erosion

Keeping a pixel only where the structuring element fits entirely inside the foreground.

Erosion of $A$ by $B$ , written $A\ominus B$ , sets output pixel $p$ to foreground only if every foreground cell of $B$ , aligned so $B$ 's origin sits at $p$ , lands on a foreground pixel of $A$ -- treating any structuring-element cell that falls outside $A$ as background, so a shape near the border erodes there too.