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Author Topic: Resolution and alaising challenge  (Read 20928 times)
Fine_Art
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« Reply #40 on: October 24, 2011, 02:39:06 PM »
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Clean your sensor Bob!

I'm curious why people pay $5k for a DSLR when a $750 camera is almost hitting nyquist on a 16MP sensor?
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Guillermo Luijk
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« Reply #41 on: October 24, 2011, 02:54:18 PM »
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As requested, the ACR rendering is on the left and the  (dcraw -v -d -r 1 1 1 1 -4 -T file.nef + 50% nearest neighbour) after a levels adjustment is on the right. I converted the ACR to monochrome for the composite.

Nice, still clearly aliased individual channels produce a zero aliased output once demosaiced. Thanks Bill.

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BartvanderWolf
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« Reply #42 on: October 24, 2011, 05:32:56 PM »
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Nice, still clearly aliased individual channels produce a zero aliased output once demosaiced. Thanks Bill.

Hi Guillermo,

That's because the channels are not aliased. We're looking at data, not images, and the data has gaps that are yet to be filled. The aliasing you are looking at is caused by the crude downsampling (by eliminating the gaps), which will cause aliasing (also when the original doesn't have aliasing), it's the same difference. When the normal output is downsampled properly to 50%, there will be hardly any aliasing.

Cheers,
Bart
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Fine_Art
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« Reply #43 on: October 24, 2011, 06:10:01 PM »
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Uploaded the whole shot to the net.
http://www.sendspace.com/file/sjqf5p

All cameras should get close to the 92 pixel limit. My blur circle @ 96 pixel dia. should be fairly standard.
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Guillermo Luijk
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« Reply #44 on: October 24, 2011, 06:16:47 PM »
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That's because the channels are not aliased. We're looking at data, not images, and the data has gaps that are yet to be filled. The aliasing you are looking at is caused by the crude downsampling (by eliminating the gaps), which will cause aliasing (also when the original doesn't have aliasing), it's the same difference. When the normal output is downsampled properly to 50%, there will be hardly any aliasing.

I know, it's just a way to see the 4 original subsets of data as independent signals sampled at half the sampling frequency. It's the existing correlation between them that allows to apply a quite simple interpolation algorithm to build an output RGB image with half the spatial interval and no visible aliasing.

I wonder which would be the result if demosaicing some RAW dataset by previously eliminating this correlation, e.g. by zeroing the G1, G2 and R channels to see how the B channel is interpolated. In this case I assume there should persist visible aliasing in the output bluish image of your card.

Regards
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BartvanderWolf
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« Reply #45 on: October 25, 2011, 03:36:21 AM »
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I know, it's just a way to see the 4 original subsets of data as independent signals sampled at half the sampling frequency. It's the existing correlation between them that allows to apply a quite simple interpolation algorithm to build an output RGB image with half the spatial interval and no visible aliasing.

Hi Guillermo,

IMHO, there is no real correlation, since the samples are taken at distictly different positions, even if we consider them to be a 4x4 bin of samples overlapping with the next bin. I haven't tried the DCraw half output size algorithm on my target, but I assume that it also cuts corners by not doing a full demosaicing and low-pass filtering before resampling. It will also be interasting to see what happens when using the "Medium Raw"resolutions that modern cameras can produce. They are pre-cooked to some extent, and downsampled. It is possible to see how well that downsampling is implemented in the ASICs of the various brands.

If one has the goal to produce such a 50% zoom output algorithm, there is possibly a shortcut that can be found that does deliver little aliasing. Downsampling to exactly 50% can be done efficiently, because the Low-pass filter can be precalculated (even for a Lanczos windowed Sinc filter which is close to optimal), and the filter kernel values optimized for fast execution (e.g. multiples of 2, so division can be avoided and replaced by a shift operator). Maybe DCraw already does that, I haven't studied the code.

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I wonder which would be the result if demosaicing some RAW dataset by previously eliminating this correlation, e.g. by zeroing the G1, G2 and R channels to see how the B channel is interpolated. In this case I assume there should persist visible aliasing in the output bluish image of your card.

I think it would not help, but one could try a quick and dirty solution by replacing the empty positions by the average of surrounding actual samples (bi-linear interpolation), and use these averages instead of the real samples. But that would still result in suboptimal results due to the lack of pre-filtering.

Cheers,
Bart
« Last Edit: October 25, 2011, 04:06:02 AM by BartvanderWolf » Logged
ErikKaffehr
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« Reply #46 on: October 25, 2011, 07:39:15 PM »
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Hi!

I just discovered a healthy amount of Moiré in one of my testshots with the Sony Alpha 55SLT, one more indication of weak OLP.


Full images and DNG from Alpha 55 and Alpha 900 are here: http://echophoto.dnsalias.net/ekr/images/Demos/ApsVsFX/


Best regards
Erik (in some need of sleep)
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Fine_Art
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« Reply #47 on: October 25, 2011, 08:43:43 PM »
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Erik,

That is definitely lightroom letting you down in the de-bayer algorithm. In RT, if I switch to one of the older de-bayer methods without changing anything else I get the same colored checkerboard on the fine lines. It is clearly inventing fake resolution in creating a grid. If it cant resolve something it should leave it grey. It's also aggressively sharpened; there is no real sign of the sine function. It looks more like a box waveform. I've never used lightroom, I would have thought from its popularity it would do a better job on detail.

If you try your IDC or RT you will get more detail back.

Having the colorchecker in the shot is a great idea BTW.
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ErikKaffehr
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« Reply #48 on: October 25, 2011, 09:12:15 PM »
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Hi,

Sony IDC at default settings also shows moiré.

Best regards
Erik
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Fine_Art
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« Reply #49 on: October 25, 2011, 10:47:46 PM »
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I was referring to the detail in the chart, I didn't look at the cloth.
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Fine_Art
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« Reply #50 on: October 25, 2011, 11:07:30 PM »
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Here is your A900 shot. It looks quite clean.
http://www.sendspace.com/file/o0was7
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Fine_Art
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« Reply #51 on: October 25, 2011, 11:30:22 PM »
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Here is your A55. Again, its clean. I turned on some detail enhancement so the lines will look a bit darker. On the A900 shot i left all that stuff off to just uplaod in a few seconds. It still looks good.

http://www.sendspace.com/file/rthhgb
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ErikKaffehr
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« Reply #52 on: October 25, 2011, 11:50:31 PM »
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Hi,

Regarding the Moiré I turned sharpening all the way down to zero in LR and it was still there, but I can see absolutely no Moiré in your image processed in RT.

Best regards
Erik


Here is your A55. Again, its clean. I turned on some detail enhancement so the lines will look a bit darker. On the A900 shot i left all that stuff off to just uplaod in a few seconds. It still looks good.

http://www.sendspace.com/file/rthhgb
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Fine_Art
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« Reply #53 on: October 26, 2011, 12:04:13 AM »
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Hi,

Regarding the Moiré I turned sharpening all the way down to zero in LR and it was still there, but I can see absolutely no Moiré in your image processed in RT.

Best regards
Erik



Some color was there, i had to use one of the remove false color settings. Please look at the difference in the resolution chart! Note the sine wave is still there.
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Fine_Art
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« Reply #54 on: October 26, 2011, 01:41:50 PM »
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I just made a $25 donation to Raw Therapee for the great detail. It is worth it. I should thank Emil as well.
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ErikKaffehr
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« Reply #55 on: October 26, 2011, 10:47:50 PM »
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Hi,

Just as a side note, the image in question was not really intended for this discussion. I have posted screenshots of Bart's images from my both cameras before. This image was intended for another comparison of scanned 67 film and digital sensors. There was a question on the forum asking for the difference between full frame and APS-C and I had the arrangement still standing, so I took another shot with the SLT55.

We have seen that the SLT had weak OLP filtering, but this image has clearly shown color Moiré, something I never have seen on my DSLRs before. So I felt that this was good information. The SLT can certainly show moiré.

Regarding the table top setup, focus is actually on the red flowers, but I tried to arrange everything to be approximately in focus.

Strong sharpening was applied to both images, essentially what I would regard as optimal for this image. The SLT image was upscaled to match the the Alpha 900 image, slight sharpening was applied after scaling. So it was sharpened in two steps.

I have looked at Raw Therapy and I guess that I also made a donation (not really sure, i made several donations to different projects). Unfortunately RT and similar tools don't really fit into my workflow, so I don't see it or some other alternatives I tried as my main application. I'm pretty much aligned to parametric editing.


Best regards
Erik

Some color was there, i had to use one of the remove false color settings. Please look at the difference in the resolution chart! Note the sine wave is still there.
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EricWHiss
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« Reply #56 on: October 27, 2011, 12:17:02 AM »
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Well, this is an interesting thread - thanks to Bart for creating the chart. 

I've just printed the chart and spent a few minutes shooting with it and my AFi-ii 12.   I'm getting just over 100 pixels blur diameter with the 5.2 micron spacing on the 80mp dalsa sensor but I haven't had a chance to really check my focus and this is with a longish 8 sec exposure in my dark office.  Shooting the 130mm chart at about 8 feet with a 90mm lens and it doesn't seem to matter what aperture same result f/4 through f/8.   
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BartvanderWolf
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« Reply #57 on: October 27, 2011, 04:02:47 AM »
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Well, this is an interesting thread - thanks to Bart for creating the chart.

Hi Eric,

You're welcome. I agree that apparently the target has helped a bit already to gain more insight in some effects that the total imaging chain can produce. It's due to the target that one can much easier drill down to the cause of things. I know that quite a few people have an allergic reaction to synthetic targets, and they prefer real life subjects to test on (and then there are those that say, get a life - start making pictures instead of shooting targets). But IMHO it is only a useful exercise if one can draw conclusions from the test that help us identify issues, that in turn hopefully can be resolved. It allows us to hone our technique.

The target gets as close to a real life object as possible (all sorts of spatial frequencies in all sorts of directions), but is also abstract enough (luminosity only, repetitive/predictable patterns allows to spot deviations, and it ilustrates loss of contrast with increasing resolution) to allow quantification for comparisons. Even the Raw Converter is challenged as is our sharpening strategy.

Quote
I've just printed the chart and spent a few minutes shooting with it and my AFi-ii 12.   I'm getting just over 100 pixels blur diameter with the 5.2 micron spacing on the 80mp dalsa sensor but I haven't had a chance to really check my focus and this is with a longish 8 sec exposure in my dark office.  Shooting the 130mm chart at about 8 feet with a 90mm lens and it doesn't seem to matter what aperture same result f/4 through f/8.

Something close to 100 pixels seems fine for an MF system, but focus will prove to be more critical than we tend to believe. Digital sensors keep a pretty good MTF/contast all the way to their limiting resolution, but defocus will quickly kill that contrast. Defocus is also a much more efficient Low-pass filter than diffraction. I'd expect little influence from diffraction at f/4 and f/5.6, because the diffraction pattern diameter is smaller than approx. 1.5 x sensel pitch, so the only difference is from residual lens aberrations that get eliminated, if present. From f/5.6 and narrower you'd see a gradual loss of micro detail contrast due to diffraction, but to a certain extent it's restorable with deconvolution sharpening (as long as noise is kept low, a lot can be recovered). It will be interesting to see how well the aliasing is handled between the optical system and the Raw converter.

I think that the target will also serve to improve the quality and predictability of our estimates of DOF, or rather a more realistic assumption of the COC, and enlargeability vs viewing distance of our images.

Cheers,
Bart
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hjulenissen
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« Reply #58 on: October 27, 2011, 03:41:01 PM »
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I did this test for my 7D and a few lenses:
50mm f/1.8 II
85mm f/1.8
17-55mm f/2.8 @55mm
70-200mm f/4.0L IS @116mm
100mm f/2.8 USM Macro

All lenses had a minimum of two shots at: maximum aperture, f/5.6 and f/7.1.

Did the test at a distance of ca 4 meters (5-6 for the telezoom) centered slightly below sensor. Using Liveview, manual focusing at 10x zoom.

Developed using my standard settings in Lightroom, WB against paper white, exposure compensated so as to nearly clip white, no NR.

I am a bit unsure how to interpret the results and how to share them. Cropped 1:1 TIF exports are ~2.6MB and there are 15 or so files.

-h
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Fine_Art
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« Reply #59 on: October 27, 2011, 04:08:27 PM »
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Highest quality jpgs shouldn't lose anything. Did you add (to the tiff) a 1 pixel wide circle of where you think it blurs out? Then save the center as jpg crop.
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