# The Noise Filter

After you have loaded or created a profile, you are ready to remove noise from the image. First, click on the Filter tab:

## The preview window

The preview window -- located in the upper right portion of the plug-in user interface -- shows the effects of the current filter settings. When you make adjustments to the settings, the preview window is updated. If you have a slow machine, you might want to choose a smaller preview size in the Preferences dialog, so the preview will be updated more quickly. The Preferences dialog also has options for animating the update of the preview window.

You can press the Before Button, located to the right of the preview window, to toggle between the original and filtered pixels. Press and hold the button to view the original pixels, and release it to display the filtered pixels.

## Removing noise

The Filter page includes three groups of controls. The luminance group affects filtering of noise in the brightness component of an image. This is usually the part of the image that contains the most real information. The colors group controls filtering of color noise, which is often the most displeasing noise. The sharpness group enhances edge sharpness using an unsharp mask.

Strength controls how aggressively the filter is applied. This is the control that you'll normally want to adjust if you don't like what you see in the preview rectangle. If you move it far to the right, the image may look too plastic-like. If you move it to the left, it will look more grainy. Often the most natural-looking results are achieved when you leave a modest amount of grain. The human eye tolerates a certain amount of noise when it is accompanied by detail, and the luminance channel contains most of the detail in an image.

Smoothness increases or decreases the noise levels estimated by the Noise Profile. Usually the default setting is adequate, but if you see isolated specks in smooth areas, try increasing the smoothness setting to see if they go away. (Here's a tip for adjusting the Smoothness slider: Move the preview window to a smooth background areas, and move the Strength slider all the way to the right so the preview is a smooth as possible. Then set the Smoothness slider to the minimum level such that the preview is still smooth (if it is too low, you will usually see isolated specks). Finally, lower the Strength slider until you like the results.)

Contrast applies a scaling factor to edges, similar to an unsharp mask, but at several resolutions simultaneously. You probably won't need to adjust this in most situations.

Sharpness amount and sharpness radius apply a conventional unsharp mask to the filtered image. Some people prefer to set the amount to zero and do sharpening later in the workflow. However, even in that case, it can be useful to preview the sharpening effect to see how it interacts with the noise reduction. The Suppress halos option eliminates unnatural highlight artifacts that can occur when high sharpening levels are used

In addition, there are some controls at the top of the panel. Filter coarse noise is occasionally useful when an image has very low-frequency noise, usually in the color channels. Turbo mode takes a few shortcuts to accelerate filtering. Usually the difference in quality is negligible, but sometimes strong diagonal edges may show some aliasing artifacts ("jaggies").

## A trick for adjusting the Filter controls

Here is a quick and easy strategy that often yields excellent results:

1. Move the preview to an area that should be smooth.
2. Move the Luminance Strength slider all the way to the right.
3. Adjust the Luminance Smoothness slider to the lowest setting possible without introducing obvious speckling. The preview area should be smooth.
4. Now, reduce the Luminance Strength slider until you like the balance between detail and smoothness.

## Saving and reusing profile and filter settings

You can save the current profile and filter settings directly to a file by clicking on the "Save" button. Use the "Open" button to load a saved settings file from disk.

## Selective filtering

The Noise Ninja plug-in filters the currently active layer and/or channel. It also respects the selection mask. So, you can select an area in PhotoShop and apply Noise Ninja only to that area. You can feather the selection for a gradual transition.

Using layer masks to emulate the Noise Brush: In the standalone version of Noise Ninja, there is a tool called the "Noise Brush" that allows you to paint on the image to selectively undo the results of filtering. What is especially powerful about the Noise Brush is that it allows you to apply the undo operation separately in the luminance and chroma channels. Because most real image detail is contained in the luminance information and the ugliest noise is often in the chroma channels, the Noise Brush can be effective at restoring detail without restoring unwanted color noise in cases where you feel the filtering algorithm has been too aggressive with fine detail.

Currently, the plug-in version of Noise Ninja does not include the Noise Brush feature. However, you can emulate the Noise Brush using layer masks in PhotoShop. Here is the basic procedure:

1) Make a duplicate of the layer containing the image to be filtered.

2) Select the duplicate layer, then choose "New Layer Mask -> Hide All" from the Layer menu.

3) Set the blend mode for the duplicate layer to "Luminosity", "Colors", or "Normal", depending on what you want to manipulate.

4) Filter the original layer with Noise Ninja as you normally would.

5) Select the layer mask for the duplicate layer.

6) Select the brush tool and paint in white to reveal the original pixel values, and paint with black to mask them and reveal the filtered pixels. (Make sure the opacity of the brush isn't too light, or it will be difficult to see the effect.)

This should allow you to blend the original and filtered pixels in a variety of ways. It's a little work to set up, but you could probably automate a few steps using a PhotoShop action.