Canon Line Offset Noise

Started Oct 20, 2009 | Discussions thread
ejmartin Veteran Member • Posts: 6,274
Re: About options

GaborSch wrote:

John used the bottom of a black frame . That works well within the same image , i.e. the banding in the black frame can be removed that way (it is demonstrable by Rawnalyze).

However, we do not have that good strip of masked pixels.

Different forms of pattern noise call for different treatments. The 7D sort seems to be a fixed repeating pattern; if so, then the preferred treatment is to generate a pattern removal template that would be applied as a subtractive offset to all images. dcraw for instance has this capability, though unfortunately most of the GUI raw converters based on it (eg RawTherapee) do not enable this capability.

A template for fixed pattern noise can be made by stacking multiple blackframes, the more the merrier; the more one stacks, the more the random read noise averages away, leaving only the fixed pattern noise. The template then with only the fixed pattern noise is subtracted from the image one wishes to convert, and removes the fixed pattern noise from it before conversion.

An example of the technique can be found at
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/#patternnoise

This technique will of course fail for random line noise which varies from frame to frame; stacking blackframes will not pick out this sort of noise, and there is no template one can build that will reliably remove this sort of noise from a given image. This is where it is useful to have a large masked border; as John has mentioned, with a sufficiently large border area one can detect the variable line noise sufficiently accurately to subtract it for the given image. However, it may not be worth it to Canon or other manufacturers to incur the cost of added sensor area that goes only to this purpose.

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