StartCamera.kt

private fun startCamera() {
        CameraX.unbindAll()

        val previewConfig = PreviewConfig.Builder().apply{
            setTargetResolution(Size(680, 400 ))
            setTargetRotation(viewFinder.display.rotation)
        }.build()

        // Build the viewFinder use case
        val preview = Preview(previewConfig)
        // Recompute layout every time viewFinder is updated
        preview.setOnPreviewOutputUpdateListener {
            val parent = viewFinder.parent as ViewGroup
            parent.removeView(viewFinder)
            parent.addView(viewFinder, 0)

            viewFinder.surfaceTexture = it.surfaceTexture
            updateTransform()

        }
        CameraX.bindToLifecycle(this, preview)
}

private fun updateTransform() {
        val matrix = Matrix()

        // calculate center of texture view
        val centerX = viewFinder.width / 2f
        val centerY = viewFinder.height / 2f

        // correct the preview output
        val rotationDegrees = when(viewFinder.display.rotation) {
            Surface.ROTATION_0 -> 0
            Surface.ROTATION_90 -> 90
            Surface.ROTATION_180 -> 180
            Surface.ROTATION_270 -> 270
            else -> return
        }
        
        // apply transformations
        matrix.postRotate(-rotationDegrees.toFloat(), centerX, centerY)
        viewFinder.setTransform(matrix)
}