From DXOMark: More pixels offsets noise3

Started May 30, 2009 | Discussions thread
John Sheehy Forum Pro • Posts: 21,615
Re: From DXOMark: More pixels offsets noise3

DSPographer wrote:

John Sheehy wrote:

[snip]
I know exactly how they can avoid banding after capture, even if they
can't do it electronically. Banding is extremely simple. I don't
know what the H*ll is wrong with companies that leave banding in
their images - it almost seems like sabotage, to bank for future
upgrade carrots.

Really? I looked at the banding noise from my 5D2 and it was not as
simple to remove in software as I expected because it wasn't as
stable across the image as I had hoped. The techniques I can think of
to reduce would either not eliminate it completely or else they could
impact the signal details in deep shadows. Do you know of a technique
that would leave the signals untouched while completely removing the
banding?

Most of the banding noise energy is completely consistent across the frame in the RAW data. Conversion, and SNRs of different tonal levels may make it seem otherwise. Radio interference, when it occurs, will also be asynchronous to the lines.

Canon RAWs have 70 to 150 pixels to the left of each row of pixels which are masked from light; you can get a rough map of horizontal banding just from that.

Canon doesn't have many blind pixels on the vertical columns, though. They only have 13 such pixels on the 5D2; not a great sample.

If the sensor had, say, 180 blind pixels on both ends of each row of pixels, and 120 at both ends of each column, the banding could be very effectively removed by converters, even if the banding offsets were on a gradient, such as a line being +0.2 ADU too high on the left end, ramping to 0.4 ADU too low on the right end, due to electrical pulses or interference.

Figuring out the banding from the image itself can be a little tricky. Some images are easy; a low-contrast shot taken with extremely weak exposure has noise and banding stronger than signal, and simply shrinking the image to one pixel wide with downsampling, and then resizing it back creates a suitable mask. If you have a vertical strip of blackness or smooth, OOF solids, you can get the banding pattern from that as well.

Sometimes you can take even a non-linear image, and do local banding removal by using a high-pass filter on a copy of the image with perfectly horizontal or vertical mothion blur, as needed. You can paint any areas on the correction frame where you don't want correction in gray 128.

The best way to subtract banding, however, is to do it in a linear state, before conversion. You can not subtract it accurately from a converted image on a tone curve.

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John

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tko
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