Improve CDEF parameter selection

[tmatth]

Paraphrasing @tterribe:

For a given frame, we can have up to 8 sets of CDEF parameters for superblocks to choose from (note: it is better to have fewer than 8 for lower bitrates, to reduce the cost of coding them per SB).
Currently these are hard coded.

We could do a feedforward approach, where for the last frame of a given type, you greedily search for better CDEF parameters, then select those for the next frame of the same type (inter/intra, pyramid level, etc.?).

We could also look at stats and try and find decent values to hardcode for lower bitrates.

Once we can buffer a frame in rav1e, that would allow to do something closer to what libaom does, where we do multiple passes of one frame and greedily search for better sets, i.e. swapping in parameters that perform better.

[tmatth]

@tterribe suggested as a preliminary step, simply varying the weights depending on quantizer. It may be worth it to dump whatever libaom's using as parameters for different QP.

[tmatth]

via @KyleSiefring:
https://aomedia.googlesource.com/aom/+/805c7d26fca822b33e22ee489b6de7ad287b6086

[tmatth]

From running this script on subset1

tmatth@hydra ~ $ cat hack_metrics.sh 
#!/bin/sh

set -e
set -u

SEQ=${1:-/mnt/raid/Videos/subset1-y4m/Air_Force_Academy_Chapel,_Colorado_Springs,_CO_04090u_original.y4m}
BASENAME_SEQ=$(basename ${SEQ})

AOMDIR=aom-master/aom_build
OUTDIR=~/out
mkdir -p ~/out

cd ${AOMDIR}

for x in  20 32 43 55 63; do
	echo $x
	OUTPUT=${BASENAME_SEQ}.$x.ivf
	./aomenc ${SEQ} --ivf --tile-rows=2 --tile-columns=2 --passes=1 --quiet --rt --cpu-used=8 --end-usage=q --cq-level=$x -o ${OUTDIR}/${OUTPUT}
done

I see that aomenc uses these CDEF parameters per QP:

QP=20 strengths[0]:0 uv_strengths[0]:0 q:22 pri_damping:3 sec_damping:3
QP=32 strengths[0]:0 uv_strengths[0]:4 q:44 pri_damping:3 sec_damping:3
QP=43 strengths[0]:4 uv_strengths[0]:4 q:95 pri_damping:4 sec_damping:4
QP=55 strengths[0]:9 uv_strengths[0]:8 q:235 pri_damping:5 sec_damping:5
QP=63 strengths[0]:22 uv_strengths[0]:13 q:465 pri_damping:5 sec_damping:5

[tmatth]

@tterribe suggests making cdef parameters depend on quantizer in terms of log_target_q instead of ac_q(fi.base_q_idx, 0, bd) as i32;), e.g.:

let quantizer = bexp64(log_target_q + scale);
rate.rs:    let quantizer_u = bexp64(log_target_q + offset_u + scale);
rate.rs:    let quantizer_v = bexp64(log_target_q + offset_v + scale);

[tmatth]

@tterribe suggests that for inter frames, we may want to search (as master is doing) but between 2 choices: strength dependent on qp vs. disabling CDEF entirely.

I also want to compare only forcing strength based on QP for keyframes (and leaving inter frames with the existing search) to see if inter frames are where objective fast is regressing.

[tmatth]

I also want to compare only forcing strength based on QP for keyframes (and leaving inter frames with the existing search) to see if inter frames are where objective fast is regressing.

CDEF strength from QP for intra frames only gives a -0.39% improvement across metrics, but no real encoder speed improvement on objective-fast-1:
https://beta.arewecompressedyet.com/?job=master-d3992e510b9c4e67ad99f8ceaa59943dc34534f7&job=pick-cdef-from-q-intra-only%402019-06-27T03%3A09%3A08.928Z

Compared to always selecting CDEF strength from QP (for inter and intra):
https://beta.arewecompressedyet.com/?job=pick-cdef-from-q-intra-only%402019-06-27T03%3A09%3A08.928Z&job=pick-cdef-from-q-always%402019-06-27T03%3A09%3A40.343Z
Here the speed savings are significant at low QP.

[tmatth]

So commit 5625ee3 is pretty restrictive in terms of CDEF search (effectively disabling it), I think the next step would be to put the CDEF from QP mode behind a speed setting for low QP.

[tmatth]

Basically this is at implementing this TODO:

rav1e/src/encoder.rs

Line 946 in dc8bb63

match self.cdef_search_method {

[tmatth]

@xiphmont I know this is going back a ways, but do you recall why these strengths are multiplied by 4?

rav1e/src/encoder.rs

Line 698 in e9be6c9

cdef_y_strengths: [