AndroidX Camera：如何实现指定区域的捏合缩放

要解决捏合缩放始终以预览中心为基准的问题，核心是将触摸区域的屏幕坐标转换为相机传感器的归一化坐标，再通过CameraX的CameraControl设置缩放中心，而非依赖默认的中心缩放。以下是具体实现步骤：

1. 监听捏合手势与触摸坐标 #

使用Android原生的ScaleGestureDetector捕获捏合操作，同时记录捏合手势的中心点屏幕坐标：

private lateinit var scaleGestureDetector: ScaleGestureDetector
private var currentZoomRatio = 1f
private lateinit var cameraControl: CameraControl
private lateinit var cameraInfo: CameraInfo

private val scaleListener = object : ScaleGestureDetector.SimpleOnScaleGestureListener() {
    override fun onScaleBegin(detector: ScaleGestureDetector): Boolean {
        // 记录当前缩放比例，避免初始缩放跳变
        currentZoomRatio = cameraInfo.zoomState.value?.zoomRatio ?: 1f
        return true
    }

    override fun onScale(detector: ScaleGestureDetector): Boolean {
        // 计算新的缩放比例并限制范围
        val maxZoom = cameraInfo.zoomState.value?.maxZoomRatio ?: 1f
        val newZoom = currentZoomRatio * detector.scaleFactor
        currentZoomRatio = newZoom.coerceIn(1f, maxZoom)

        // 获取捏合中心点的屏幕坐标
        val touchScreenX = detector.focusX
        val touchScreenY = detector.focusY

        // 将屏幕坐标转换为相机传感器归一化坐标
        val previewView = findViewById<PreviewView>(R.id.preview_view)
        val previewMatrix = Matrix()
        previewView.getPreviewMatrix(previewMatrix)
        
        // 反转矩阵，将屏幕坐标映射到预览纹理坐标
        val invertedMatrix = Matrix()
        if (previewMatrix.invert(invertedMatrix)) {
            val touchPoints = floatArrayOf(touchScreenX, touchScreenY)
            invertedMatrix.mapPoints(touchPoints)
            
            // 转换为传感器归一化坐标（0-1范围，(0,0)为传感器左上角）
            val sensorNormX = touchPoints[0] / previewView.width
            val sensorNormY = touchPoints[1] / previewView.height

            // 设置缩放中心与缩放比例
            cameraControl.setZoomCenter(PointF(sensorNormX, sensorNormY))
            cameraControl.setZoomRatio(currentZoomRatio)
        }
        return true
    }
}

2. 绑定触摸监听与Camera实例 #

在CameraX初始化完成后，绑定预览View的触摸事件，并获取CameraControl和CameraInfo实例：

// 假设已通过CameraProvider绑定相机生命周期
val camera = cameraProvider.bindToLifecycle(
    this, CameraSelector.DEFAULT_BACK_CAMERA, previewUseCase
)
cameraControl = camera.cameraControl
cameraInfo = camera.cameraInfo

// 初始化手势检测器并绑定触摸监听
scaleGestureDetector = ScaleGestureDetector(this, scaleListener)
previewView.setOnTouchListener { _, event ->
    scaleGestureDetector.onTouchEvent(event)
    true
}

3. 适配传感器方向（可选） #

部分设备的相机传感器方向与屏幕方向不一致，可能导致缩放中心偏移。可通过cameraInfo.sensorRotationDegrees调整坐标：

// 根据传感器旋转角度调整归一化坐标
val sensorRotation = cameraInfo.sensorRotationDegrees
val adjustedX: Float
val adjustedY: Float
when (sensorRotation) {
    90 -> {
        adjustedX = 1f - sensorNormY
        adjustedY = sensorNormX
    }
    180 -> {
        adjustedX = 1f - sensorNormX
        adjustedY = 1f - sensorNormY
    }
    270 -> {
        adjustedX = sensorNormY
        adjustedY = 1f - sensorNormX
    }
    else -> {
        adjustedX = sensorNormX
        adjustedY = sensorNormY
    }
}
cameraControl.setZoomCenter(PointF(adjustedX, adjustedY))

关键说明 #

• 必须使用PreviewView.getPreviewMatrix()获取预览的转换矩阵，该矩阵包含了预览的缩放、旋转、裁剪等变换信息，反转后才能将屏幕坐标正确映射到相机传感器坐标系。
• 缩放比例需限制在1f到maxZoomRatio之间，避免超出相机硬件支持的范围。
• 可通过cameraInfo.zoomState的观察者实时更新当前缩放比例，确保手势开始时的初始值准确。

内容的提问来源于stack exchange，提问作者Rahul Mishra