Adobe After Effects CS3 - Manual (Page 461)

AFTER EFFECTS CS3
User Guide
456
• To increase the effect’s interaction speed and RAM preview duration, use the Region Of Interest feature in the
Composition panel to reduce the area that’s processed. (See “Work with the region of interest” on page 111.)
• To retain an image of the current frame in its current state, click Take Snapshot in the Composition panel. You
can subsequently click and hold down Show Last 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 “Work with snapshots” on page 125.)
• To compare the preview region with and without the grain effect, click the Effect switch (small f icon) next to the
grain effect’s name 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 Last
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 very 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 instance, 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, you must precompose or prerender 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 infor-
mation.
1 In the Effect Controls panel, choose Noise Samples from the Viewing Mode menu.
The samples appear as small white squares (24 x 24 pixels) overlaid on the source image.