text_src/15.00__vp8-bitstream__loop-filter.txt

### Section 15: Loop Filter                                {#h-15-00}


Loop filtering is the last stage of frame reconstruction and the
next-to-last stage of the decoding process.  The loop filter is
applied to the entire frame after the summation of predictor and
residue signals, as described in Section 14.

The purpose of the loop filter is to eliminate (or at least reduce)
visually objectionable artifacts associated with the semi-
independence of the coding of macroblocks and their constituent
subblocks.

As was discussed in Section 5, the loop filter is "integral" to
decoding, in that the results of loop filtering are used in the
prediction of subsequent frames.  Consequently, a functional decoder
implementation must perform loop filtering exactly as described here.
This is distinct from any postprocessing that may be applied only to
the image immediately before display; such postprocessing is entirely
at the option of the implementor (and/or user) and has no effect on
decoding per se.

The baseline frame-level parameters controlling the loop filter are
defined in the frame header (Section 9.4) along with a mechanism for
adjustment based on a macroblock's prediction mode and/or reference
frame.  The first is a flag (`filter_type`) selecting the type of
filter (normal or simple); the other two are numbers
(`loop_filter_level` and `sharpness_level`) that adjust the strength or
sensitivity of the filter.  As described in Sections 9.3 and 10,
`loop_filter_level` may also be overridden on a per-macroblock basis
using segmentation.

Loop filtering is one of the more computationally intensive aspects
of VP8 decoding.  This is the reason for the existence of the
optional, less-demanding simple filter type.

Note carefully that loop filtering must be skipped entirely if
`loop_filter_level` at either the frame header level or macroblock
override level is `0`.  In no case should the loop filter be run with a
value of `0`; it should instead be skipped.

We begin by discussing the aspects of loop filtering that are
independent of the controlling parameters and type of filter chosen.