Log2 issue in CCLM

[zhangkai]

In 8.4.5.2.13 Specification of INTRA_LT_CCLM, INTRA_L_CCLM and INTRA_T_CCLM intra prediction mode
...
– If diff is not equal to 0, the following applies:
diffC = maxC − minC (410)
x = Floor( Log2( diff ) ) (411)
normDiff = ( ( diff << 4 ) >> x ) & 15 (412)
x += ( normDiff != 0 ) ? 1 : 0 (413)
y = Floor( Log2( Abs ( diffC ) ) ) + 1 (414)
a = ( diffC * ( divSigTable[ normDiff ] | 8 ) + 2y − 1 ) >> y (415)

...

Here, diffC may be equal to zero. Log2(0) is meaningless.
eq (414) should be changed to

y = diffC > 0 ? Floor( Log2( Abs ( diffC ) ) ) + 1 : 0

[fbossen]

The suggested fix is incorrect as it will produce an incorrect result for negative values of diffC.

Alternate fixes to avoid Log2(0) would be:
y = Ceil( Log2( Abs( diffC ) + 1 ) ) )
or
y = diffC == 0 ? 0 : Floor( Log2( Abs ( diffC ) ) ) + 1

Also note that in (415) 2^y-1 should probably be replaced by ( ( 1 << y ) >> 1 ) such that the result is 0 when y is 0.

[zhangkai]

Yes, I agree. My initial intention is to change it to
y = Abs(diffC) > 0 ? Floor( Log2( Abs ( diffC ) ) ) + 1 : 0
Abs is missing in the ticket

[bbross]

Thanks for reporting!

This has been fixed in JVET-Q2001-v5.