Raw channel noise reduction

[Beep6581]

Originally reported on Google Code with ID 2679

This is related to issue 2624.

I'm no expert on noise reduction so this may be a bad idea. However it seems to me
a bit backwards to only apply noise reduction after demosaicing, so here's the idea:

Noise in an image is roughly read noise from the sensor A/D conversion and photon shot
noise. Both are dependent on exposure level, ie how close to clipping the signal is.
After demosaicing and white balancing etc raw channels are scaled and mixed so you
don't have the same direct connection to signal level. The red channel may have been
scaled up one stop compared to the green for example.

I'd like to test and see what happens if you apply noise reduction pre-demosaic (as
a raw preprocess step in the raw tab) per raw channel, and with a curve to control
noise reduction amount based on exposure level (ie typically no noise reduction on
the top 4-5 stops or so, and then increase the amount towards the bottom). This would
give you direct control to apply noise reduction based on how much noise there actually
is in the signal.

With the current noise reduction tool I lack control to adjust noise reduction based
on how noisy the signal is, and I can't leave well-exposed areas without noise reduction.
There is some control, but not enough for my own needs which is about refining low
ISO images, not make high ISO images better.

Reported by [email protected] on 2015-02-20 10:55:12