​https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104591

When you enable the QPA algorithm (Perceptually optimised QP adaptation, -qpa 1 --SliceChromaQPOffsetPeriodicity=1) is enabled and the internal bit-depth parameter is the default set in the main .cfg files (--internal-bitdepth=10), the result is as expected, a considerable rate saving and an improvement in perceptual quality.

However, if we force the internal bitdepth to be 8 (--internal-bitdepth=8), the result is a considerable increase in bitrate compared to not enabling the QPA algorithm.

As an example, I have encoded frames 1, 101, 201, 301, 401 and 501 of the BQTerrace sequence in All Intra mode, with base QPs 22, 27, 32, 37 and 42. Here are the average results:

InternalBitDepth=8, QPA off

base QP	Mbps	MS-SSIM
22	2.659	0.996
27	0.989	0.986
32	0.503	0.979
37	0.268	0.966
42	0.138	0.942

InternalBitDepth=10, QPA off

base QP	Mbps	MS-SSIM
22	2.724	0.996
27	1.021	0.987
32	0.517	0.979
37	0.275	0.967
42	0.142	0.944

InternalBitDepth=8, QPA on

base QP	Mbps	MS-SSIM
22	3.388	0.998
27	1.561	0.992
32	0.601	0.983
37	0.309	0.972
42	0.167	0.952

InternalBitDepth=10, QPA on

base QP	Mbps	MS-SSIM
22	2.454	0.996
27	0.820	0.986
32	0.408	0.977
37	0.210	0.963
42	0.115	0.935

As can be seen, for a base QP of 22, enabling the QPA algorithm increases the bit rate by almost 1 Mbps if we compare the bit depth level of 8 versus 10, when it would be expected to have a very similar value.

I attach the RD curves corresponding to the tables, where you can visually appreciate the mentioned rate increase.

I suspect that the problem, if any, may be in the calculation of picture or block energy (activity), or in the algorithm that modifies the Lagrangian during the rate control.

As per the meeting notes following elements of DMVR is adopted

• Early termination w/ (0,0) position SAD between list0 and list1
• Block sizes for DMVR W*H>=64 && H>=8
• Split the CU into multiple of 16x16 sub-blocks for DMVR of CU size > 16*16
• Reference block size (W+7)*(H+7) (for luma)
• 25 points SAD-based integer-pel search (i.e. (+-) 2 refinement search range, single stage)
• Bilinear-interpolation based DMVR
• MVD mirroring between list0 and list1 to allow bilateral matching
• “Parametric error surface equation” based sub-pel refinement
• Luma/chroma MC w/ reference block padding (if needed)
• Refined MVs used for MC and TMVPs only

In current VTM version, Adaptive Search pattern is used instead of 25 points raster scan order based exhaustive search for +/- 2 refinement.