AV1技术学习：Constrained Directional Enhancement Filter

CDEF允许编解码器沿某些(可能是倾斜的)方向应用非线性消阶滤波器。它以8×8为单位进行。如下图所示，通过旋转和反射所示的三个模板来定义八个预设方向。

Templates of preset directions and their associated
directions. The templates correspond to directions of 45◦, 22.5◦, and
0◦, as shown by the dash lines. Each preset direction d ∈ {0,...,7}
can be obtained by using the template directly, rotating the
template by 90◦ clockwise (marked by †) or reflecting the template
along the horizontal axis (marked by ρ)

 解码器利用重建像素通过最小化下式来选择向索引

式中xp为像素p的值，Pd,k为沿d方向k行的像素，μd,k为Pd,k的平均值;

一个主要的滤波器是沿着所选的方向应用，而一个次要的滤波器沿着主方向的偏移45◦的方向应用。像素p(x, y)的滤波运算公式为 

其中 wp d,m,n 和 ws d,m,n 分别为与主滤波器和次滤波器相关联的滤波器系数，如下图所示。其中Sp、Ss为主要滤波器和次要滤波器的强度指标，D为阻尼系数。f()是一个分段线性函数 

Primary filter templates associated with direction
d ∈ {0,...,7} (subject to rotation and reflection), where w1  4/16
and w2  2/16 for even strength indexes, and w1  w2  3/16 for
odd strength indexes.

Secondary filter templates associated with each direction
(subject to reflection), where w3  2/16 and w4  1/16. The
secondary filter is applied along the direction 45◦ off the
corresponding primary direction d.

它排除了值远离p(x, y)的参考像素。注意，参考像素p(m, n)是在应用去块滤波器之后，但在应用CDEF滤波器之前重建的像素。

Frame Header中传输多达八组滤波器参数，包括亮度和色度分量的主滤波器和次滤波器强度指标。每个64 × 64块从预设中选择一组来控制滤波器操作。