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 information.
- To move a noise sample, do one of the following:
-
Click the point parameter
for
the noise sample in the Noise Sample Points controls group. A cross
hair appears in the composition, centered on that sample. Click
the desired location in the Composition panel to place the sample. -
Using the Selection tool
, drag
the sample point in the Composition panel to the desired location. -
Enter the desired horizontal and vertical coordinates in the Effect Controls panel.
Note: The number of sample points that are enabled corresponds to the current value of the Number Of Samples.
-
Change the sampling source frame
By default, the Remove Grain and Match Grain effects take noise samples from the first frame of the sequence, but you can choose to sample the noise from a different frame. Changing the frame might be useful if large lighting or exposure variations occur between frames within the sequence.
Change the noise sample box color
You can set the Remove Grain or Match Grain effect’s viewing mode to Noise Samples to see the areas sampled by the effect. Sampled areas are automatically framed with a white outline. If you prefer, you can change the color of these noise sample boxes.
Next to the Sample Box Color control in the Sampling
controls group, do one of the following:-
Click the color swatch, and select a color in the Color Picker.
-
Click the eyedropper, and click a color anywhere in the application window.