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

Started Aug 17, 2009 | Discussions thread
GordonBGood Veteran Member • Posts: 6,308
Re: Magenta edge tinging at high ISO and a possible solution...

dlacoutu wrote:

K20 users may have experienced strange magenta tinging at the top and bottom edges of their pics when using high ISOs...
... (removed for brevity, GBG)

... became this one once the black pixels were (crudely) averaged and subtracted:

(the pic took a slight green tinge because the whole pic in fact suffered from magenta tinging...)

Now, in the following thread :
http://forums.dpreview.com/forums/readflat.asp?forum=1036&thread=32600106&page=1

GordonBGood nicely proposed to try to deliver a small application that will remove this magenta tinge right in the PEF/DNG, thus greatly simplifying the post-processing steps with high ISO!

Hi Dominique (dlacoutu) et al, and I am indeed working on this mini-project, with one day expended for what should be about a two day project. If I don't have too many interruptions today, I should have something for your testing by tomorrow. I only became aware of this problem as a result of Dominique's post to the thread linked above, as I do not have a K20D and haven't looked that much into the use of ISO 6400 images, although I suspect that there is the same problem to a lesser degree at lower ISO's, as well.

The user interface works something like the Adobe DNG Converter application, with the user selecting the source and destination folders (no options necessary, at least as yet) and pressing the "do-it" button. I have gone a little beyond just a simple program in that it will correct all the K20D DNG's that it finds in the given source folder if they haven't already been corrected and run the real work as a background task so that the program does not appear to go unresponsive (which is where part of the time went yesterday.

The first version will only support K20D DNG's and only uncompressed DNG's as in preferably the original DNG's from the K20D, although I can also support the uncompressed DNG's from the Adobe DNG Converter application. It is a fair amount of extra work to support all the variations of DNG loss less compression although not so much to support just the one (simplest) variation as produced by the K-7, if it appears that the K-7 has this same magenta edges problem. I will also add PEF file support for both the K20D or the K-7 once Dominique is happy with the results for the DNG files.

The program is written for the DotNet Framework 2.0 and accordingly can be run on Linux or Mac platforms using Mono, which I have tested.

As to the green WB tinging of the result as in Dominique's example above, once the correction is proven to work I can add an automatic White Balance (WB) correction based on the amount of magenta that is removed as compared to the strongest green channel readings in the image, and that will likely take care of it. I could do this one of two ways (at least) in that I could just scale the green channel down in the raw data or I could just modify the "As-Shot WB" tag in the respective files when the camera is in Auto WB (AWB) mode, with each having advantages and disadvantages. I think the latter approach may be the better one, in that this green tinge should mostly only be present for AWB situations unless the user has tried to compensate for the magenta tinge with Fine Tuning of the other presets.

For K-7 users, do you have this same problem of magenta (or other coloured) tinged edges at high ISO or for highly exposure boosted images?

Regards, GordonBGood

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