Boundary Issues

Clipping

Treats the non-existent pixels as black.

Wrapping

Uses the opposite edge of the image to continue the edge. It was intended for periodic functions and is useful for seamless images, but looks noticeably bad for non-seamless images.

Extending

Copies a chunk of the edge to fit the filter kernel.

Reflecting

Copies a chunk of the edge like the previous method, but it mirrors the edge like a reflection.