just because I saw it on the other site…
GpuMat has upload and download methods. use them to copy data from host (main memory) to device (GPU memory) and back. do not mess around with that pointer. it’s an implementation detail that happens to be exposed but you don’t need it, unless you are going to use CUDA functions on the host/CPU side, and need to get at the cuda object that’s contained in the GpuMat (OpenCV thing).
I am very sure that you would benefit from working through this “tutorial”. it contains the basics of OpenCV’s CUDA modules but isn’t specifically made for the purpose.
https://docs.opencv.org/master/dd/d3d/tutorial_gpu_basics_similarity.html
and here’s how you’d write a kernel. a kernel is code that runs on the GPU. it’s called “kernel” because usually it’s the code of an “inner loop”, and CUDA takes care to throw it at every element of an array that lives in GPU memory. code running on the CPU can’t touch data on the GPU. code on the GPU can’t (easily) touch host memory. some piece of code can’t “pick” where it runs, nobody can. it’s written for the host or for the GPU, and it only runs where it’s written to run.
an introduction to CUDA or GPU programming in general might be a good idea, if any of those statements surprised you. there’s a ton more to know (streams at least) to make actual use of a GPU.
I’m not claiming to be correct in every aspect but the gist of it should be right.