Erosion is a fundamental morphological operator: each pixel is replaced by the minimum value in its structuring element neighborhood. With a 3x3 square SE, isolated white pixels are completely removed and thin lines are broken. FPGA implementation uses a cascaded comparator min-tree with line-buffer sliding window.

I_out(x,y) = min({ I(x+dx, y+dy) | (dx,dy) ∈ SE }})
Square SE: all 9 pixels; Cross SE: 5 center pixels