text_src/15.04__vp8-bitstream__calculation-of-control-parameters.txt

#### 15.4 Calculation of Control Parameters                {#h-15-04}


We conclude the discussion of loop filtering by showing how the
thresholds supplied to the procedures above are derived from the two
control parameters `sharpness_level` (an unsigned 3-bit number having
maximum value 7) and `loop_filter_level` (an unsigned 6-bit number
having maximum value 63).

While the `sharpness_level` is constant over the frame, individual
macroblocks may override the `loop_filter_level` with one of four
possibilities supplied in the frame header (as described in
Section 10).

Both the simple and normal filters disable filtering if a value
derived from the four pixels that straddle the edge (2 either side)
exceeds a threshold / limit value.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/* Luma and Chroma use the same inter-macroblock edge limit */
uint8 mbedge_limit = ((loop_filter_level + 2) * 2) +
  interior_limit;

/* Luma and Chroma use the same inter-subblock edge limit */
uint8 sub_bedge_limit = (loop_filter_level * 2) + interior_limit;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


The remaining thresholds are used only by the normal filters.  The
filter-disabling interior difference limit is the same for all edges
(luma, chroma, inter-subblock, inter-macroblock) and is given by the
following.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
uint8 interior_limit = loop_filter_level;

if( sharpness_level)
{
    interior_limit  >>=  sharpness_level > 4 ?  2 : 1;
    if( interior_limit > 9 - sharpness_level)
        interior_limit = 9 - sharpness_level;
}
if( !interior_limit)
    interior_limit = 1;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


Finally, we give the derivation of the high edge-variance threshold,
which is also the same for all edge types.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
uint8 hev_threshold = 0;

if( we_are_decoding_akey_frame)   /* current frame is a key frame */
{
    if( loop_filter_level >= 40)
        hev_threshold = 2;
    else if( loop_filter_level >= 15)
        hev_threshold = 1;
}
else                            /* current frame is an interframe */
{
    if( loop_filter_level >= 40)
        hev_threshold = 3;
    else if( loop_filter_level >= 20)
        hev_threshold = 2;
    else if( loop_filter_level >= 15)
        hev_threshold = 1;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}