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Author Topic: Sharpening ... Not the Generally Accepted Way!  (Read 54222 times)

digitaldog

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #80 on: August 10, 2014, 06:17:42 pm »

That is not an article, it is a series. If you go through it it shows a comparison of the PS smart sharpen with richardson-lucy here: http://www.clarkvision.com/articles/image-restoration2/index.html
First thing I see is: In this example, we will start with a high signal-to-noise ratio image, then intentionally blur it. I try to never intentionally blur my images from the get go.
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Yes, that is what Doug Kerr discusses in his article (if you take the time to read it).
I'm a fan of Doug's work and will, but I think what I've seen even before that is anyone can call a routine USM and they all produce different results. In the test I did today, pretty significant visual differences! In fact, if I showed you the two side by side and said one was USM and the other a vastly different approach (dare I say deconvolve), an observer could come to many of the same conclusions as to what is 'better' as we see on the various pages referenced here. One looks quite less sharp than the other and that suggests to me, a setting of USM in Photoshop may not produce the same level of sharpness as another product presumably using the same sharpening process (they share the same name). If USM in PS set to the same values we read on Clark's page look soft compared to the settings in Graphic Converter, does that mean one should up the values? USM isn't USM it appears, all things are not equal.
« Last Edit: August 10, 2014, 06:19:34 pm by digitaldog »
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bjanes

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #81 on: August 10, 2014, 06:29:50 pm »

First thing I see is: In this example, we will start with a high signal-to-noise ratio image, then intentionally blur it. I try to never intentionally blur my images from the get go.

The point of Bart's demonstration of blurring an image with Gaussian blur and restoring it with deconvolution is to demonstrate that deconvolution works very well if you know the PSF, but I agree that it best to work with real world images. The PSF can often be estimated or approximated by a Gaussian PSF.

If you use USM, you are blurring the image as part of the process, although you might not be aware of it. :)

Regards,

Bill
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Mark D Segal

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #82 on: August 10, 2014, 06:58:56 pm »

That is not an article, it is a series. If you go through it it shows a comparison of the PS smart sharpen with richardson-lucy here: http://www.clarkvision.com/articles/image-restoration2/index.html

There are already several deconvolution threads on the site so to me remaining unconvinced= remaining in the dark. Your choice.

Mr. Clark's work shows two things: (i) yes, he obtained good image detail from the deconvolution technique he used, and (b) at normal viewing distance it is unlikely to look much better than the best result he got from PS Smart Sharpen. He hasn't published tests versus Photokit Sharpener or Nik Sharpener Pro, arguably the best conventional sharpening tools other than dedicated deconvolution techniques. I really cannot justify putting in the time to do this, but for me the determinative tests would be to use PKS and NIK at their best versus deconvolution at its best on properly focused photographs that come out of a good DSLR (full-frame or APS-C in the 20~24 MP range) with a good lens and see what differential quality of prints in the 13*19 to 17*22 size range they produce. The kind of evaluation basis I have in mind would be to observe any differences in the definition and the natural character of that definition of fine textural detail (i.e. relative to how we humans see textured objects without loupes), as well as for "easier" objects such as wires against sky, etc. If anyone reading this thread has done this kind of comparison or can point me to one it could be of considerable interest.

As for remaining in the dark, keep your personal slurs to yourself. This is about science.
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digitaldog

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #83 on: August 10, 2014, 07:28:12 pm »

The point of Bart's demonstration of blurring an image with Gaussian blur and restoring it with deconvolution is to demonstrate that deconvolution works very well if you know the PSF, but I agree that it best to work with real world images.
Yes and the demo IS impressive in handling blurred images. By most of mine are not blurred, my current workflow is to use LR for capture sharpening on images that are not out of focus. Going back full circle to the comment that Chances are people using deconvolve methods can beat anything done in LR/ACR. That may be true, but the demo's provided thus far have two issues as I see it. First, the images being used are blurred, out of focus. Next, one has to wonder if the USM examples are handled ideally (well no, as none so far are used on real world non blurry images). Clark shows one example with one setting of USM, no question it doesn't look as sharp (on-screen which is rarely my final goal) as the others. He does suggest upping the settings would look sharper but produce other issues and it would have been nice to see that. Just today's test using USM in two different products, something I've never looked at, gives me the impression that there are vast differences in just what someone calls USM! That the same settings are not ideal in both cases. That it would be useful for someone to really attempt to produce the best possible results with the tools provided on good images in the first place, then show me a scan of good output such that I could evaluate what the results would mean in a real would context.
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Fine_Art

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #84 on: August 10, 2014, 08:18:49 pm »

Yes and the demo IS impressive in handling blurred images. By most of mine are not blurred, my current workflow is to use LR for capture sharpening on images that are not out of focus. Going back full circle to the comment that Chances are people using deconvolve methods can beat anything done in LR/ACR. That may be true, but the demo's provided thus far have two issues as I see it. First, the images being used are blurred, out of focus. Next, one has to wonder if the USM examples are handled ideally (well no, as none so far are used on real world non blurry images). Clark shows one example with one setting of USM, no question it doesn't look as sharp (on-screen which is rarely my final goal) as the others. He does suggest upping the settings would look sharper but produce other issues and it would have been nice to see that. Just today's test using USM in two different products, something I've never looked at, gives me the impression that there are vast differences in just what someone calls USM! That the same settings are not ideal in both cases. That it would be useful for someone to really attempt to produce the best possible results with the tools provided on good images in the first place, then show me a scan of good output such that I could evaluate what the results would mean in a real would context.

Upping the settings in USM creates halos. I bet you, as a photographer, have seen countless images on the web with them. When you start to see images sharpened without halos you see that defect as rather nasty, in that it is no longer required based on the methods available. I have grown to abhor halos over the last few years. Now, deconvolution can create ring artifacts when taken to the level that would make a high contrast print. They can be wiped out by blending back to the original.

IMO the biggest easy improvement for LR/ACR would be to have listed deconvolve methods in a sub dialog box. People should be able to use several in a sequence they choose. The methods are well documented non-proprietary scientific algorithms. There is no reason not to make them available. Adobe always seems to want to say they have a secret sauce. They marketing power that convinces many people whatever they do is best. Test it. If they included these methods along with several NR methods I would buy it. For NR DxO seems to be taking the prize. If Adobe doesn't move they will soon take the prize in sharpening with deconvolution set for specific lenses.

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digitaldog

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #85 on: August 10, 2014, 08:35:50 pm »

Upping the settings in USM creates halos. I bet you, as a photographer, have seen countless images on the web with them.
Yes, I'm keenly aware that over sharpening can cause visible halos on output, that's not what I'm suggesting.
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IMO the biggest easy improvement for LR/ACR would be to have listed deconvolve methods in a sub dialog box.
I'll let the engineers who handle this within the product comment, I'm not qualified to suggest they do or do not do this, I'll bet they are pretty aware of this possibility.
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The methods are well documented non-proprietary scientific algorithms.
Why do you suppose we are not seeing this in said products?
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There is no reason not to make them available.
Again, with no knowledge of the processing or specifics of this product, I'm not willing to accept that at face value, I'd certainly prefer to hear what an engineer would have to say about this.
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Adobe always seems to want to say they have a secret sauce. They marketing power that convinces many people whatever they do is best.
Ah sure OK. That seems like a pointless area to speculate about.
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Fine_Art

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #86 on: August 10, 2014, 09:21:08 pm »

Here is a real world RAW from Imaging Resource
http://www.imaging-resource.com/PRODS/nikon-d800/D800PINE.NEF.HTM

Anyone can download it, then sharpen however they want.

The reason to look at the methods astronomers use is that they have the most difficult problem. Very faint data with a variety of problems like the atmosphere. They somehow have to get an improved image while retaining accurate data. Wavelets, then deconvolution are the methods they came up with.
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Fine_Art

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #87 on: August 10, 2014, 10:26:20 pm »

Here is a crop from the image. screenshot pasted to MS Paint, saved as JPG. A PNG was too big.

This has Adaptive Richardson-Lucy in Gaussian 5x5 then 3x3 pixels.
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Robert Ardill

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #88 on: August 11, 2014, 07:27:01 am »

Hi Robert,

Yes, deconvolution is perfect for Capture sharpening, and it's also very good for restoration of some of the upsampling blur, and yes these can also be combined if one wants to avoid upsampling any artifacts. For workflows involving Photoshop, I can recommend FocusMagic. What my analysis has shown, Capture sharpening should be 'focused' at Aperture dictated blur (not image detail as suggested in 'Real world Image sharpening'). The amount of blur (in the plane of best focus) is largely Gaussian in nature, due the the combination of several blur sources (which tends to combine into a Gaussian distribution), and varies with aperture.


I'm very interested in this and if the only thing that comes from this thread is a better way of doing capture sharpening then I, for one, will be very happy indeed.  I've had a try with FocusMagic and it looks very good at first sight.  I added it in to the Ps action I'm using to compare different methods.

For FM capture sharpen using default settings (the filter estimates the blur distance), my first conclusion (based on a sample of 1) is that FM does a much better job of capture sharpening than does Lr.    

My second conclusion is that if the Lr image is then resized (after capture sharpen) and compared to the FM image sharpened after resize (the FM filter estimates the blur distance at twice the blur distance for the normal size image, which is pretty impressive), the improvement is really significant, with virtually no artifacts (halos and noise) in the FM-sharpened image but with significant artifacts in the Lr image.  

The FM sharpened (after resize image) is also better than the Ps sharpened after resize image, with cleaner edges and more detail.  

The FM sharpened (after resize image) is also slightly better than the Ps sharpened once for output image with a little more detail, but the differences are pretty subtle.

My test is not very fair though, because the sharpening in Lr/ACR used Masking, whereas the FM sharpening was just default sharpening with no edge mask.

So I repeated the test with Masking removed and then the advantage swings much further towards the FM sharpening as the ACR sharpening also sharpens noise whereas the FM sharpening doesn't (and this is with a test version of FM which doesn't include noise reduction).

So, based on this I'm forking out $65 for Focus Magic ... so I can try it out properly.

This forum is costing me money!

Robert
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Robert Ardill

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #89 on: August 11, 2014, 07:53:36 am »

Here is a crop from the image. screenshot pasted to MS Paint, saved as JPG. A PNG was too big.

This has Adaptive Richardson-Lucy in Gaussian 5x5 then 3x3 pixels.

And here is the same crop with sharpening using FocusMagic, default settings.  Pretty impressive IMO.



(To view the image at 100%, right-click on it and select View Image ... no doubt there's a better way of doing this on this forum, in which case perhaps someone would enlighten me :)).

I also did a test, with the same image, but this time capture sharpening and then upsizing x 2,  compared to upsizing x 2 and then sharpening (with FocusMagic using default settings).  I found the capture followed by resize a bit sharper, but over-sharpened to my taste.  I preferred the resize-then-sharpen with the amount dialled up by 25%.

Robert
« Last Edit: August 11, 2014, 08:29:36 am by Robert Ardill »
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Bart_van_der_Wolf

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #90 on: August 11, 2014, 09:08:21 am »

I'm very interested in this and if the only thing that comes from this thread is a better way of doing capture sharpening then I, for one, will be very happy indeed.  I've had a try with FocusMagic and it looks very good at first sight.  I added it in to the Ps action I'm using to compare different methods.

For FM capture sharpen using default settings (the filter estimates the blur distance), my first conclusion (based on a sample of 1) is that FM does a much better job of capture sharpening than does Lr.

Hi Robert,

Since you are new to FocusMagic, allow me to share a tip (or two). FocusMagic does try to estimate the best blur width setting, but may fail at getting it right for the best focused part of the image (also depends on where you exactly set the preview marker). I tend to increase the Amount setting to its maximum of 300%, and set the Blur width to 0. Then increase the blur width by 1 at a time. There will be a point where most images will suddenly start to produce fat contours/edges instead of sharper edges. That's where you back-off 1 blur width click, and dial in a more pleasing amount (larger radii tolerate larger amounts). For critical subsequent upsampling jobs, I then use a Layer Blend-if setup, or I first upsample and then (WYSIWYG) sharpen that.

Here's a generic Blend-if setup I use in an action that creates a duplicate (sharpening) layer in Photoshop and gives a useful way to throttle the possible artifacts if the amount settings are taken to extremes:


It basically reduces the amount of sharpening by the top layer as the local contrast is already high, and near clipping. The start/end points of the gradual decrease/increase for shadows/highlights can be used for further fine-tuning, as can the layer's opacity.

FocusMagic usually strikes a very nice balance between enhancing signal/sharpness and constraining noise, but for larger (>4) blur width settings it also allows to manually switch noise suppression on/off. One can instead also do modest noise reduction before sharpening with a dedicated noise reduction plugin.

It is of course also possible to use multiple runs of FM, usually starting at the larger required radius, and finishing off with the smaller/smallest radius, with adjusted amounts. That allows to optimize for more complex PSF shapes than whatever Focusmagic uses by default. There are also differences between the different image source methods/models, although Digital camera and Forensic produce very similar/close results.

Cheers,
Bart
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AFairley

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #91 on: August 11, 2014, 10:15:23 am »

Thank you all for a most informative thread.  Can you tell me whether there is any inherent advantage to performing capture sharpening as step in the demosaicing process as opposed to on a tiff "developed" without sharpening?
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Robert Ardill

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #92 on: August 11, 2014, 10:42:24 am »

Hi Robert,

Since you are new to FocusMagic, allow me to share a tip (or two). FocusMagic does try to estimate the best blur width setting, but may fail at getting it right for the best focused part of the image (also depends on where you exactly set the preview marker). I tend to increase the Amount setting to its maximum of 300%, and set the Blur width to 0. Then increase the blur width by 1 at a time. There will be a point where most images will suddenly start to produce fat contours/edges instead of sharper edges. That's where you back-off 1 blur width click, and dial in a more pleasing amount (larger radii tolerate larger amounts). For critical subsequent upsampling jobs, I then use a Layer Blend-if setup, or I first upsample and then (WYSIWYG) sharpen that.


Thank you Bart - you are very helpful as usual! I'll give that a go.   I often use a Layer Blend-if setup similar to yours to soften halos in sharpening. 

Focus Magic can't be used as a smart filter which is a real pity ... and right now it doesn't seem to install for CC 2014 (which isn't such a great surprise as I'm having problem installing plugins). Hopefully they will fix that in a future release.

Robert
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Robert Ardill

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #93 on: August 11, 2014, 10:50:47 am »

Thank you all for a most informative thread.  Can you tell me whether there is any inherent advantage to performing capture sharpening as step in the demosaicing process as opposed to on a tiff "developed" without sharpening?

I can't say if there's an inherent advantage, but I can say that empirically there is no advantage.  If you think about it, in order to sharpen, Lightroom has to render the image, so the sharpening is applied after the demosaicing. So whether the sharpening is done in Lr or in Ps won't make any difference (although I don't know at what point Lr applies the sharpening ... for example, before or after applying Clarity) so there might be some small differences.  Also, if you want to keep things like to like you would be better opening/exporting the image as ProPhoto.

It's easy to check using the Ps Camera Raw filter (so apply the sharpening in Lr, open the image in Ps without sharpening, then use the Camera Raw filter to apply exactly the same sharpening as in Lr, and compare the two images).

Robert
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Bart_van_der_Wolf

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #94 on: August 11, 2014, 11:05:20 am »

Thank you all for a most informative thread.  Can you tell me whether there is any inherent advantage to performing capture sharpening as step in the demosaicing process as opposed to on a tiff "developed" without sharpening?

Hi Alan,

I'm not sure at which point LR applies the parametric sharpening settings when it ultimately renders the image. Maybe some is done in Raw (e.g. some noise reduction), but chance has it that it's most likely after demosaicing to RGB. So at that stage it would make little difference, other than the sharpening algorithm used, whether one uses the LR Capture sharpening or another application.

Of course, LR's parametric adjustments do add onto each other, so it would not be the same to just switch it off before export and sharpen elsewhere as it would be if you skipped it all together from the start of your tweaking of other LR parameters.

At this point in time, the capabilities of sharpening outside of LR make it worth considering. Maybe not on a routine basis, but when you want the best of the best you might. Especially if one also has Photoshop at ones disposal, there is a lot that can be done there, that LR was not designed for.

Cheers,
Bart
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Robert Ardill

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #95 on: August 12, 2014, 08:30:17 am »

I would really appreciate a bit of help understanding the whole concept of deconvolution.  BTW, I see there was a massive thread 4 years ago, here: http://www.luminous-landscape.com/forum/index.php?topic=45038, started by Bill. (Which humbles me a bit as I can see you guys have been talking about this for ages!). I've read some of it and while it's very interesting, with something like 18 pages it takes some plowing through!  Still, I will get to it.

From a mathematical point of view it seems straightforward enough: the signal f is convolved with another signal g to yield h.  If we know g then we can find its inverse and so recover f.  If we donít know g then we can guess it or estimate it and so attempt recovery of f.

Noise messes things up a bit because itís added to the convolved signal Ö so how do we remove it from h before doing the deconvolution?  Well, one way would be to add some blur to h (in other words convolve it further, which isnít a brilliant idea if the g was a blur function to start off with!).

Anyway, moving on to imaging, I assume that all filters convolve the image (essentially one function applied to another).  If we convolve the image with a blur filter and then apply the inverse filter (a sharpening filter?) then we are convolving the image twice, but the second convolution is also a deconvolution.  Is that correct?

Looking at the Ps Custom filter, itís easy enough to apply a blur and then apply the inverse (so where the adjacent pixel was added, we now subtract it).  The effect is to remove the blur Ö but it also introduces the beloved halo!

So I guess I must be missing something fundamental!  Or not using the Ps Custom filter correctly, which is also highly likely!

But assuming that Iím not entirely off the mark, when Jeff says that the Lr sharpen is effectively a USM-type sharpening when used with a low Detail setting, but becomes a deconvolution filter with high Detail settings Ö Iím both puzzled and lost.  Iím puzzled as to how a Detail setting of 0 gives USM (which to my mind is a deconvolution if its intention is to remove blur) while at 100 itís a deconvolution.  

If I take an image and blur it with a Gaussian blur, radius 3, and then sharpen using the ACR sharpen, moving the Detail to 100% certainly gives more sharpening, but it also gives a nice (NOT) halo Ö it certainly doesnít recover the image to the pre-blur version.

Here is a very simple test image (real-life) that can be used to try out the different techniques:

http://www.irelandupclose.com/customer/LL/sharpentest.tif

Iíve tried various methods (after applying a Gaussian blur of 3) and none of them seem to be particularly effective.  I would very interested indeed if you have a filter, or multiple filters, or filter applied multiple times, that can (within reason) restore the image to the original.

And I would be very grateful for clarification on deconvolution (and correction of my understanding, particularly on how it is normally applied to digital images).  There's a lot of talk about deconvolution, but I doubt that there are too many of us who understand it (me included)!  

Robert
« Last Edit: August 12, 2014, 08:41:00 am by Robert Ardill »
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bjanes

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #96 on: August 12, 2014, 09:11:31 am »

Thank you Bart - you are very helpful as usual! I'll give that a go.   I often use a Layer Blend-if setup similar to yours to soften halos in sharpening. 

Focus Magic can't be used as a smart filter which is a real pity ... and right now it doesn't seem to install for CC 2014 (which isn't such a great surprise as I'm having problem installing plugins). Hopefully they will fix that in a future release.

Robert

Robert,

FM works fine on my Windows 8 machine with PS CC ver 2014.1.0. I can't remember how I installed it, whether with the installer or merely copying the plugin from a previous version of CC. FocusMagic64.8bf resides in c:\\ProgramFiles\Adobe\Adobe Photoshop CC 2014\Plug-ins.

Regards,

Bill
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Robert Ardill

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #97 on: August 12, 2014, 10:03:18 am »

Robert,

FM works fine on my Windows 8 machine with PS CC ver 2014.1.0. I can't remember how I installed it, whether with the installer or merely copying the plugin from a previous version of CC. FocusMagic64.8bf resides in c:\\ProgramFiles\Adobe\Adobe Photoshop CC 2014\Plug-ins.

Regards,

Bill

Cool, Bill!  Thanks ... stupid, I should have checked the plug-in folder.  The FM installation just doesn't (currently at least) install into the CC 2014\Pluging.

Robert
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TonyW

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #98 on: August 12, 2014, 10:30:49 am »

Here is a very simple test image (real-life) that can be used to try out the different techniques:

http://www.irelandupclose.com/customer/LL/sharpentest.tif

Iíve tried various methods (after applying a Gaussian blur of 3) and none of them seem to be particularly effective.  I would very interested indeed if you have a filter, or multiple filters, or filter applied multiple times, that can (within reason) restore the image to the original.

And I would be very grateful for clarification on deconvolution (and correction of my understanding, particularly on how it is normally applied to digital images).  There's a lot of talk about deconvolution, but I doubt that there are too many of us who understand it (me included)!  

Have been following this thread with some interest and as far as deconvolution goes I am sure that Bart and others knowledge and experience of this aspect will prove very useful for you.

Couple of things I picked up on and it is my opinion that maybe you are making things a little more difficult than they need to be to get excellent result whichever sharpening route you choose.

1.  Your test file of the power/telephone line is not a particularly good choice as presented due to purple green CA.  IMO this should be removed first during raw processing to give a meaningful view of sharpening options.

2.  As you started the thread with PS have you tried the Smart Sharpen / Lens Blur / More Accurate checked?  This AFAIK is deconvolution sharpening (particular parameters unknown) and offers quite a lot in the way of control.  Not as many options of course as in other software but sometimes this maybe enough?

By chance I had also played with the sample NEF image in ACR using Amt=50 Rad= 0.7 Detail = 80  and seems to be pretty close to your FM example although that was not my intention.  Seems to me in this case that a little tweaking ACR would narrow the differences even further
« Last Edit: August 12, 2014, 10:32:54 am by TonyW »
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Bart_van_der_Wolf

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Re: Sharpening ... Not the Generally Accepted Way!
« Reply #99 on: August 12, 2014, 12:34:47 pm »

I would really appreciate a bit of help understanding the whole concept of deconvolution.  BTW, I see there was a massive thread 4 years ago, here: http://www.luminous-landscape.com/forum/index.php?topic=45038, started by Bill. (Which humbles me a bit as I can see you guys have been talking about this for ages!). I've read some of it and while it's very interesting, with something like 18 pages it takes some plowing through!  Still, I will get to it.

From a mathematical point of view it seems straightforward enough: the signal f is convolved with another signal g to yield h.  If we know g then we can find its inverse and so recover f.  If we donít know g then we can guess it or estimate it and so attempt recovery of f.

Hi Robert,

Deconvolution is a pretty simple operation (mathematically speaking). In the spatial domain it just allows to restore the original signal that was intended for a single pixel, but was also spread over a range of surrounding pixels instead, by subtracting that spread signal from the neighbors and adding it back to its intended location. However, also that pixel carries parts of signal from all surrounding source pixels, so that needs to be subtracted and added back to those other pixels.

The distribution of that 'stray information' is mathematically described by a Point Spread Function (PSF), which will allow to subtract the correct amounts of information from its neighbors. For a completely accurate description of that PSF one would need infinite precision (because the amounts get smaller per neighbor, but there will be also many more neighbors, as the distance increases), and no noise to disturb the smaller and smaller true amounts of signal as we get further away from the source pixel position.

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Noise messes things up a bit because itís added to the convolved signal Ö so how do we remove it from h before doing the deconvolution?  Well, one way would be to add some blur to h (in other words convolve it further, which isnít a brilliant idea if the g was a blur function to start off with!).

Given that we already have to deal with a signal made from (Poisson distributed) shot noise and a few other sources of electronic noise, we will not be able to have such a perfect restoration, but by adding some clever statistical procedures (which know how to deal with noise and probability distributions) to the concept of deconvolution, one can devise rather successful algorithms for image restoration, luminance resolution (and color) restoration in our case.

When we try to reduce noise, we also reduce or dilute signal which was made up from photon shot-noise, so we need to understand the statistical properties of our (mostly Poisson distributed) noisy signal, and knowledge about the PSF, to have a better chance of beating the odds in the reconstruction of what is called, a solution to an "ill posed problem".

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Anyway, moving on to imaging, I assume that all filters convolve the image (essentially one function applied to another).  If we convolve the image with a blur filter and then apply the inverse filter (a sharpening filter?) then we are convolving the image twice, but the second convolution is also a deconvolution.  Is that correct?

Here is where we need to distinguish between a masking type of filter, like USM and other acutance enhancing filters, and a deconvolution type of filter. A mask is just an overlay, that selectively attenuates the transmission to underlying layers. It adds a (positive or negative) percentage of a single pixel to a lower layer's pixel. A deconvolution on the other hand adds weighted amounts of surrounding pixels to a central pixel, for all pixels (a vast amount of multiplications/additions is required for each pixel) in the same layer.

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Looking at the Ps Custom filter, itís easy enough to apply a blur and then apply the inverse (so where the adjacent pixel was added, we now subtract it).  The effect is to remove the blur Ö but it also introduces the beloved halo!


Halo only occurs if we use the wrong amounts to add back from many surrounding pixels. When we add back the correct (positive and negative) amounts from the neighbors, we will have a perfect restoration (within the limitations of noise and calculation precision). It's due to those limitations that we cannot have a perfect restoration, although we may get close enough for our usually 8-bit/channel output requirements to make positive difference.

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So I guess I must be missing something fundamental!  Or not using the Ps Custom filter correctly, which is also highly likely!

Assuming you used the correct kernel values to reverse the operation, Photoshop offers a limited precision of calculation (integer values as input, being divided by scaling integers, with limited calculation precision and rounding or truncation of intermediate values), and it is also limited to small 5x5 kernel sizes. So expect less than perfect results form that operator.

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But assuming that Iím not entirely off the mark, when Jeff says that the Lr sharpen is effectively a USM-type sharpening when used with a low Detail setting, but becomes a deconvolution filter with high Detail settings Ö Iím both puzzled and lost.  Iím puzzled as to how a Detail setting of 0 gives USM (which to my mind is a deconvolution if its intention is to remove blur) while at 100 itís a deconvolution.

If I take an image and blur it with a Gaussian blur, radius 3, and then sharpen using the ACR sharpen, moving the Detail to 100% certainly gives more sharpening, but it also gives a nice (NOT) halo Ö it certainly doesnít recover the image to the pre-blur version.

I assume it is just a gradual blend between USM and a sort of deconvolution, and the deconvolution part is not as powerful as e.g. FocusMagic, to allow faster execution. The deconvolution method used, will quickly create more artifacts than restored signal, and the required amount setting is a complete guess (zero guidance is offered, other than eyeballing the resulting effect).

Cheers,
Bart
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