[Elphel-support] mage Noise problem

[Andrey Filippov]

>
> ...so my interest in RAW is mainly to understand current JPEG
> behavior/difference with RAW and being able to compare things to "original"
> image to make sure everything goes as it should.
>


Alexander,

Yes, it is exactly the purpose of the "raw" images in the camera

>
>  2. However, resulting RAW and JPEG images have considerably different
> brightness/contrast, suggesting that some parts of processing are done
> differently to RAW than JPEG?
>


> http://www.ndl.kiev.ua/downloads/elphel/image.jpg
> http://www.ndl.kiev.ua/downloads/elphel/image_8.raw
> http://www.ndl.kiev.ua/downloads/elphel/image_16.raw
>


> Gamma is set to 1.0 and black level to zero in parameters, that's why
> image_8.raw completely matches image_16.raw after normalization. However,
> JPEG image image.jpg clearly differs from both of RAW images. I've tried to
> perform the same experiment with "factory default" settings (only set WB
> off, auto exposure off, color to 0), but result is similar.
>
>
I used your files, converted raw to png with
  convert -size 2596x1940 -depth 8 gray:image_8.raw image_8.png
  convert -size 2596x1940 -depth 16 gray:image_16.raw image_16.png
Then opened them all using "open as layers" in gimp. Then shifted JPEG image
by 2 pixels horizontally and 2 - vertical as raw has 2-pixel border and so
total image dimension differs by 4 pixels each direction (these extra pixels
are used in color conversion). and - subtracted (shifted) image.jpg from
image_8.raw - it matched pretty good - the result seemed black (I admit - I
did not add fat zero, so half pixels should be zeros instead of negative).
So I incresed contrast with "levels" 32 times, and the result is here:

http://community.elphel.com/files/raw/image_diff_contrast.jpeg

It is obvious that those were different exposures - you can see there is
amplified noise - lower in dark areas, higher - in bright ones (Shot noise)
- that is another illustration why it is appropriate to use non-linear
compression of the dynamic range. You also can see some dim light near the
bottom - I think it is because of 100Hz (or 120Hz if it was in US)
modulation of the light source intensity.

Anyway - I do not see any difference between raw and JPEG in this eaxample,
the difference between shots is definitely larger. You may want to use test
modes - it is possible to generate uniform gradient instead of the sensor
data (before any conversions) by the FPGA (sensor-independent) or use some
test patterns that sesnor can generate itself - both modes are supported
through parameters (i.e. with parsedit.php).

Andrey
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