Adobe After Effects - Manual (Page 597)

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pixels. (Larger preview regions can result in slower rendering.)
4. Select Show Box if you want to outline the preview region in color. If you want to change the outline color, next to Box Color do one of the
following:
Click the color swatch and select a color in the Color Picker dialog box.
Click the eyedropper button, and click a color anywhere on the screen.
5. View the results:
To view the fine detail of the noise structure, zoom into the preview region.
To examine the noise in each channel independently, click the corresponding color channel icon in the Composition panel.
To increase the interaction speed and RAM preview duration, use the Region Of Interest feature in the Composition panel to reduce the
area that’s processed. (See Region of interest (ROI).)
To retain an image of the current frame in its current state, click Take Snapshot in the Composition panel. You can then click Show
Snapshot to view the most recent snapshot instead of the active composition, and to toggle between the current and previous states
of the preview region. This technique is extremely useful for evaluating subtle adjustments. (See Snapshots.)
To compare the preview region with and without the grain effect, click the Effect switch next to the name of the grain effect in the
Effect Controls panel to temporarily disable the effect. Click Take Snapshot in the Composition panel, click the Effect switch again to
re-enable the effect, and then click and hold down Show Snapshot to display the snapshot of the image without the effect.
Work with noise samples in Grain effects
Noise sampling is the first and most important step in removing noise from an image or in matching the noise of one image in another image.
Normally, this process is entirely automatic. For fine control, you can switch to Manual mode and adjust the samples using the Sampling controls
group in the Effect Controls panel.
A noise sample should be a solid block of uniform color that clearly displays the noise pattern present in the image. The objective is to extract
samples of pure noise, without any image features that the algorithm could misconstrue as grain. For example, extract samples from a piece of
sky, a background wall, or an area of fleshtone. All samples should be selected from the normal range of the film, DV, or video stock. Avoid
underexposed or overexposed areas lacking in information, especially areas where pixel values have been clipped to pure black or white. Within
this normal exposure range, it’s best to select samples with various RGB values and colors—for example, one sample from a bright area, one from
a dark area, and one from an area with midtones.
The number of samples in automatic mode is high to ensure that the algorithm has enough good noise data, even if finding good samples in a
particular image is difficult. In addition, automatic mode may override the number of samples you’ve set if the effect can’t find enough good
samples. You can vary the size of the samples in either automatic or manual mode; however, increasing sample size doesn’t guarantee better
results, especially if the resulting samples include more substantial variations in RGB values. Sample size should be reduced if a particular image
doesn’t contain sufficiently large areas of constant color values. Conversely, increasing the sample size may give better results if the image
contains large featureless areas.
Manually reposition noise samples
Automatic grain or sample selection generally gives acceptable results for the Match Grain or Remove Grain effect, but you can choose to
manually position and resize each sample or change the sample number. For example, you may want to reposition samples if the automatic
sampling selected a uniform area that is underexposed or overexposed and that lacks information about grain structure.
Noise samples for the Match Grain and Remove Grain effects are always extracted from the source layer without regarding any effects or masks
already applied to the layer; this method results in more accurate sampling. If you want the samples to include the existing effects, precompose or
pre-render the source layer with the effects and then apply the grain effect to the resulting source layer.
Avoid sample areas with the following characteristics: sharp edges, color gradients, highlights, textures such as grass or water ripples, fine detail
such as hair or tree leaves, and overexposed or underexposed areas lacking in information.
1. In the Effect Controls panel, choose Noise Samples from the Viewing Mode menu.
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