Cast CvCameraViewFrame to Mat throws Error. What to do instead?

edit

public Mat onCameraFrame(CvCameraViewFrame inputFrame) {              
    Imgproc.cvtColor((Mat)inputFrame, grayscaleImage, Imgproc.COLOR_RGBA2RGB);



E/AndroidRuntime: FATAL EXCEPTION: Thread-2912
              java.lang.ClassCastException: org.opencv.android.JavaCameraView$JavaCameraFrame cannot be cast to org.opencv.core.Mat
                  at mzx.chn.lanedetection.LaneDetectActivity.onCameraFrame(LaneDetectActivity.java:242)
                  at org.opencv.android.CameraBridgeViewBase.deliverAndDrawFrame(CameraBridgeViewBase.java:391)
                  at org.opencv.android.JavaCameraView$CameraWorker.run(JavaCameraView.java:350)
                  at java.lang.Thread.run(Thread.java:856)