Remove Grain effect

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Remove noise or grain from an image
Add temporal filtering to a sequence
Sharpen an image with Unsharp Mask controls

To remove grain or visual noise, use the Remove Grain effect. This effect uses sophisticated signal processing and statistical estimation techniques in an attempt to restore the image to how it would look without the grain or noise. While many techniques, such as applying a mild Gaussian Blur effect or the Median effect, reduce the visibility of noise in an image, the tradeoff is an unavoidable loss of sharpness and highlights. The Remove Grain effect, in contrast, differentiates fine image detail from grain and noise and preserves the image detail as much as possible.

The Remove Grain effect provides several options to precisely balance the reduction in noise and the amount of sharpness retained in the image. Additionally, the Remove Grain effect can analyze the differences between frames to further improve noise reduction and sharpness; since this process operates over time, it is called temporal filtering.

Note: Good degraining depends on good noise sampling. The results of the automatic sampling depend on the image content and noise type. You can also change the number, size, and position of the samples to get the best results for a particular image.

The Temporal Filtering controls of the Remove Grain effect use a statistical algorithm to blend the current frame with previous and next frames. These controls are especially effective in removing compression artifacts from DV or video footage. Because temporal filtering works on the basis of differences between frames, it is useful only for sequences.

To properly evaluate the results of this filter, the sequence must be viewed in real time, either with a RAM preview or by viewing a movie rendered to a file.

To increase the speed of the Remove Grain effect preview, adjust the Remove Grain controls in order in the Effect Controls panel. Specifically, the most efficient workflow is to find effective degraining settings first and to adjust the last three controls last.

This effect works with 8-bpc and 16-bpc color.

Original (left), and with effect applied (right)

Remove noise or grain from an image

Select the layer, and choose Effect > Noise & Grain > Remove Grain.
Adjust any of the following using the Noise Reduction Settings controls group:
- To adjust the overall amount of noise in the image, adjust the Noise Reduction value.
- To adjust the amount of noise on each channel individually, adjust the Red, Green, and Blue Noise Reduction values in the Channel Noise Reduction controls.
  
  The blue channel often has the most pronounced grain in a chemical film-based image. Try reducing the noise in only the blue channel to retain all image detail in the other two channels.
Adjust the Passes value to control the maximum noise radius that can be detected:
- If your grain is large and chunky, try increasing the Passes value. A higher number of passes reduces larger-sized noise.
- If your render time is longer than desired because your file size is large, try lowering the number of passes to reduce the memory usage and render time.
  
  Note: Once the optimum number of passes is applied, additional passes have no effect.
Choose one of the following from the Mode pop-up menu:
Multichannel

Degrains all channels of a color image together, which generally produces the best results on color images. This mode takes advantage of correlations between channels to improve the accuracy of the denoising process.
Single Channel

Degrains each channel independently. Use this mode for a monochromatic image or if Multichannel causes objectionable color artifacts.
Adjust any of the following in the Fine Tuning controls group to improve the balance between noise reduction and retained sharpness:
Chroma Suppression

Suppresses some of the chroma from the noise to clean up the image. If the noise is colorful, increasing this control can help remove it. Setting the amount too high may strip some chroma from the image itself. (Chroma Suppression has no effect on grayscale images and isn’t available if the Noise Reduction Settings Mode is Single Channel.)
Texture

Controls the amount of low-level noise that passes through to the output. This setting is especially useful to reduce objectionable artifacts or to retain finely textured areas such as wood grain or brick. Lower values result in a smoother, possibly artificial-looking result. Higher values may leave the output unchanged from the input.
Noise Size Bias

Controls how the noise reduction process responds to variations in noise size within the same image. The default value of zero treats all sizes equally. Negative values leave larger residual noise and more aggressively remove smaller-sized grain. Positive values leave smaller noise and more aggressively remove noise of larger size.
Clean Solid Areas

Controls the extent to which adjacent pixels with low variations in value are smoothed out by the noise reduction process. This setting is helpful for large areas of solid color that need to be as clean as possible. Settings that are too high can smooth out nearly solid areas of the image, resulting in an artificial appearance.
Adjust the Unsharp Mask controls to return subtle edge detail that the degraining removed.
If you are applying the effect to a sequence of frames, use the Temporal Filtering controls to perform interframe noise reduction.
To change the effect view, choose any of the following from the Viewing Mode pop-up menu:
Noise Samples

Shows the areas that have been sampled to extract the current noise model.
Preview

Displays the current settings of the applied effect in a 200x200 pixel area.
Blending Matte

Shows the current color matte or mask, or the combination of both, which results from the current settings of the Blend With Original controls group.
Choose Final Output from the Viewing Mode control.

Add temporal filtering to a sequence

Apply the Remove Grain effect to your image.
Place the Remove Grain preview region over the area of the image that has the most subtle changes from frame to frame or that has the most moving image detail.
Select Enable in the Temporal Filtering controls.
Adjust the Amount value to 100%.
Create a RAM preview of the sequence or render it.
If you see unwanted streaking or blurs around moving objects, reduce the Motion Sensitivity value, and then preview or render it again.
Try the following techniques if you want to improve the results:
- To quickly reduce the noise of a sequence that has a lot of buzzing noise, set the Noise Reduction value to zero and the Temporal Filtering Amount to 100%, and render the sequence.
- To speed up previews, apply temporal filtering to your sequence after all the settings for a single frame have been adjusted.
- To retain effects on a layer and also apply temporal filtering to it, precompose the selected layer (choose Layer > Precompose), and then apply the Remove Grain effect to that layer.

Sharpen an image with Unsharp Mask controls

The Remove Grain effect contains Unsharp Mask controls, which increase the contrast of edges and fine details to help restore some of the sharpness that may have been lost during the grain reduction process.

Do any of the following:

Increase the Unsharp Mask controls Amount value to obtain acceptable sharpening without generating undesirable artifacts or bringing back too much grain.
Increase the Threshold value to remove any unwanted artifacts that resulted from the sharpening.
Adjust the Radius to change the area over which Unsharp Mask finds details.
Adjust the Noise Reduction value until the image begins to lose sharpness; then decrease the value a little, and then apply the Unsharp Mask controls to sharpen the image.