I have some 1 band image. it is periodically (step=8) brighter in rows and cols. I want to suppress this periodical trait to remove the "grid". How do I do it in the most perfomant way?

I thought I should start with summing by rows and cols. I tried to do it by resizing but for some reason I've got values of -1 -- is it ok or is it some corner case bug?

irb(main):062:0> t.resize(1r/t.width, vscale: 1).to_a.take(10)
=> [[[-1]], [[-1]], [[-1]], [[-1]], [[-1]], [[-1]], [[-1]], [[1]], [[1]], [[-1]]]
irb(main):064:0> t.resize(1, vscale: 1r/t.height).to_a[0].take(10)
=> [[-1], [-1], [-1], [-1], [-1], [-1], [-1], [-1], [-1], [-1]]

UPD: I suppose I've found a proper method:

irb(main):066:0> t.shrinkv(t.height).to_a[0].take(10)
=> [[0], [0], [0], [0], [0], [0], [0], [0], [0], [0]]
irb(main):069:0> t.shrinkh(t.width).to_a.take(10)
=> [[[0]], [[0]], [[0]], [[0]], [[0]], [[0]], [[0]], [[1]], [[1]], [[0]]]

I wish there was a fast method to do the opposite. resize back to original size (to then subtract/divide/whatever) seems to be slow.