ACDNR

Adaptive Contrast-Driven Noise Reduction

The process works using a low-pass filter that smooth's the image by removing small-scale structures ( noise ) , and the edge protection device prevents image structures from being damaged during low-pass filtering.

ACDNR offers two identical sets of parameters: one for the luminance and another for the chrominance of color images. the eye see's detail ( and therefore noise ) much more in the luminance than the colour so we have to be less aggressive with the settings in the lum channel than the colour , fortunately ACDNR lets us do both at the same time

Also the use of a luminance mask in the tool enables great control of where noise reduction is applied

I will start by looking at all the settings in the tool , buts its not as bad as it looks most of the settings work great in default

Below is open control panel of ACDNR

 

Most used options

A) Luminance and chrominance--Selection Tabs for each channel , options the same for each

B) Apply--check to apply or not to each channel

C) Std. Dev.: Standard deviation of the low-pass filter (in pixels).. This parameter controls the size in pixels of the kernel used. The kernel size directly defines the sizes of the image structures that the low-pass filter will tend to remove. For example setting of 1 to 2 are used to remove small scale noise ( a setting used a lot ) and larger scales 4 to 6 can be used on low signal areas such as the background The Std Dev is what you will mainly use for setting the aggressiveness of the process

D) Amount-- Sets the implementation of the tool , setting of 0.5 to 1 are often used can be left default for a lot of the time

E) Iterations: This is the number of times that the low-pass filter is applied. The ACDNR filter is much more efficient when applied iterativly common settings of 3 to 5 , can be left default for a lot of the time

Q) Check to apply luminance mask that you will create with the bottom part of the tool, This is something you will use virtually all the time as with the use of a mask you control where you want the noise reduction

 

The above settings are what you will use most of the time, these are what adjust and control the low pass filter

 

Next I will have a look at the luminance Mask settings as these are used a lot

 

Edge Protection

O) Preview --Check to enable preview of luminance mask ( Also click on real time preview on bottom bar to activate )

P) Midtones / shadows / highlights --Adjust these to create your luminance mask, remember the darker = more protection and white= no protection

 

The remaining settings you can class as advanced and can be left default for 99% of the time

 

F ) Prefilter: If necessary, ACDNR can apply an initial filtering process to remove small-scale structures from the image. This can help to achieve a more robust edge protection , Its suggested only use if you have a large amount of noise ( If you need to use this , you need to get some more subs )

G) Robustness: When ACDNR's edge protection has to operate in presence of strong small-scale noise, it may have a hard time defining accurate edges of significant structures. For example, isolated noisy pixels can be very bright or dark, and their contributions to the definition of protected edges can be relevant. Robustness refers here to the ability of ACDNR to become immune to small-scale noise when discriminating significant image structures. Two robustness enforcing methods have been implemented: weighted average and morphological median. In both methods, for each pixel a neighborhood is defined and a robust reference value is calculated from the neighbor pixels, which is then used to command the edge protection device. Both methods have their strong points. The method based on the morphological median is especially good to preserve sharp edges. On the other hand, the weighted average method can yield more natural-looking images. You can try both of them and see which is best for your specific needs, according to your preferences.

H) Structure size: Minimum structure size used

I) Symmetry: Use the same setting for both the dark side ( No darth vader is not here ) and bright side edge protection .suggest leave as default ( checked )

J) Enable checkbox for bright sides--Leave this on

K) Threshold-- This parameter defines the relative brightness difference that triggers the edge protection mechanism. Low numbers offer higher protection--suggest leave as default

L) Overdrive-- This parameter controls the strength of edge protection. When overdrive is zero (its default value), edge protection just tries to preserve the existing pixel values of protected edges. Larger numbers is more aggressive -- suggest leave as default

M) Star protection--Check box , leave checked

N) Star threshold: As part of the bright sides edge protection parameters, Star threshold allows us to define a star edge -- suggest leave as default

Lu

How to use ACDNR

 

To start with we need to create a luminance mask to protect areas of the image that

1) Do not need noise reduction - Protect with black

2) Need some noise reduction-protect with gray

3) need large amount of noise reduction--do not protect leave white

So under the luminance mask I have checked the preview and activated the real time preview , and then adjusted the sliders to get the the desired mask , see the image below for details

After you are happy with the mask disable the preview and close down the real time preview

 

 

Click on Image to enlarge

 

I then used the following setting, and all I have really done is set the std dev of two for the lum and 3 for the chrom ( don't forget we can be more aggressive with colour ) , checked the lum mask for protection and lowered the amount a little in luminance all the rest of the settings have been left at the default

 

 

Now I am going to use a preview with ACNR this means I can try the settings on a small part of the screen, so I can fine tune the settings

I have drawn one below ( using the shortcut alt N ) trying to cover a area with low and high signal , so I can judge the results

To do this click on the the preview tab on the side bar of the image ( preview will come to the front ) and apply the process to the preview

You can then undo redo the preview with the icon on the top bar for comparison

 

Click on image to enlarge

 
To look at the before and after roll your mouse over the image below
As you can see noise reduction has been carried out where it is needed, if not adjust your luminance mask to suit , when you are happy with the settings reselect the main image and apply
 

You can noise reduction with a much more aggressive setting on the background , but you will need to create a heavy mask to protect everything except the the back ground and apply with a std dev of around 5 to the lum and 6 to the chrom

See below for the idea

I would say though be careful with your noise reduction , its not magic and sometimes a little noise is better than lumpy images
 

 

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