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

Started Aug 17, 2009 | Discussions thread
GordonBGood Veteran Member • Posts: 6,308
Re: PROGRESS: Haven't forgetten you...

GordonBGood wrote:

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.

I am almost completed the correction algorithm now, although it didn't help that I spend several hours yesterday debugging it when it didn't need to be debugged: I mistakenly was comparing an ISO 6400 image in my analysis tools to an ISO 200 image for the correcter application - just too tired, I guess. At any rate, it looks like there is a tiny little bit of correction even for ISO 200 images. So back to testing and comparing ISO 6400 images....

But today I am hosting a birthday party, so it will be at least tomorrow.

Once I finish the correction algorithm (only about 4 to 6 hours of programming away), I have only about another 20 hours or so to make the program support PEF files (about 6 hours) and compressed DNG files (about 14 hours) in case some have used Lightroom to look at the images with the resulting default behaviour that the files are converted to the Adobe version of lossless compression for DNG's. It isn't time wasted, in that I have been meaning to support lossless compression for DNG files for a while now anyway for my own raw converter program.

At some stage in there, I will also look into removing the gross vertical banding that godfrog showed here.

Regards, GordonBGood

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