Magenta edge tinging at high ISO and a possible solution...

Started Aug 17, 2009 | Discussions thread
GordonBGood Veteran Member • Posts: 6,308
Re: PROGRESS: You might like to try this...

dlacoutu wrote:

Gordon, your remark made me think... Maybe a better strategy would be to add magenta to the center of the pic instead of removing it from the border, to avoid accented contrast...

But this would complicate the algorithm, as each line would now have to know the greatest value found across the pic, instead of being self-sufficient.

This would maybe have the added benefit of a lesser impact on the WB...

This is exactly how I am changing the correction algorithm now: I am determining the slope of the trend between the center of the sensor and the edges for each of the colour channels by linear regression by the simple least squares method as well as the average offset along each of the vertical (most important) and horizontal (less strong but there is an effect) axes for landscape orientation. I then correct the data with the corrections added or subtracted to the center of the sensor and the opposite to the edges so that the average offset is not changed at all and there is no stitching across the different trend lines at the center of the sensor. Thus I am just moving the offsets of the borders and the center towards each other so that the average stays the same. Note that this works when the actual tinging is not magenta as is apparently sometimes the case as each of the colour channels are detected and adjusted independently.

In this way, I won't have to smudge the White Balance (WB) at all as the average black levels won't change for any channel.

For later work, I am thinking that I may be able to fix godfrog's type of vertical banding problem by looking at a sliding window of what looks like 256 photosites wide and determining a integrating correction as compared to the trend line above. I don't think that this would be for general consumption, as it could increase the narrow variation of Vertical Pattern Noise (VPN) while it decreases the wider bands.

Best regards, GordonBGood

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