Mat::convertto() not working in javacv, tutorial 2 mixed

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I am developing an android opencv apps. Based on Opencv4android SDK tutorial 2- mixed process.

in the frame processing functionpublic Mat onCameraFrame(CvCameraViewFrame inputFrame) {}

since the Frame is RGBA and I want to make RGB by doing show

 mRgba = inputFrame.rgba();
 mGray = inputFrame.gray();

 Mat mRgb=new Mat(640,480,CvType.CV_8UC3);

 mRgba.convertTo(mRgb, CvType.CV_8UC3);

 PinkImage(mRgba.dataAddr());

I debug and log the things I passed to jni part, and I find it's not working at all. mRgb is CV_8UC4 even after the converto();

any idea for this?

point me out please, I write C++ opencv for a while and remember just a bit of java.