Fix negative MV diff rate calculation when allow_high_precision_mv is false

[redzic]

Bit shifting for division by a power of 2 causes incorrect rounding for negative numbers. This should very slightly increase quality as a result of more accurate rate distortion calculations.

[codecov-commenter]

Codecov Report

Base: 86.52% // Head: 86.48% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (9d50709) compared to base (c113c00).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3003      +/-   ##
==========================================
- Coverage   86.52%   86.48%   -0.04%     
==========================================
  Files          89       89              
  Lines       34046    34046              
==========================================
- Hits        29457    29445      -12     
- Misses       4589     4601      +12     

Impacted Files	Coverage Δ
src/me.rs	95.10% <100.00%> (-0.49%)	⬇️
src/tiling/tile_state.rs	89.13% <0.00%> (-0.73%)	⬇️
src/api/internal.rs	97.47% <0.00%> (-0.23%)	⬇️
v_frame/src/plane.rs	88.78% <0.00%> (-0.23%)	⬇️
src/header.rs	68.93% <0.00%> (-0.13%)	⬇️
src/deblock.rs	95.43% <0.00%> (ø)
src/encoder.rs	87.09% <0.00%> (ø)

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[barrbrain]

The logic is sound. I started an AWCY run on objective-1-fast at default speed:

PSNR Y	PSNR Cb	PSNR Cr	CIEDE2000	SSIM	MS-SSIM	PSNR-HVS Y	PSNR-HVS Cb	PSNR-HVS Cr	PSNR-HVS	VMAF	VMAF-NEG
0.0260	0.0176	-0.0239	0.0250	0.0430	0.0421	0.0239	-0.0521	-0.0485	0.0202	0.1038	0.1044

[barrbrain]

I think the confounding factor is that this function is called for motion estimation on subsampled frames.

[redzic]

@barrbrain I also saw your other commit (barrbrain@5b45ccf)

At least for me, it wasn't obvious at first that it changes the rate curve itself rather than just the rounding (see below)

https://www.desmos.com/calculator/ekdizwd572

perhaps the rate curve could somehow be adjusted for better gains? I'll look into it further

[barrbrain]

@barrbrain I also saw your other commit (barrbrain@5b45ccf)

This was a mistake, I shouldn't have changed the scale.

I tried another experiment which gave modest gains on top of the rounding change, barrbrain@7baa93f :
"Do not round in get_mv_rate except for original resolution"

PSNR Y	PSNR Cb	PSNR Cr	CIEDE2000	SSIM	MS-SSIM	PSNR-HVS Y	PSNR-HVS Cb	PSNR-HVS Cr	PSNR-HVS	VMAF	VMAF-NEG
-0.0248	0.1399	0.0763	-0.0095	-0.0751	-0.0429	-0.0080	0.1179	0.1972	-0.0085	-0.1348	-0.1132

However, the more surprising result was that if that change were applied first, the rounding change had no impact on decisions.

Compared to the baseline, results for this experiment were a wash:

PSNR Y	PSNR Cb	PSNR Cr	CIEDE2000	SSIM	MS-SSIM	PSNR-HVS Y	PSNR-HVS Cb	PSNR-HVS Cr	PSNR-HVS	VMAF	VMAF-NEG
0.0010	0.1568	0.0512	0.0156	-0.0321	-0.0008	0.0159	0.0657	0.1465	0.0116	-0.0314	-0.0091

[redzic]

Hmm, I'll mark this as draft for now then unless I can find an improvement later on