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Author Topic: Question about Radius Control for Sharpening  (Read 3331 times)

One Frame at a Time

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Question about Radius Control for Sharpening
« on: May 29, 2012, 09:19:57 pm »

Hi,

I am most of the way through the LL LR4 tutorial.  Learned a bunch but I don't understand one important part of the Sharpening Controls.
Does the radius slider work as an edge finder or to set the pixel width of the sharpening effect?  Jeff seems to describe it in a way that makes me think a lower setting (described as high frequency?) finds more edges in an image to apply the effect.  I always thought the slider set the size of the effect (ie the pixel width of the effect on either side of a determined edge line).  This is an important difference, that I would like to understand better.

Any help clarifying this would be appreciated.  Thanks!

Paul

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Schewe

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Re: Question about Radius Control for Sharpening
« Reply #1 on: May 29, 2012, 10:33:14 pm »

Does the radius slider work as an edge finder or to set the pixel width of the sharpening effect?  Jeff seems to describe it in a way that makes me think a lower setting (described as high frequency?) finds more edges in an image to apply the effect.

The sharpening in ACR/LR is based on edges...so it finds and sharpens edges. The radius controls how far on either side of an edge the sharpening will be applied to. If set low (below 1) it doesn't 'find" more edges, it just applies the sharpening mich tighter to the edge. Higher radius is the opposite.
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One Frame at a Time

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Re: Question about Radius Control for Sharpening
« Reply #2 on: May 29, 2012, 11:05:36 pm »

Thanks for the reply.  The conversation you shared in the tutorial about hi vs low frequency images got me thinking there was more to this control.  If you make some additions to the series,  could you expand on the capture sharpen discussion?
I got something from it, but don't feel I understand all the controls and how they interact as much as I'd like.  Thanks again.

Paul Podell
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Schewe

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Re: Question about Radius Control for Sharpening
« Reply #3 on: May 30, 2012, 12:01:46 am »

I got something from it, but don't feel I understand all the controls and how they interact as much as I'd like. 

It takes practice...that will teach you more than further vids...
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One Frame at a Time

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Re: Question about Radius Control for Sharpening
« Reply #4 on: May 30, 2012, 10:35:36 am »

Thanks again Jeff.  Really appreciate all the help contained in the Tutorials you and Michael created.  Not to stretch this out but your comment about practice is OK, but for me (and others I'm guessing) it really helps to understand what the controls really do to the image pixels.  As you point out in the Tutorial, some of the labels are vague in their function.  To be expected, as complicated as they are.

For me to function at my best, clear understanding of cause and effect is essential to get the most out of the tools we use.  No amount of practice will allow a deep understanding of the complex tools we have at our disposal.  They need some explaining.  (Hope this does not come off as argumentative.  That is not my intention!) 

Thanks,

Paul
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gholleman

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Re: Question about Radius Control for Sharpening
« Reply #5 on: May 30, 2012, 11:07:54 am »

I would highly recommend Real World Image Sharpening with Adobe Photoshop, Camera Raw, and Lightroom (2nd Edition) By Bruce Fraser and Jeff Schewe. Doesn't get any better than to get the info straight from the creators. Very detailed on all aspects of sharpening.
« Last Edit: May 30, 2012, 12:15:23 pm by gholleman »
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Bart_van_der_Wolf

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Re: Question about Radius Control for Sharpening
« Reply #6 on: May 30, 2012, 12:14:51 pm »

Does the radius slider work as an edge finder or to set the pixel width of the sharpening effect?

Hi Paul,

Actually, it works like this. The central pixel in a circle has its brightness modified based on all the surrounding pixels that are mostly inside that circle's given radius. Then the next pixel is adjusted based on its neighbors in the circle radius, then the next, etc.

So effectively, the radius determines the size of the area that is involved to determine the new central pixel's value, for each and every pixel in turn. After that, the original pixel values are replaced by their modified versions upon final rendering.

So the radius setting doesn't 'find' edges, it just restricts itself to the edge region, or a larger region when a larger radius is selected, when it uses its specific contrast enhancement algorithm based on local (inside the circle) contrast. The contribution of the pixels inside the circle radius is not uniform, they are weighted depending on the algorithm used, and even pixels outside the radius will contribute some, but not much. The contribution weights decrease with distance to the central pixel with the pixels inside the radius contributing most of the effect.

Cheers,
Bart
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One Frame at a Time

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Re: Question about Radius Control for Sharpening
« Reply #7 on: May 31, 2012, 09:46:53 am »

Thanks guys!! 

Bart,

I read your response 4 times and I THINK? I understand your explanation.  Going out on a limb, what I think your saying is: the  Radius control does not work directly as a control for the output of the sharpening edge adjustment.  But, more like a sampling control, that controls the edge sampling.  That in turn is used to calculate the applied (output) edge adjustment.  I am going to postulate that the width of the applied sharpening, is controlled most directly by the Amount slider?  The Detail slider is closest to an "edge finder" control?

Since its clear you have a really deep understanding of the tool, can you explain (My last request, I promise) why the radius is in values of 1/10s of a pixel?  Is going below 1.0 making some kind of pivotal change in the sharpening as compared to values over 1.0?

Thanks once again,

Paul
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Bart_van_der_Wolf

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Re: Question about Radius Control for Sharpening
« Reply #8 on: May 31, 2012, 11:08:06 am »

Thanks guys!!  

Bart,

I read your response 4 times and I THINK? I understand your explanation.  Going out on a limb, what I think your saying is: the  Radius control does not work directly as a control for the output of the sharpening edge adjustment.  But, more like a sampling control, that controls the edge sampling.

Hi Paul,

Yes, a sampling control in the sense that it determines which surrounding pixels are included in the modification of the central pixel that's being processed at that moment, and how much weight is attached to those pixels (less with distance).

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That in turn is used to calculate the applied (output) edge adjustment.

Not only edge, could also be any other detail or noise or smooth area, although the latter will then undergo little change. Actually, the underlying algorithms are not very intelligent, they don't see edges or other structures. They just see a pixel surrounded by other pixels within a circle radius, and calculate a new value for the central pixel.

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I am going to postulate that the width of the applied sharpening, is controlled most directly by the Amount slider?

No, 'Amount' is more of an amplifier to boost the amount of contrast enhancement. If the calculations with the surrounding pixels result in a brightness increase of e.g. 2 of the central pixel, then an Amount of 200 will result in an increase of 4 instead of 2, and an Amount of 50 will result in an increase of 1 instead of 2. The actual boost amount may be a bit different but you can view the Amount as a percentage.

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The Detail slider is closest to an "edge finder" control?

Not as you think it does. It is a control that mixes between two sharpening algorithms used. At its minimum position it only uses something like a traditional USM sharpening algorithm, and at its maximum position it only uses a sort of 'deconvolution' sharpening algorithm. The in between positions of the Detail control produces a weighted mix between the two methods.

At its minimum position the sharpening algorithm basically boosts edge and point contrast by subtracting a blurred version of the original image and adds that result back on top of the original (while multiplying the 'amount' it will add back). It boosts local contrast and that tricks the human eye into a suggestion of sharpness.
At its max position the deconvolution algorithm attempts a real restoration of sharpness, by subtracting some of the blurred content from surrounding pixels, and adding it back to the central pixel, for each pixel in turn. It really restores original detail that was spread/blurred around. Unfortunately it cannot distinguish noise from detail, so it might increase noise as much as detail if one's unlucky.  

Quote
Since its clear you have a really deep understanding of the tool, can you explain (My last request, I promise) why the radius is in values of 1/10s of a pixel?  Is going below 1.0 making some kind of pivotal change in the sharpening as compared to values over 1.0?

No problem, just ask if needed. Blur is not something that abruptly stops at a pixel boundary, and it is also not constant within the radius. There is usually an underlying model that describes how much influence/weight the surrounding pixels have on the central pixel that is being processed. that model usually resembles a Gaussian curve. Most of the contribution comes from the pixels that are close by (near the 'peak' of the bell shaped curve), and progressively less contribution comes from more distant pixels.

Therefore, a radius of 1.0 doesn't mean that only one pixel will have influence, because that wouldn't change anything. It just means that say, 75% of the result is dominated by the influence of the 9 central pixels (inside the radius of 1), and 25% by all other pixels outside the radius (although very distant pixels will have negligible weight/effect or even none). When the radius becomes smaller than one, then that contribution of surrounding pixels is further reduced, but is not yet zero (after rounding). When the radius gets smaller than approx. 0.3 then hardly any noticeable effect can be expected from surrounding pixels anymore.

Cheers,
Bart
« Last Edit: May 31, 2012, 06:57:34 pm by BartvanderWolf »
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One Frame at a Time

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Re: Question about Radius Control for Sharpening
« Reply #9 on: May 31, 2012, 11:45:41 am »

Bart,

Outstanding!!  Thank you so much for that thorough explanation.  Read it 4 times and will probably read it another 4 - to let it all sink in.  Very much appreciated!

So amazing to get a glimpse of what is going on in the software we use.  I always think of the PC (and now my smart phone too) as one of the most incredible devices ever created by man.  When you think about the collaborative number of man hours that build upon each other (dating back to the 50's) to design the current OS, and then the actual software on top of that.  Add to that: all the time and research producing the hardware.....  Each time we boot up, its the result of countless collective man hours - putting the results, literally at our finger tips.  Truly astounding when you think about it.  Thanks again!

Paul Podell
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