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小米845系列机器类原生调用红外摄像头进行人脸识别 背景 小米845系列的小米8、小米8屏幕指纹版有一颗前置红外摄像头用于面部识别，这样即使在无光的环境下仍可以使用面部解锁。然而如果使用LineageOS的设备树不加修改编译出来的类原生无法使用红外摄像头，只能使用普通的摄像头，而PixelExperience的设备树是可以的，故探究其原因。 原因 CameraID 首先是需要指定人脸识别模块使用红外摄像头。翻阅PixelExperience的相关设备树源码可以发现：在overlay中设置了调用CameraID为5的摄像头用作人脸识别。 https://github.com/PixelExperience-Devices/device_xiaomi_dipper/blob/fourteen/overlay/packages/apps/FaceUnlockService/app/src/main/res/values/config.xml <?xml version="1.0" encoding="utf-8"?> <resources> <integer name="override_front_cam_id">5</integer> <bool name="use_alternative_vendor_impl">true</bool> </resources> PixelExperience使用的是motorola的人脸解锁方案，而现在很多类原生使用的是AOSPA的ParanoidSense，因此上述设置的overlay并不适用。 查阅ParanoidSense源码可以看到：使用了Properties属性ro.face.sense_service.camera_id标识使用的摄像头。 https://github.com/AOSPA/android_packages_apps_ParanoidSense/blob/uvite/src/co/aospa/sense/camera/CameraUtil.kt#L32 val cameraIdProp = SystemProperties.get("ro.face.sense_service.camera_id") 因此理论上只要设置ro.face.sense_service.camera_id属性为5即可使其调用红外人脸摄像头。 vendor.camera.aux.packagelist属性 如果仅仅设置上述CameraID相关属性，那么人脸模块调用时会直接报错。翻阅Frameworks部分的代码可以得知：系统默认隐藏了除主前摄和主后摄之外的其他辅助摄像头（即AUXCamera），而红外摄像头也属于AUXCamera之一。 取LineageOS的Framework代码作为例子： https://github.com/LineageOS/android_frameworks_base/blob/lineage-21.0/core/java/android/hardware/Camera.java#L295-L301 /** * Returns the number of physical cameras available on this device. * The return value of this method might change dynamically if the device * supports external cameras and an external camera is connected or * disconnected. * * If there is a * {@link android.hardware.camera2.CameraCharacteristics#REQUEST_AVAILABLE_CAPABILITIES_LOGICAL_MULTI_CAMERA * logical multi-camera} in the system, to maintain app backward compatibility, this method will * only expose one camera per facing for all logical camera and physical camera groups. * Use camera2 API to see all cameras. * * @return total number of accessible camera devices, or 0 if there are no * cameras or an error was encountered enumerating them. */ public static int getNumberOfCameras() { int numberOfCameras = _getNumberOfCameras(); if (!shouldExposeAuxCamera() && numberOfCameras > 2) { numberOfCameras = 2; } return numberOfCameras; } 不难看出，即使设备有大于2颗摄像头，若函数shouldExposeAuxCamera()返回为False则会强制指定摄像头数量为2，阻止程序访问CameraID>2的摄像头；再来看看这个判断函数的定义： https://github.com/LineageOS/android_frameworks_base/blob/lineage-21.0/core/java/android/hardware/Camera.java#L264-L278 /** * @hide */ public static boolean shouldExposeAuxCamera() { /** * Force to expose only two cameras * if the package name does not falls in this bucket */ String packageName = ActivityThread.currentOpPackageName(); if (packageName == null) return true; List<String> packageList = Arrays.asList( SystemProperties.get("vendor.camera.aux.packagelist", packageName).split(",")); List<String> packageExcludelist = Arrays.asList( SystemProperties.get("vendor.camera.aux.packageexcludelist", "").split(",")); return packageList.contains(packageName) && !packageExcludelist.contains(packageName); } 可以看出，假如包名在属性vendor.camera.aux.packagelist中并且不在vendor.camera.aux.packageexcludelist中就会返回为True，否则False。 那么解决方案就很明了了：将ParanoidSense的包名加到vendor.camera.aux.packagelist属性中即可让其能够调用到红外摄像头。 解决 将ParanoidSense的包名co.aospa.sense加到vendor.camera.aux.packagelist中： https://github.com/YoriInstitute/crDroidAndroid_device_xiaomi_sdm845-common/commit/28fe5bc41d49b31b4acb840bf167b70d70a40c61 设置ro.face.sense_service.camera_id属性以指定摄像头： https://github.com/YoriInstitute/crDroidAndroid_device_xiaomi_equuleus/commit/67518f130650e4592b5f4c7210248072058d48cc https://github.com/YoriInstitute/AOSPA_device_xiaomi_equuleus/blob/topaz/device.mk#L397-L399 在crDroid魔改的ParanoidSense中，因指定了其位于system_ext（https://gitlab.com/crdroidandroid/android_packages_apps_FaceUnlock/-/commit/545688260eb32ba19f348e84e3cae89ba29f20d1），故将这个属性加到system_ext的prop文件中。