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.
The handbook · 7 volumes
Fora Soft Video Engineering Handbook
The deblocking filter hides block edges inside the prediction loop, but set too strong it trades blocking for softness. We wrote seven volumes covering the whole video stack, and for the deblocking filter start with Volume 1: in-loop filters, transforms and quantization, rate control and artifact metrics in H.264, HEVC and AV1. Written by Nikolay Sapunov, CEO at Fora Soft.
Fighting blocking without making the picture soft — is your deblocking filter set right?
Share sample encodes and your encoder config: we tune the deblocking filter and related settings and show the before and after with metrics. 250+ video products shipped since 2005.
