<TCam>   connect(int cameraId,                 const String16& clientPackageName,                 int clientUid);  ……}

class DeathRecipient : public virtual RefBase{public:  virtual void binderDied(const wp<IBinder>& who) = 0;};

回到Camera::connect()的实现上,其中,new TCam(cameraId)生成BnCameraClient对象,BnCameraClient定义在ICameraClient.h文件中,继承自模板类BnInterface。getCameraService()方法返回CameraService的服务代理BpCameraService,BpCameraService同样继承自模板类BnInterface。然后通过Binder通信发送CONNECT命令,当BnCameraService收到CONNECT命令后调用CameraService的connect()成员函数来做相应的处理。

template <typename TCam, typename TCamTraits>sp<TCam> CameraBase<TCam, TCamTraits>::connect(int cameraId,                        const String16& clientPackageName,                        int clientUid){  ALOGV("%s: connect", __FUNCTION__);  sp<TCam> c = new TCam(cameraId); // BnCameraClient   sp<TCamCallbacks> cl = c;  status_t status = NO_ERROR;  const sp<ICameraService>& cs = getCameraService(); // BpCameraService  if (cs != 0) {    TCamConnectService fnConnectService = TCamTraits::fnConnectService;    status = (cs.get()->*fnConnectService)(cl, cameraId, clientPackageName, clientUid,                       /*out*/ c->mCamera);  }  if (status == OK && c->mCamera != 0) {    c->mCamera->asBinder()->linkToDeath(c);    c->mStatus = NO_ERROR;  } else {    ALOGW("An error occurred while connecting to camera: %d", cameraId);    c.clear();  }  return c;}

class BnCameraClient: public BnInterface<ICameraClient>{public:  virtual status_t  onTransact( uint32_t code,                  const Parcel& data,                  Parcel* reply,                  uint32_t flags = 0);};

class BpCameraService: public BpInterface<ICameraService>{public:  BpCameraService(const sp<IBinder>& impl)    : BpInterface<ICameraService>(impl)  {  }  ……}

注:connect()函数在BpCameraService和BnCameraService的父类ICameraService中声明为纯虚函数,在BpCameraService和CameraService中分别给出了实现,BpCameraService作为代理类,提供接口给客户端,真正实现在BnCameraService的子类CameraService中。

在BpCameraService中,connect()函数实现如下:

// connect to camera service (android.hardware.Camera)  virtual status_t connect(const sp<ICameraClient>& cameraClient, int cameraId,               const String16 &clientPackageName, int clientUid,               /*out*/               sp<ICamera>& device)  {    Parcel data, reply;    data.writeInterfaceToken(ICameraService::getInterfaceDescriptor());    data.writeStrongBinder(cameraClient->asBinder());    data.writeInt32(cameraId);    data.writeString16(clientPackageName);    data.writeInt32(clientUid);    remote()->transact(BnCameraService::CONNECT, data, &reply); // BpBinder的transact()函数向IPCThreadState实例发送消息,通知其有消息要发送给binder driver
    if (readExceptionCode(reply)) return -EPROTO;    status_t status = reply.readInt32();    if (reply.readInt32() != 0) {      device = interface_cast<ICamera>(reply.readStrongBinder()); // client端读出server返回的bind    }    return status;  }

首先将传递过来的Camera对象cameraClient转换成IBinder类型,将调用的参数写到Parcel(可理解为Binder通信的管道)中,通过BpBinder的transact()函数发送消息,然后由BnCameraService去响应该连接,最后就是等待服务端返回,如果成功则生成一个BpCamera实例。

真正的服务端响应实现在BnCameraService的onTransact()函数中,其负责解包收到的Parcel并执行client端的请求的方法。

status_t BnCameraService::onTransact(  uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){  switch(code) {  
  ……   case CONNECT: {      CHECK_INTERFACE(ICameraService, data, reply);      sp<ICameraClient> cameraClient =          interface_cast<ICameraClient>(data.readStrongBinder()); // 使用Camera的Binder对象生成Camera客户代理BpCameraClient实例       int32_t cameraId = data.readInt32();      const String16 clientName = data.readString16();      int32_t clientUid = data.readInt32();      sp<ICamera> camera;      status_t status = connect(cameraClient, cameraId,          clientName, clientUid, /*out*/camera); // 将生成的BpCameraClient对象作为参数传递到CameraService的connect()函数中       reply->writeNoException();      reply->writeInt32(status); // 将BpCamera对象以IBinder的形式打包到Parcel中返回       if (camera != NULL) {        reply->writeInt32(1);        reply->writeStrongBinder(camera->asBinder());      } else {        reply->writeInt32(0);      }      return NO_ERROR;    } break;  ……  }}

主要的处理包括:

  1. 通过data中Camera的Binder对象生成Camera客户代理BpCameraClient实例;
  2. 将生成的BpCameraClient对象作为参数传递到CameraService(/frameworks/av/services/camera /libcameraservice/CameraService.cpp)的connect()函数中,该函数会返回一个BpCamera实例;
  3. 将在上述实例对象以IBinder的形式打包到Parcel中返回。

最后,BpCamera实例是通过CameraService::connect()函数返回的。CameraService::connect()实现的核心是调用connectHelperLocked()函数根据HAL不同API的版本创建不同的client实例(早期版本中好像没有connectHelperLocked()这个函数,但功能基本相似)。

status_t CameraService::connectHelperLocked(    /*out*/    sp<Client>& client,    /*in*/    const sp<ICameraClient>& cameraClient,    int cameraId,    const String16& clientPackageName,    int clientUid,    int callingPid,    int halVersion,    bool legacyMode) {  int facing = -1;  int deviceVersion = getDeviceVersion(cameraId, &facing);  if (halVersion < 0 || halVersion == deviceVersion) {    // Default path: HAL version is unspecified by caller, create CameraClient    // based on device version reported by the HAL.    switch(deviceVersion) {     case CAMERA_DEVICE_API_VERSION_1_0:      client = new CameraClient(this, cameraClient,          clientPackageName, cameraId,          facing, callingPid, clientUid, getpid(), legacyMode);      break;     case CAMERA_DEVICE_API_VERSION_2_0:     case CAMERA_DEVICE_API_VERSION_2_1:     case CAMERA_DEVICE_API_VERSION_3_0:     case CAMERA_DEVICE_API_VERSION_3_1:     case CAMERA_DEVICE_API_VERSION_3_2:      client = new Camera2Client(this, cameraClient,          clientPackageName, cameraId,          facing, callingPid, clientUid, getpid(), legacyMode);      break;     case -1:      ALOGE("Invalid camera id %d", cameraId);      return BAD_VALUE;     default:      ALOGE("Unknown camera device HAL version: %d", deviceVersion);      return INVALID_OPERATION;    }  } else {    // A particular HAL version is requested by caller. Create CameraClient    // based on the requested HAL version.    if (deviceVersion > CAMERA_DEVICE_API_VERSION_1_0 &&      halVersion == CAMERA_DEVICE_API_VERSION_1_0) {      // Only support higher HAL version device opened as HAL1.0 device.      client = new CameraClient(this, cameraClient,          clientPackageName, cameraId,          facing, callingPid, clientUid, getpid(), legacyMode);    } else {      // Other combinations (e.g. HAL3.x open as HAL2.x) are not supported yet.      ALOGE("Invalid camera HAL version %x: HAL %x device can only be"          " opened as HAL %x device", halVersion, deviceVersion,          CAMERA_DEVICE_API_VERSION_1_0);      return INVALID_OPERATION;    }  }  status_t status = connectFinishUnsafe(client, client->getRemote());  if (status != OK) {    // this is probably not recoverable.. maybe the client can try again    return status;  }  mClient[cameraId] = client;  LOG1("CameraService::connect X (id %d, this pid is %d)", cameraId,     getpid());  return OK;}

可见,在CAMERA_DEVICE_API_VERSION_2_0之前使用CameraClient进行实例化,之后则采用Camera2Client进行实例化。以CameraClient为例,其initialize()函数如下:

status_t CameraClient::initialize(camera_module_t *module) {  int callingPid = getCallingPid();  status_t res;  LOG1("CameraClient::initialize E (pid %d, id %d)", callingPid, mCameraId);  // Verify ops permissions  res = startCameraOps();  if (res != OK) {    return res;  }  char camera_device_name[10];  snprintf(camera_device_name, sizeof(camera_device_name), "%d", mCameraId);  mHardware = new CameraHardwareInterface(camera_device_name);  res = mHardware->initialize(&module->common);  if (res != OK) {    ALOGE("%s: Camera %d: unable to initialize device: %s (%d)",        __FUNCTION__, mCameraId, strerror(-res), res);    mHardware.clear();    return res;  }  mHardware->setCallbacks(notifyCallback,      dataCallback,      dataCallbackTimestamp,      (void *)(uintptr_t)mCameraId);  // Enable zoom, error, focus, and metadata messages by default  enableMsgType(CAMERA_MSG_ERROR | CAMERA_MSG_ZOOM | CAMERA_MSG_FOCUS |         CAMERA_MSG_PREVIEW_METADATA | CAMERA_MSG_FOCUS_MOVE);  LOG1("CameraClient::initialize X (pid %d, id %d)", callingPid, mCameraId);  return OK;}

上述函数中,主要注意以下流程:

  1. 加粗的代码CameraHardwareInterface新建了了一个Camera硬件接口,当然,camera_device_name为摄像头设备名;
  2. mHardware->initialize(&module->common)调用底层硬件的初始化方法;
  3. mHardware->setCallbacks将CamerService处的回调函数注册到HAL处。

CameraHardwareInterface定义了Camera的硬件抽象特征,由此进入到HAL。

 

四、HAL:CameraHardwareInterface

CameraHardwareInterface的作用在于链接Camera Server和V4L2,通过实现CameraHardwareInterface可以屏蔽不同的driver对Camera Server的影响。CameraHardwareInterface同样虚拟继承自RefBase。

class CameraHardwareInterface : public virtual RefBase {public:  CameraHardwareInterface(const char *name)  {    mDevice = 0;    mName = name;  }  ……}

CameraHardwareInterface中包含了控制通道和数据通道,控制通道用于处理预览和视频获取的开始/停止、拍摄照片、自动对焦等功能,数据通道通过回调函数来获得预览、视频录制、自动对焦等数据。当需要支持新的硬件时就需要继承于CameraHardwareInterface ,来实现对应的功能。CameraHardwareInterface提供的public方法如下:

1258.tmp

在前一节中,initialize()函数调用了mHardware->initialize和mHardware->setCallbacks,下面来看下CameraHardwareInterface.h对其的实现。

status_t initialize(hw_module_t *module)  {    ALOGI("Opening camera %s", mName.string());    camera_module_t *cameraModule = reinterpret_cast<camera_module_t *>(module);    camera_info info;    status_t res = cameraModule->get_camera_info(atoi(mName.string()), &info);    if (res != OK) return res;    int rc = OK;    if (module->module_api_version >= CAMERA_MODULE_API_VERSION_2_3 &&      info.device_version > CAMERA_DEVICE_API_VERSION_1_0) {      // Open higher version camera device as HAL1.0 device.      rc = cameraModule->open_legacy(module, mName.string(),                        CAMERA_DEVICE_API_VERSION_1_0,                        (hw_device_t **)&mDevice);    } else {      rc = CameraService::filterOpenErrorCode(module->methods->open(        module, mName.string(), (hw_device_t **)&mDevice));    }    if (rc != OK) {      ALOGE("Could not open camera %s: %d", mName.string(), rc);      return rc;    }    initHalPreviewWindow();    return rc;  }

在initialize()方法中,通过cameraModule->open_legacy打开摄像头模组,initHalPreviewWindow()用于初始化Preview的相关流opspreview_stream_ops,初始化hal的预览窗口。

void initHalPreviewWindow()  {    mHalPreviewWindow.nw.cancel_buffer = __cancel_buffer;    mHalPreviewWindow.nw.lock_buffer = __lock_buffer;    mHalPreviewWindow.nw.dequeue_buffer = __dequeue_buffer;    mHalPreviewWindow.nw.enqueue_buffer = __enqueue_buffer;    mHalPreviewWindow.nw.set_buffer_count = __set_buffer_count;    mHalPreviewWindow.nw.set_buffers_geometry = __set_buffers_geometry;    mHalPreviewWindow.nw.set_crop = __set_crop;    mHalPreviewWindow.nw.set_timestamp = __set_timestamp;    mHalPreviewWindow.nw.set_usage = __set_usage;    mHalPreviewWindow.nw.set_swap_interval = __set_swap_interval;    mHalPreviewWindow.nw.get_min_undequeued_buffer_count =        __get_min_undequeued_buffer_count;  }

/** Set the notification and data callbacks */  void setCallbacks(notify_callback notify_cb,           data_callback data_cb,           data_callback_timestamp data_cb_timestamp,           void* user)  {    mNotifyCb = notify_cb;    mDataCb = data_cb;    mDataCbTimestamp = data_cb_timestamp;    mCbUser = user;    ALOGV("%s(%s)", __FUNCTION__, mName.string());    if (mDevice->ops->set_callbacks) {      mDevice->ops->set_callbacks(mDevice,                  __notify_cb,                  __data_cb,                  __data_cb_timestamp,                  __get_memory,                  this);    }  }

set_callbacks中,__notify_cb、__data_cb、__data_cb_timestamp和__get_memory分别消息回调,数据回调,时间戳回调,以及内存相关操作的回调。

 

以上通过简略分析应用层调用Camera.open()之后在Framework、ART、Library以及HAL层的响应,来说明Android中Camera系统的整体架构,希望对读者能有一定的帮助,后续将在理解Camera整体架构的基础,探索更加高效的Preview方式,敬请期待!