Convert Vizier RD parameters to normalized factors

This patch converts the Vizier custom RD multipliers, to factors
that adjust each RD multiplier either side of its default value, where
a factor of 1.0 will give the previous default  behavior.

Ultimately I would like to replace the multiple RD multipliers
triggered at different Q thresholds (eg, low, medium, high q)
with a function that adjusts the rd behavior smoothly as Q
changes.

Vizier could then be presented with a single adjustment control
for each of key frame and inter frame rd.

The current behavior is problematic.

Firstly having hard threshold Q values at which rd behavior changes
may cause anomalies in the rate distortion curve, where in some
situations, raising  Q, for example,  may not cause the expected drop
in rate and rise in distortion, because we have crossed a threshold
where the rate distortion multiplier changes sharply and this alters
the balance of bits spent in the prediction and residual parts of the
signal.

Having a single  value that is used for a range of Q index values
(eg 0-64), (65-128)  may also cause problems and over-fitting in
the context of the Vizier ML project. This project tries to optimize
the values for each Q range, for various YT formats, but does so
by analyzing the results of single point encodes on a set of clips.
For a given format all the clips are encoded with the same parameters
(target rate etc) so  there is likely to be clustering in regards to the
Q values used. For example the training set may give a new value
for the Q  range 0-64 but most of the data points used may have Q
close 64.

It will likely require several iterations working with the Vizier team
to get this right. This patch just gives an initial framework for
testing.

Change-Id: Iaa4cd5561b95a202bcae7a1d876c4f40ef444fa2
2 files changed