The deblocking filter is a normative in-loop step, defined inside the H.264/AVC and HEVC standards, that smooths block boundaries after a frame is reconstructed to hide blocking. In-loop is the key word: the filtered frame goes back into the prediction loop and becomes the reference for later frames, so encoder and decoder must apply the identical filter or they drift apart - unlike a post-processing deblock applied only at display. It is deliberately cautious so it does not erase real detail: in H.264 it runs along each transform-block edge and decides, per edge, how hard to smooth using a boundary strength from 0 to 4, adjusting at most three pixels each side. HEVC simplifies this to an 8x8 grid with three strength levels and adds Sample Adaptive Offset; AV1 chains deblocking with CDEF and loop restoration. The filter trades visible tiles for mild softness - so when bits run out, blocking becomes blur - and the H.264 designers reported up to about 9% bitrate savings at equal quality.
