translate example into Java

I'm trying to translate this demo into Java http://docs.opencv.org/3.2.0/d5/dc4/tutorial_video_input_psnr_ssim.html but I'm stuck on the lines using overloading of `-=` on `Mat`. My understanding of C++ is very limited, but I expected to see an `operator-=()` here http://docs.opencv.org/3.2.0/d3/d63/classcv_1_1Mat.html but there isn't one. What is the Java equivalent of `-=` on a `Mat`? Note that the Java docs for 3.2.0 are not available, so consult 3.1.0 instead http://docs.opencv.org/java/3.1.0/