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

Started Aug 17, 2009 | Discussions
dlacoutu Contributing Member • Posts: 811
Magenta edge tinging at high ISO and a possible solution...

Hello all...

K20 users may have experienced strange magenta tinging at the top and bottom edges of their pics when using high ISOs...

This pic was taken with the cap on, @6400 (shutter speed has no effect on this)...

Some time ago (and on "another" forum ) I presented a possible solution by using the masked black pixels available in both PEF and DNG files of the k20.

So thic pic:

... 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!

Big thanks again to him, and let's wait for the results!

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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

jamesm007 Veteran Member • Posts: 5,663
Re: Magenta edge tinging at high ISO and a possible solution...

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.

Yes the K20D has it fairly strong at iso3200 as well (nothing like iso6400) under the right conditions and perhaps a little lower. Can you offer a user adjustable strength slider? I know you really love this more work. Or is it necessary?

And to the forum what do you think if Gordon succeeds? And does the K-7 have this problem or is it gone? Look at RAW files. Why has no one responded to a wonderful solution to a real problem at high ISO at least with the K20D.

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Marius Shanzer Forum Member • Posts: 53
Re: Magenta edge tinging at high ISO and a possible solution...

I don't dare to shoot at 6400 but I am forced to shoot a lot at 3200, typically photos of dancers in poorly lit studios and clubs. The end result is that I have high levels of blue chroma noice that is most visible in the shadows. My solution has been dial in a correction of -30 Shadow Tint on Camera Calibration tab in Bridge which neutralizes the shadows and brings noise levels down a point where Noiseware can handle it. Sometimes, cutting down the Green and/or Red saturation is also helpful. The result is a much cleaner image with somewhat odd colors in Photoshop where I use selective color correction and HSL to restore them -- something that isn't always 100% possible.

Is the chroma noise that I am describing similar to what you and Gordon and working on?
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OP dlacoutu Contributing Member • Posts: 811
Re: Magenta edge tinging at high ISO and a possible solution...

Great news, Gordon!

Regarding the WB, I'd say I don't care (shooting UniWB, the "as shot" WB is pretty useless for me), but I think your second proposal is the best : modifying the "as-shot" WB to remove the green tinge would appeal to the majority of users, without messing the RAW data more than necessary.

Maybe just a little request for your application : could you add an ISO threshold control? Meaning that only DNGs above the selected ISO will be treated. This would remove the need to move high-iso DNGs to a separate folder to be handled by you application.

Thanks again for the effort!

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OP dlacoutu Contributing Member • Posts: 811
Re: Magenta edge tinging at high ISO and a possible solution...

Marius :

Well, not, this is not related to the blue specks that appear at high-ISO... But from experience, these are really easy to handle through the Chroma noise settings of all the RAW converters around...

But you should really try 6400 one day... With proper exposure (sometimes a +2Ev is needed, especially under tungsten lights), it can do wonders...

Aside note : ACR is not so great with noise, it produces a rather large and blotchy noise pattern, quite displeasing at high iso.

RawTherapee and Bible are really better in the noise dept., as they produce a finely grained image, even at 6400...

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GordonBGood Veteran Member • Posts: 6,308
Re: Magenta edge tinging at high ISO and a possible solution...

jamesm007 wrote:

Yes the K20D has it fairly strong at iso3200 as well (nothing like iso6400) under the right conditions and perhaps a little lower. Can you offer a user adjustable strength slider? I know you really love this more work. Or is it necessary?

James, a strength slider isn't really necessary as the correction as determined by Dominique is based on the improperly corrected masked black offset determination photosites. These should be at zero other than for some random noise, but instead they have an average positive level that varies apparently linearly over the width and height of the image. The correction consists of determining the trend lines associated with this offset drift and subtracting these offsets from the rows and columns of all the exposed raw image data.

The correction applied probably will improve images down to about ISO 800 or so, as it looks from the K20D samples that I have that the offset error is about 10 levels out of the full 12-bit range of 4096 levels and the error will vary linearly with ISO sensitivity; however, you would likely never notice the fringing at ISO 800 and it would be pretty slight at ISO 1600.

Regards, GordonBGood

awaldram
awaldram Forum Pro • Posts: 13,263
Re: Magenta edge tinging at high ISO and a possible solution...

GordonBGood wrote:

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.

Your a star

Thank You

Regards, GordonBGood

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GordonBGood Veteran Member • Posts: 6,308
Re: Magenta edge tinging at high ISO and a possible solution...

dlacoutu wrote:

Great news, Gordon!

Yes, the project is moving along, but I did get interrupted today so I only got to work on it for about 4 hours. The user interface is complete (other than for adding any options ) and I am reading in the DNG files, parsing them a little to be sure of correct model number and etc., that the files haven't been changed by a DNG converter (I don't yet handle loss less compressed DNG's), unpacking the data, and writing the (presumed) corrected data back into the exact same formating of output file. Much of this was fairly easy as I just cribbed code from my raw conversion program, then stripped out quite a bit of parsing that isn't necessary as this program doesn't do any raw conversion whatsoever. I could have made it even simpler by looking for data at fixed file offsets but that wouldn't have been as universally friendly to different cameras such as the K-7 or if a firmware update changes location in the slightest way. When you see the project, you will see that it looks fairly professional, as I am so confident that this is going to work well that I have been coding for production.

Regarding the WB, I'd say I don't care (shooting UniWB, the "as shot" WB is pretty useless for me), but I think your second proposal is the best : modifying the "as-shot" WB to remove the green tinge would appeal to the majority of users, without messing the RAW data more than necessary.

Yes, I don't really want to modify the raw data any more than necessary and in fact there are ways not to modify it all all using horizontal and vertical gradient curves that are a feature of DNG. However, I want the technique to work for PEF files as well and they don't have such gradient definitions and there is a possibility that some raw converters may not fully implement the DNG specification and support gradients properly, so I'll stick to just subtracting variable offsets from the raw data and likely just smudging the As-Shot White Balance tags.

Maybe just a little request for your application : could you add an ISO threshold control? Meaning that only DNGs above the selected ISO will be treated. This would remove the need to move high-iso DNGs to a separate folder to be handled by you application.

As I told James above, this correction might still have a slight effect at ISO 800, so there is quite a wide range of ISO's where it would be desired. My thinking so far has been that it won't change the raw data at all or in very faint ways when the correction is not necessary as in ISO 400 and under, so it wouldn't hurt to apply it to all K20D (and maybe eventually K-7) raw files. There is a good possibility that one won't really need to keep the original files after the correction, as I am thinking that there may be no adverse effects on the image quality due to the correction. It shouldn't add any pattern noise or banding as my offset corrections are determined from a best fit straight line with all noise variations eliminated. But if necessary, I can add an ISO threshold for the correction, although this will make me have to parse the EXIF data for the ISO. I think we'll wait for past the alpha/beta stage to make a decision on this.

Thanks again for the effort!

You're most welcome!

Best regards, GordonBGood

brady38 Regular Member • Posts: 143
Re: Magenta edge tinging at high ISO and a possible solution...

tagged
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Critiques of my images are always welcome

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GordonBGood Veteran Member • Posts: 6,308
Tagging is no longer necessary, use Bookmark instead...

brady38 wrote:

tagged

Tagging is no longer necessary, use Bookmark instead. This will leave more room in the thread for peoples comments.

Regards, GordonBGood

GordonBGood Veteran Member • Posts: 6,308
Re: Magenta edge tinging at high ISO and a possible solution...

dlacoutu wrote:

Thanks again for the effort!

Dominique, just a request for a little support from you if you get some time in the next day or so: Could you email me (email address in profile) with a link where I can download one of your ISO 6400 lens cap on DNG shots for use to confirm that the program does what I think it does, as I don't have a K20D myself. You may as well send me a PEF file as well so I can do the same for it, too, as I plan to support PEF in the near future. I can confirm that the general correction algorithm is working from the raw sample images available from Imaging Resources ( http://www.imaging-resource.com/PRODS/K20D/K20DTHMB.HTM ) and can even see that my White Balance (WB) "smudge" is effective as there are lots of white areas in test samples, but I can't really prove that the magenta is greatly reduced unless the erroneous magenta is about the only thing in the image.

Also, you mentioned that you shoot images with UniWB (for others, do a search on this forum or the Internet to see what that means) so that As-Shot WB values don't have much meaning to you. Given this, what program do you use for raw conversion, how do you determine WB, and how did you show that after correction there was left a green tint. It would seem to me that this would indicate that you implemented the test correction in JPEG colour space after you had already developed the image with some sort of manual WB?

Briefly, the magenta edge tinging is caused by positive drift of the black offset of the colour channel raw data, which is not being properly compensated in-camera to have an average = mean =~ median value of zero, especially with regards to this offset drift.

As this is turning into a bit of a development blog for the project, I will describe the correction algorithm in a little detail, as follows:

  1. Read in the raw file format and parse it enough to confirm that the file is from a supported camera (currently K20D and maybe later the K-7) and that the tags are complete enough to be able to read the raw data, as well reading the ISO sensitivity from the EXIF (if necessary) and the As-Shot Neutral (WB) values for the RGB channels as well as the location in the file where the As-Shot Neutral and the raw data are located.

  2. Read the raw data and decompress/unpack it as necessary.

  3. Apply the correction using the masked black photosites on the right to be applied to the horizontal rows in landscape orientation and (possibly, if effective, the same correction using the masked black photosites at the bottom of the sensor in landscape orientation. It appears that the K20D scans some channels horizontally and some channels vertically, and this is the likely explanation for why there is both Horizontal and Vertical Pattern Noise (HPN and VPN) . It appears that the red and blue channels are read by horizontal rows and that the green channel is read by vertical columns, with the red and blue channels read from the center of the sensor toward the upper and lower edge while the green channel is read from the right to the left, all for landscape orientation.

  4. For both, the correction consists of determining the best fit straight lines for the channel data as to change of true mean = median, from the center to the top and bottom vertically and right to left horizontally in landscape orientation. Since this data appears to always have the median level slightly offset positively, the median can be determined as that level for which 50% of the readings are either at the level or lower.

  5. The best fit trend line is calculated on the basis of a running window of 256 samples across the black masked photosites of a given colour channel, which will produce several straight line trend lines of which likely only the green(s) will be used horizontally and the vertical four trend lines will be used vertically from center to top and center to bottom for each of the blue and red channels.

  6. The results of these trend lines will be subtracted from all of the rows for the red and blue raw data and from all the columns for the green.

  7. The resulting raw data will be repacked/re-compressed in the identical way as for the original raw data format.

  8. The As-Shot Neutral values will be "smudged" according to the average correction applied.

  9. The magenta corrected data will be written out to a file that is identical to the original file other than the corrected raw data and the "smudged" As-Shot Neutral values will be patched in.

Toward the end of today (as in about six hours from now), I will try to provide a screen shot of what the program looks like and a few comments on its use.

Regards, GordonBGood

OP dlacoutu Contributing Member • Posts: 811
Re: Magenta edge tinging at high ISO and a possible solution...

Great news!

DNG and PEF links send !

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GordonBGood Veteran Member • Posts: 6,308
Re: Magenta edge tinging at high ISO and a possible solution...

dlacoutu wrote:

DNG and PEF links send !

Dominique, files received and testing continues.

Running out of time today, but as promised here is the progress report:

1) Tested reading and writing of files with correction forced to zero and did a binary file compare to prove and nothing out of the ordinary was happening (fc b %f1 %f2) with the result that there were no differences as there should not be.

2) It looks like the download size of the program will be only well under 15 KBytes in a '.zip' file, so isn't going to put much load on a Live Spaces account where I will public share it.

3) To run it under Windows with at least DotNet Framework 2.0 installed, you just extract the '.exe' file from the '.zip' and double click it where you extract it to. There is no installation/uninstallation. To run it under Mono depends on your platform but you will just run it from a command line or batch file as "mono %path to executable"\"program file name"

4) When you run it, you select the Source Folder, the Destination Folder (which defaults to the Source Folder if it is selected first), with "Browse" buttons provided for both of these or one can manually enter the folder paths, and press the "Correct" button. The "Correct" button changes it's label and function to a "Cancel" Button and a list of "Pending: " files are shown in the log scrollable text box, which change to "Corrected: -> mc_" as the files are processed by a background task or to "Error correcting " messages and a reason for any problems.

5) It looks like it will take about one second per file to correct, including file read and write times, on a reasonably up-to-date computer.

6) When all files are complete, there is a completed message and a total time expended shown.

7) Currently, the program screenshot on completion of one test file looks like as follows:

Just give me another few hours tomorrow for final testing of the correction algorithm.

Regards, GordonBGood

OP dlacoutu Contributing Member • Posts: 811
Re: Magenta edge tinging at high ISO and a possible solution...

Seems to be going pretty good!

Already got a dumb question : what happens if you "Correct" an already corrected file?

The obvious answer must be "nothing, black pixels are now neutral", but I had to ask!

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godfrog Senior Member • Posts: 1,463
Re: Magenta edge tinging at high ISO and a possible solution...

Just took a look at what ISO 6400 looks like on my GX-20, shooting a lens cap in RAW, developed in Lightroom. Images are posted below, the the resized shots have +4 exposure (aka this is ISO 102400) and +150 brightness, to better show what the pattern looks like. The 100% crop shot does not have the +150 brightness, only +4 exposure.

Looks like correcting for this problem might be difficult on my camera, as the tinging varies from shot to shot (look at the top left corner, it has lots of green in the second shot). A greater issue is the hard vertical edge where the noise changes intensity. Ive included a real shot to show the effect this has, this shot is ISO 4500.

1. ISO 6400 lens cap shot, +4 exposure and +150 brightness

2. ISO 6400 lens cap shot, +4 exposure and +150 brightness

3. 100% crop of the noise (taken from picture 2, near top left corner, centered around the noise "edge), +4 exposure only

4. Real shot, no postprocessing besides cropping, ISO 4500

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GordonBGood Veteran Member • Posts: 6,308
Answer to double correction possibility...

dlacoutu wrote:

Already got a dumb question : what happens if you "Correct" an already corrected file?

The obvious answer must be "nothing, black pixels are now neutral", but I had to ask!

Actually, that's a good question and one that I've already taken care of. First, double correction should be impossible as I check for file names starting with "mc_" for the source files and also that the equivalent name starting with "mc_" in the destination folder does not exist before proceeding with correction for each file. Second, as you say, on the off chance that the user renamed a corrected file to remove the "mc_", the correction should do nothing as the masked black areas with be corrected to appear as expected "normal' by the correction algorithm; thus, the correction should end up to be zero. I will check this, however.

Regards, GordonBGood

jamesm007 Veteran Member • Posts: 5,663
Re: Magenta edge tinging at high ISO and a possible solution...

I was just playing around taking iso6400 pics with the lens cap on and using slow shutter NR and times of 1/80 then about one second.

The point, I forgot to use the viewfinder cover its a must at iso6400 and did change the results. The results were cleaner with the viewfinder cover on, as it was allowing light to enter the dark chamber altering the results.

I don't know if you did or did not, this is to remind everyone doing the lens cap test.
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GordonBGood Veteran Member • Posts: 6,308
Re: Magenta edge tinging at high ISO and a possible solution...

godfrog wrote:

Just took a look at what ISO 6400 looks like on my GX-20, shooting a lens cap in RAW, developed in Lightroom. Images are posted below, the the resized shots have +4 exposure (aka this is ISO 102400) and +150 brightness, to better show what the pattern looks like. The 100% crop shot does not have the +150 brightness, only +4 exposure.

Looks like correcting for this problem might be difficult on my camera, as the tinging varies from shot to shot (look at the top left corner, it has lots of green in the second shot). A greater issue is the hard vertical edge where the noise changes intensity. Ive included a real shot to show the effect this has, this shot is ISO 4500.

As you say, it looks like your camera has two problems, and only one of these will this current correction program eliminate, as follows:

1) Your camera has the hard vertical edge at the left side in landscape mode which is likely due to some small defect in your sensor, most likely a hot/stuck photosite in the masked black photosite strip along the bottom of the sensor (landscape) used for black offset determination that is causing an improper offset for this whole strip. You could look for a hot pixel in that area using a program that shows the full area of the sensor such as Faststone ( http://www.faststone.org ) at high zooms. As to correcting for it, the current program does not do it as it assumes that the changes across the sensor will be gradual and not abrupt and would eliminate any hottish photosites before it does the trend line calculation anyway. Full correction for this would require that one knows exactly how the camera does black offset compensation, and although I have made some guesses in this thread, no one but Pentax/Samsung will know for sure. However, your camera may offer some clues, especially if there are some hot photosite(s) in the mentioned black strip, so if you would care to send me a link to download one or both of the raw files for the first two examples, I can take a look. If I can come close to guessing how the camera algorithms work, I can improve the algorithm as to correcting the things Pentax/Samsung miss and possibly add something that will automatically take care of this hard band along with the second problem already described.

Now looking harder at the hard edge from dark to bright going from right to left at the left edge of your samples, one might think that the error is that the left edge is too bright, but in actual fact the edge might be caused by the next rightmost band being too dark, which is what I would expect from hot/stuck photosites in the next rightmost band in the bottom border black band. Downloading your 100% crop, I already see that the bands are about 335 pixels wide which mean the image is divided into 14 vertical correction bands, but I would like to confirm the exact pattern from the raw data. Based on what I see here, I should be able to take care of this problem just as I do for the second problem already described as long as I can establish the exact strip boundaries and a reasonable guess as to the Pentax correction algorithm, much as I was able to reduce the K10D/GX10's Vertical Pattern Noise (VPN) due to hot pixels in this masked strip for firmware version 1.0.

Question at large: do some samples of the K20D also sometimes show these vertical strips when push processed as described here? Running your camera in Live View mode and thus heating the sensor is likely to make the problem worse, as it would increase the presence of hot photosites, as would longer exposure times up to the point where Dark Frame Subtraction (DFS) cuts in. I see that these images were taken at a one second exposure but the MakerNotes tag has been stripped from the EXIF information and I can't see the sensor temperature (PhotoMe - http://www.photome.de ).

2) The fringing toward the top and bottom edges in landscape orientation is exactly what this correction program is designed to correct and the fact that the fringe colour varies from shot to shot doesn't concern me in the least as the correction is designed to be self adjusting due to being based on the trends established from the masked black offset determination photosites for each colour channel on the right and bottom sides of the sensor (landscape orientation).

Regards, GordonBGood

GordonBGood Veteran Member • Posts: 6,308
Re: Magenta edge tinging at high ISO and a possible solution...

jamesm007 wrote:

I was just playing around taking iso6400 pics with the lens cap on and using slow shutter NR and times of 1/80 then about one second.

The point, I forgot to use the viewfinder cover its a must at iso6400 and did change the results. The results were cleaner with the viewfinder cover on, as it was allowing light to enter the dark chamber altering the results.

I don't know if you did or did not, this is to remind everyone doing the lens cap test.

According to what I know and the camera manual, light from the viewfinder is not supposed to be able to reach the sensor due to the state of the mirror box while the image is shot. According to the manual, the reason one uses the viewfinder cover for low light shots is to keep the light from the viewfinder from affecting the separate exposure meter sensor. If you manually set the ISO sensitivity, aperture, and (most importantly) the shutter speed, then any light from the viewfinder should not affect the image at all. Are you finding that the light from the viewfinder is actually affecting the image pattern, contrary to what this says?

Regards, GordonBGood

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