Improve raw noise model for N6.

Replace the current raw noise model, calibrated using Android's script, with an improved noise model, using an
updated version of that script.

The new raw noise model has a different parameterization in terms
of ISO:

- The formula for 'S' scales with ISO. While shot noise should
  be zero for a completely dark signal, we still include an offset
  here to help compensate for black level variations with ISO.

- The new formula for 'O' scales with ISO^2, not ISO. This form is
  required because 'O' corresponds to the *variance* due to read
  noise. The new formula also treats analog and digital gain
  separately. While pre-ADC read noise is affected by both types
  of gain, post-ADC read noise is affected *only* by digital gain.

Bug: 16273313
Change-Id: Iec5c7854c1d529b97567235cda9a8041b0ec0914
1 file changed