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Archive 2012 · Occurance of Aliasing (Moire) and Postprocessing...

  
 
Tommy_D
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p.2 #1 · p.2 #1 · Occurance of Aliasing (Moire) and Postprocessing...


sjms wrote:
actually it has quite a bit


What you are seeing in the image above is merely a function of the image size in relation to the resolution of your monitor. If you look at the 100% file on Nikon's website, there really is next to no moire.



Feb 09, 2012 at 09:57 AM
sandycrane
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p.2 #2 · p.2 #2 · Occurance of Aliasing (Moire) and Postprocessing...


Initially when I viewed the image there was a ton of moire, but as I scrolled the screen it disappeared. A very interesting and complex optical effect!


Feb 09, 2012 at 11:39 AM
bbvaj
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p.2 #3 · p.2 #3 · Occurance of Aliasing (Moire) and Postprocessing...


@ monito: Thanks for the explanation. can you elaborate a little bit on "Expect to see more "noise" in shadow areas like grass"

@rico: The example in #3 is great. But could not understand where do i see moire in #4?

@elkhornsun: Thanks for sharing your experience. When you say you can take 10 shots a person in pin-stripped shirt and have Moire visible in only one of the pictures. i think it falls more into category #3 explained by rico. Only when the focus is right on the shirt and it has enough high frequency it shows moire, if the focus is slightly off the shirt (enough DOF from face to shirt) that slight blur on the shirt can avoid moire.



Feb 09, 2012 at 12:12 PM
bbvaj
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p.2 #4 · p.2 #4 · Occurance of Aliasing (Moire) and Postprocessing...


Also can anyone with experience touch on how much more resolution we gain without AA.

Also does it matter only at very large prints or are there any other implications/advantages of no-AA



Feb 09, 2012 at 12:14 PM
Airphoto
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p.2 #5 · p.2 #5 · Occurance of Aliasing (Moire) and Postprocessing...


Thanks Monito. Nice explanation!!


Feb 09, 2012 at 01:10 PM
Monito
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p.2 #6 · p.2 #6 · Occurance of Aliasing (Moire) and Postprocessing...


I think the reason that pictures like the one posted here work (very little aliasing visible) is that the D800E has enough pixels that it can rely on the lack of quality of the Nikon lenses to act as an anti-aliasing filter.

That was a tad provocative, since the Nikon (Nikkor) lenses are very good quality and are generally as good as any in their class or so near that the differences have little consequence. The same principles would apply to other leading lens manufacturers.

Thus permit me to expand on my remarks. The high quality Nikkor lenses have limits (like every lens ever manufactured or that will be manufactured). Given the prices they must be sold at, those limits are high, but not stupendous.

I wasn't able to find lists of specific resolutions at line pairs per millimeter (lpm) of Nikkors, and MTF contrast at the lpm is important too. But I have some indication that top quality Nikkors can do about 80 lpm at 50% MTF contrast.

Think of a line pair as a single sine wave cycle, which is what it approximates at 50% MTF: gray to black to gray to white to gray. Thus it gives us the spatial frequency or rather the wavelength at that spatial frequency. One millimetre divided by 80 lpm = 12.5 micrometres per line pair.

The thing to note is that the dimensions of the D800E sensor are 7360 x 4912 pixels and (close to or exactly) 36 x 24 mm. Now, 36 mm = 36,000 micrometres. 36,000/ 7,360 = 4.89 micrometres per sensel (pixel) side.

The Nyquist frequency is half the sampling frequency. The sampling frequency is the pixel density. So two pixels (sensels) side by side must cover no more than one line pair or there will be aliasing and false image data created.

Two sensels span 2 x 4.89 = 9.8 or 10 micrometres. This is less than 12.5 micrometres per line pair. This is good.

Thus, it seems to me, even the best Nikkor lenses are good enough to challenge the sensor with no AA filter, but provide enough blur to act as the AA filter.

In practice, there is some resolution at less than 30 % MTF and thus some small amount of signal (image) will get aliased. But it will generally be overwhelmed by stronger signals of bolder image data or will be weak enough that the aliasing will not be visible under conditions that would cause aliasing with weaker sensors.

In practice, there may not be much difference between the D800 and D800E images. Where there is a difference it is likely to be unfavourable (jaggies and moire and aliasing artifacts).



Feb 09, 2012 at 06:44 PM
gfiksel
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p.2 #7 · p.2 #7 · Occurance of Aliasing (Moire) and Postprocessing...


Monito wrote:
I think the reason that pictures like the one posted here work (very little aliasing visible) is that the D800E has enough pixels that it can rely on the lack of quality of the Nikon lenses to act as an anti-aliasing filter.


Add diffraction to that, which will become comparable to the pixel size at approximately f/11.



Feb 09, 2012 at 08:07 PM
Ruahrc
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p.2 #8 · p.2 #8 · Occurance of Aliasing (Moire) and Postprocessing...


Monito wrote:
Thus, it seems to me, even the best Nikkor lenses are good enough to challenge the sensor with no AA filter, but provide enough blur to act as the AA filter.


Say what? Either the blur is too "fine" to be picked up by the sensor, in which case it is not acting as an AA filter at all, or the blur is "coarse" enough to be seen by the sensor, and you can conclude that the sensor is capable of outresolving the lens. There is no middle ground.



Feb 09, 2012 at 08:37 PM
Monito
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p.2 #9 · p.2 #9 · Occurance of Aliasing (Moire) and Postprocessing...


Ruahrc wrote:
Say what? Either the blur is too "fine" to be picked up by the sensor, in which case it is not acting as an AA filter at all, or the blur is "coarse" enough to be seen by the sensor, and you can conclude that the sensor is capable of outresolving the lens. There is no middle ground.


You got it!

The blur is coarse enough to be seen by the sensor and you can conclude that the sensor is capable of outresolving the lens.

There is always a middle ground despite the legions and billions who wish the world were as simple as black and white. This death grip on the wish for sharp dividing lines is particularly prevalent in politics and particularly destructive in that sphere.

To begin with, "resolving" and "outresolving" is not cut and dried. The MTF contrast has to be specified, which means that below that contrast there is some resolution and above that contrast there is not enough to count. If a lens resolves 80 lpm at 50% contrast then it will resolve more lpm at 30% contrast and fewer lpm at 80% contrast.

Even the filter is not 100%, nor is the frequency cut off sharp. There is some amount of roll-off.

The use of the word "challenge" is a tip of the hat to the quality of the Nikkor lenses, meaning that they are sharp enough to give a D800 sensor a run for the money.



Feb 09, 2012 at 08:47 PM
rico
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p.2 #10 · p.2 #10 · Occurance of Aliasing (Moire) and Postprocessing...


bbvaj wrote:
... But could not understand where do i see moire in #4?

#4 was a production example of my fabric shoots (i.e. successful). I had to dig through my archive to present a useful example of failure. Ignore creases on the scarf below - it was a test for lighting and focus. Given the moire generated, I think focus was nailed. In the first figure below, both images are scaled by 0.20x, meaning the darkfield version was further away. It is also a correct representation of the design color when you inspect figure #2 which is a 1x-scale crop. The brightfield version was shot closer and has a different background, but same lighting scheme. The explosion in moire evident in figure #3, also at 1x scale, was an unpredicted coincidence of 1Ds sensor and fabric thread pitches. Even when reduced in size (figure #1), the moire is noticeable as blood-red flecks and splotches - unacceptable.
http://patternassociates.com/rico/fm/moire/moire100.jpg
http://patternassociates.com/rico/fm/moire/moire101.jpg
http://patternassociates.com/rico/fm/moire/moire102.jpg



Feb 10, 2012 at 02:55 AM
HerbChong
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p.2 #11 · p.2 #11 · Occurance of Aliasing (Moire) and Postprocessing...


there are a lot fewer of them than people would like to think. i don't own a Nikkor prime wider than 45mm for a reason. for some lenses, the D3 was easily showing their deficiencies. moving to the D3X only made it slap in the face obvious for some lenses. even the 45P is mostly for my film bodies as it is noticeably less sharp than my Zeiss 50MP at every aperture. since it's the focal length i use for my multirow panorama stitching, the 50MP is gets the nod almost all the time. going past 50mm, there are a lot more Nikkor primes that shine.

Herb...

Monito wrote:
I wasn't able to find lists of specific resolutions at line pairs per millimeter (lpm) of Nikkors, and MTF contrast at the lpm is important too. But I have some indication that top quality Nikkors can do about 80 lpm at 50% MTF contrast.




Feb 10, 2012 at 10:01 AM
ecidi
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p.2 #12 · p.2 #12 · Occurance of Aliasing (Moire) and Postprocessing...


drdrew wrote:
sometimes less is moire....


I like that comment!

How about this: "When the moon hits your eyes with a big pizza pie, that's a moire` "



Feb 10, 2012 at 10:12 AM
bbvaj
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p.2 #13 · p.2 #13 · Occurance of Aliasing (Moire) and Postprocessing...


@Monito: Thanks for the detailed explanation.

@Rico: great example. I assume the 1Ds has AA filter and still the moire shows in this specific case.

Is it possible to fix this photo in Post?



Feb 10, 2012 at 10:56 AM
TSY87
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p.2 #14 · p.2 #14 · Occurance of Aliasing (Moire) and Postprocessing...


bbvaj wrote:
Also can anyone with experience touch on how much more resolution we gain without AA.

Also does it matter only at very large prints or are there any other implications/advantages of no-AA


There are comparisons of the d800/e online of the same shot and zoomed in all the way...

honestly, the difference is negligible and to me, not worth the price or potential issues with moire.



Feb 10, 2012 at 01:35 PM
nolaguy
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p.2 #15 · p.2 #15 · Occurance of Aliasing (Moire) and Postprocessing...


Nominates this for one of the top 100 threads on FM. Nice job, guys. Thanks.

Pin it?



Feb 10, 2012 at 02:44 PM
plasticmotif
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p.2 #16 · p.2 #16 · Occurance of Aliasing (Moire) and Postprocessing...


Good read here.

I found this interesting article -

http://mansurovs.com/nikon-d800-vs-d800e

http://mansurovs.com/wp-content/uploads/2012/02/Nikon-D800-vs-D800E-Low-Pass-Filter.jpg


As light rays reach the first “horizontal low-pass filter”, they get split in two, horizontally. Next, they go through an infrared absorption filter (illustrated in green color). After that, the light rays go through the “second vertical low-pass filter”, which further splits the light rays vertically. This light ray conversion process essentially causes blurring of the details. Now with the Nikon D800E model, Nikon took an interesting approach. We know that the full low-pass filter cannot be completely removed, because it would cause the focal plane to move; plus, the camera still needs to be able to reflect infrared light rays....Show more

I'm interested in seeing how it actually handles jaggies, artefacting and moire in real world usage.



Feb 10, 2012 at 03:23 PM
John Wheeler
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p.2 #17 · p.2 #17 · Occurance of Aliasing (Moire) and Postprocessing...


Hi Rico
Thanks for the permission to use your images and attempt some reduction in the anti-aliasing and Moire patterns.

No question in my mind that once the subject detail drops to the resolution of the camera sensor, a lot of that detail information will not be recorded in a way that is recoverable. Best approach I believe is tethered shooting, checking the images at 100% and reshoot if needed.

In a worst case situation where taking an additional shot is not feasible, the backup strategy IMHO would be to employ software techniques. What can be recovered is never as good as what can be done just by a different shot. That said, in spite of the antialiasing, there is sometimes information left by the demosaiciing algorithm both in the color channels and spatially to adjacent pixels. That can sometimes be used to reduce the artifacts introduced by the anti-aliasing (it cannot recreate the detail that was already below the resolution of the sensor of course). Here is an example of what can be done within Photoshop with your fabric image. Shown is a GIF animation of at the same magnification you provided (100% I assume) cycling through before and after. This allows closer examination of the artifact reduction:

CLICK ON IMAGE TO GET 100% VIEW
http://i774.photobucket.com/albums/yy26/thebestcpu/NAPP%20General/moire102-SWF-1.gif

Also, here is another view reduced down by 4X similar to your small image example:
http://i774.photobucket.com/albums/yy26/thebestcpu/NAPP%20General/moire102fsm-version-SWF.jpg

The steps I took were to first remove both the Color and Luminosity Moiré banding through approaches I have already posted. Then, the second step was trying to reduce the visibility of the tight anti-aliasing patterns (basically adjacent pixels being assigned luminosity incorrectly by the demosaicing algorithm). It may appear that the sharpness has decreased (I did not add any sharpening). That is partly because the anti-aliasing with the high contrast artifacts gives a false data impression of sharpness that is not really there.

As mentioned in the beginning. It is best to take steps to avoid Moiré from the beginning. You cannot recover to the same quality that you can achieve with a good original image.

As an aside about the D800 vs the D800E. From examples that Nikon has provided of apples to apples sharpness comparisons between the two, it appears that the extra sharpness is on the order of about 1/2 pixel (just my judgement no hard measurements yet). To put this in perspective, if you blew up an image to 40x60 inches and viewed it from 10 inches away (pretty extreme) two side by side 1/2 pixels would create a spot that is only 0.2mm in size. This is right at the threshold of perception by the human eye. For me personally, I would rather have the protection of less Moiré with the associated less post processing to do. I personally would go for the D800.





Feb 14, 2012 at 04:20 AM
Monito
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p.2 #18 · p.2 #18 · Occurance of Aliasing (Moire) and Postprocessing...


John Wheeler wrote:
As mentioned in the beginning. It is best to take steps to avoid Moiré from the beginning. You cannot recover to the same quality that you can achieve with a good original image.

As an aside about the D800 vs the D800E. From examples that Nikon has provided of apples to apples sharpness comparisons between the two, it appears that the extra sharpness is on the order of about 1/2 pixel (just my judgement no hard measurements yet). To put this in perspective, if you blew up an image to 40x60 inches and viewed it from 10 inches away
...Show more

+1



Feb 14, 2012 at 08:34 PM
John Wheeler
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p.2 #19 · p.2 #19 · Occurance of Aliasing (Moire) and Postprocessing...


Hi Rico
Per your PM suggestion I took your shots of the Moiré that showed up on your patterned test images and ran them through the artifact reduction process. I left the lower right image alone since it looked pretty good. For the other 3, they each individually were run through color and luminosity stripe reduction. Then for all 3 together, I ran them through a process that that reduces the tight adjacent pixel pattern artifacts. You can see that they are reduced and not as obtrusive at 100% viewing yet if you enlarge to above 100% then you can still see some traces of the artifacts. In the steps taken I did not use any of the PS or Plug-In noise reduction tools nor any of the sharpening tools. Either or both could be applied to see if a better result could be achieved.

As you can see, it reduces the visibility of the artifacts yet you can only recover so far. There is information in the color channels and spatially with adjacent pixels that is used to help with this process yet just a better shot to begin with will do a lot better. Such post processing steps IMHO should be used only if you cannot have a "do-over." If one cannot get another shot, then these examples show what can be done if needed.

Below is first your original followed by the post processing as outlined above:

Original

http://i774.photobucket.com/albums/yy26/thebestcpu/ReTouch%20Pro/frames3456-Original.jpg

Post Processed

http://i774.photobucket.com/albums/yy26/thebestcpu/ReTouch%20Pro/frames3456-Post-Processed.jpg

Thanks for allowing me to use your images as demonstration.



Feb 15, 2012 at 12:33 AM
rico
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p.2 #20 · p.2 #20 · Occurance of Aliasing (Moire) and Postprocessing...


John, your results are a-mazing if you'll pardon the pun! This pattern was a severe test. The lower right image is a reference ideal, having been digitally reduced from a version of the pattern shot closer, and shown partly in the original post: it should not be achievable by the camera I used (Canon 1Ds, FF 11MP sensor w/Bayer mosaic). I do have an even more nasty pattern in mind, so expect a future challenge (insert evil laugh here). Meanwhile, how does your algorithm fare on an image with no aliasing present. In other words, do you have to steer the process toward problem areas and away from the good? This is a question about detection of sample ambiguity, which theory says cannot be done.


Feb 15, 2012 at 01:36 AM
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