There are two main filters in Neat Image: noise reduction filter and sharpening filter. These two filters can be used together and each of them can be used alone. You can enable / disable and adjust both filters using the guidelines in the subsections below.
The noise filter has separate settings for the luminance and chrominance components of the input image. Noise reduction applied to one image component is controlled by two settings: noise level and noise reduction amount. Such a pair of settings associated with an image component (here, either with the luminance or chrominance component) is fundamental for Neat Image noise reduction.
Let us return for a moment to the noise analysis and noise profile. When you did noise analysis and built the noise profile, Neat Image measured levels of noise in each component of the analyzed image. The noise level in an image component can be low or high depending on the strength of visible noise in this component. Neat Image analyses noise and measures the noise level, which results in a number saved in the noise profile. You can see the measured noise levels using the Profile Viewer window (it is accessible via the Profile | Profile Viewer menu item or the Ctrl-I shortcut).
For example, noise
level in the luminance channel could be measured at 8.55 units. This figure
tells the noise filter which image elements should be considered noise and which
- image details: the image elements that are weaker than 8.55 units are considered
noise and reduced by the noise filter; the image elements that are stronger
than 8.55 units are considered details and not reduced. In this sense, the noise
level (8.55 units in this example) may be considered a kind of threshold if
you compare it with other filters you may use (e.g., Unsharp Mask).
If you do not change the default noise filter settings (Noise Levels: Luminance channel: +0%) then noise reduction is completely determined by the above figure from the noise profile. However, if you do adjust the filter setting for the noise level of the luminance channel, then this adjustment is taken into account too. For example, if you set the Noise Levels: Luminance channel control to +15% then what is used by the noise filter as the noise level is:
8.55 * (100% + 15%) => 9.83 units
With this adjustment, the image elements in the luminance channel that are weaker than 9.83 units are considered noise and reduced and elements that are stronger than 9.83 units are not reduced.
As you have just learned, the noise level of a specific image component tells the noise filter what should be considered noise and what - important image details in this component. The noise reduction amount related to the same component tells the noise filter how much of the found noise should be reduced. For example, if the noise reduction amount is set to 50% then all image elements that are weaker than the noise level (in the above example with noise level adjustment it is 9.83) are reduced (made weaker) in half. The noise reduction amount value of 100% tells the filter to remove the found noise completely.
Thus, with a noise level - noise reduction amount pair of settings you can adjust what should be considered noise in a component of the input image and how much of this noise should be reduced. In the standard control set, you have access to two such pairs in the noise filter: one for the luminance channel of the input image and one for the chrominance channels.
Because the noise level estimations used by the filter are based on the noise profile, the default filter settings usually produce accurate results (provided the noise profile is accurate). When the noise level controls are adjusted in some direction, the noise level estimations are raised or lowered accordingly. A noise level can be in the range from -100%, which means no image elements are considered noise, and therefore, no noise reduction is applied in the corresponding image component; to +150%, which means noise reduction is applied to the image elements that are weaker than 250% of the noise profile's noise level.
Noise reduction amount controls determine how much reduction is applied to the image elements identified as noise. Noise reduction amounts can be in the range from 0% (none of the detected noise is removed) to 100% (all the detected noise is removed). By default, the noise filter removes 60% of detected noise in the luminance channel of the input image and 100% of noise in the chrominance channels. Our experience shows that the default noise reduction amounts generally provide a good balance between preserving image details and noise removal.
Decreasing the noise reduction amounts may have a positive effect if the input image contains some natural noise. For example, when you are filtering images of asphalt, sand, or anything else that contains fine natural noise-like features, it may be helpful to reduce amounts down to 40-50% (see Partial filtration for additional tips).
(It is advised to disable the sharpening filter when adjusting the noise filter. To disable the sharpening filter, set the Sharpening amount: Luminance channel to 0%.)
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As human vision is not very sensitive to variations of colors, strong filtration in the chrominance channels does not noticeably distort an image, but efficiently removes color noise. |
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The higher a specific noise level, the more image elements in the corresponding image component are considered noise. Be careful, setting a noise level too high can lead to removal of important image details. Setting a noise level too low can lead to incomplete filtration: residual noise and compression artifacts may remain in the output image.
As a rule, if the device noise profile has been built properly, it is not
necessary to increase the noise levels by more than 50%.
If adjusting noise levels still does not help and some noise elements remain
in the image, probably the device noise profile is not good at all. Return
to Stage II. Prepare a device
noise profile and additionally fine-tune the device noise profile or
simply rebuild the profile from scratch.
The Component Viewer is intended for detailed examination of channel components of the processed image. Find more details about using this tool in the Component Viewer subsection.
The Variant Selector is designed designed to compare several variants of filtration side-by-side to find the optimum filter settings easier and faster. More information about this tool is available in the Variant Selector subsection.
The sharpening filter increases image sharpness without increasing the noise strength.
The sharpening filter is disabled (sharpening amount is set to 0%) by default. Adjust the sharpening amount if you want to sharpen the image. Like with any sharpening method, you have to balance the sharpening amount to avoid over-sharpening.
Use the preview when adjusting the sharpening settings.
As soon as you are happy with the preview results regarding both noise reduction and sharpening, proceed to the subsection about saving the filter settings into a preset or to the Stage IV. Apply filter to the input image.