H.266/VVC code learning 42: ALF notes in VTM7.0

In order to facilitate understanding and subsequent learning, a statistics is performed.
The main print some data, respectively apsIdx, apsIdxChroma, alfCtuFilterIndex[i], m_ctuAlternative[1][i], m_ctuAlternative[2][i].

As shown in the figure above, it is assumed that there are n APS indexes for brightness (n=1 in the figure above):
1. The available number of APS is controlled at the slice level, not CTU, and the type of filter used by CTU control.
2. The number of brightness APS is 16+1 alfCtuFilterIndex[i]; the number of chroma APS is only 1, but there are multiple filters in chroma APS. m_ctuAlternative[1][i]
3. For each frame, the available APS types are 16 indexed from 0 to 15 Fixed filter, plus an APS filter with index 16 (there was an operation to store the coefficients of the APS index into 16).
4. Perhaps the usage rate of APS filters is higher than that of fixed filters?
5. Whether to enable ALF and chroma for the luminance CTU, this CTU does not affect it.

In the above figure, 2 kinds of brightness APS are used, so there are 16 and 17 indexes.

The above figure shows that APS may not exist in chroma, and ALF is not used in chroma channels.