Passing Array to rsForEach in Renderscript Compute

I found there's lacking good documentation in RenderScript, for what I know, forEach in RS is to execute the root() for each individual item in the allocation.

I am trying to make a library for Renderscript that does Image processing, as a starting point, I reached this great answer. But the problem, is that the blur operation is on Each pixel and each pixel requires another loop (n with blur width) of calculation. Although running on multi-core, it is still a bit too slow.

I am trying to modify it to allow (two-pass) box filter, but that requires working on a single row or column instead of cell. So, is there any way to ask foreach to send an array to root()?