The complexity of the Grain indicates the number of 'layers' of grain applied to the image. With a complexity of 1, only one grain layer is calculated and applied to the image. When complexity is set to 4, the tool calculates four separate grain layers and applies the mean combined result of each pass to the final image. Higher complexities produce visually more complex results, without the apparent regularity often perceivable in digitally produced grain.

When the Alpha Multiply check-box is enabled, the Film Grain tool will multiply it's results by the source images alpha channel. This is necessary when working with post multiplied images to ensure that the grain does not affect areas of the image where the alpha is 0.0 (transparent). NOTE: Since it is impossible to say what the final value of semi-transparent pixels in the image are until after they are composited with their background, you should avoid applying log-processed grain to the elements until after they have been composited. This will ensure that the strength of the grain is accurate.

When this check-box is enabled (default), the grain applied to the image will have it's intensity applied non-linearly to match the grain profile of most film. Roughly speaking, the intensity of the grain will increase exponentially from black to white. When this check-box is disabled the grain will be applied uniformly, regardless of the brightness of the affected pixel. One of the primary features of grain in film is that the appear of the grain varies radically with the exposure, so that there appears to be very little grain present in the blacks, with the amount and deviation of the grain increasing as the pixels exposure increases. In a film negative the darkest portions of the developed image will appear completely opaque, and this obscures the grain. As the negative becomes progressively clearer, more of the grain becomes evident in the result. Chemical differences in the response to light of the red, green and blue layers of the film also cause each color component of the film to present a different grain profile, typically with the blue channel presenting the largest amount of grain. The original Grain tool in Fusion provided a series of LUT curves that could help the artist come close to matching the response curve of any film stock. However, hitting the right set of curves to match a specific film stock turned out to be more a matter of trial and error. In an extensive analysis of a large number of reference and production film plates, eyeon's development team determined that the log-exposure nature of films provides the majority of the grain strength characteristic, enough to almost overshadow any other variable which might have an impact. As a result, the most important control in the new Film Grain tool is the 'Log Processing' check-box, which should be enabled when matching film, and disabled when working images which require a more linear grain response. Having this check-box enabled will closely mimic the results of preceding the old grain tool with a Linear->Log conversion, and following Log->Linear conversion immediately after.

NOTE: A still generalized, but somewhat more technical explanation can be found in the post titled "Grain's Anatomy", on eyeon's developers blog. http://eyeondev.blogspot.com/2010/03/grainsanatomy. html.

The Seed slider and Randomize button are presented whenever a Fusion tool relies on a random result. Two tools with the same seed values will produce the same random results. Click on the Randomize button to randomly select a new seed value, or adjust the slider to manually select a new seed value.

When the Monochrome check-box is enabled (default) the grain will be applied to the Red, Green and Blue color channels of the image equally. When deselected, individual control over the Size, Strength and Roughness of the grain in each channel becomes possible.

Deselect the Lock Size X/Y check-box to control the size of the Grain along the X and Y axis individually.

The Grain Size is calculated relative to the size of a pixel, so that changing the resolution of the image does not impact on the relative appearance of the grain. The default Grain Size of 1.0 will produce grain kernels that cover roughly 2 pixels.

Grain is expressed as a variation from the original color of a pixel. So the stronger the grains strength, the wider the possible variation from the original pixel value. For example, given a pixel with an original value of p, and a grain tool with complexity=1; size=1; roughness=0; log processing=off; the grain will produce an output value of p +/- strength. In other words a pixel with a value of 0.5 with a grain strength of 0.02 could end up with a final value between 0.48 and 0.52. Once again, that's a slight over simplification, especially when the complexity exceeds 1. Enabling the Log Processing check-box will also cause that variation to be affected such that there will be less variation in the blacks and more variation in the whites of the image. NOTE: When visualizing the effect of the grain on the image, the more mathematically inclined may find it helps to picture a sine wave, where each lobe of the sine wave covers 1 pixel when the Grain Size is 1.0. The Grain Size controls the frequency of the sine wave, while the Grain Strength controls its amplitude. Again, this is something of an oversimplification.

The Roughness slider applies low frequency variation to give the impression of clumping in the grain. Try setting the Roughness to 0, and you will observe that the grain produced has a very even luminance variation across the whole image. Increase the roughness to 1.0 and you will observe the presence of 'cellular' differences in the luminance variation.

The offset control helps to match the intensity of the grain in the deep blacks by offsetting the values before the intensity (strength) of the grain is calculated. So an offset of 0.1 would cause a pixel with a value of 0.1 to receive grain as if it's value was actually 0.2.

In the default setting the different amounts of Grain are applied to the blacks and the whites of the image.

When Log processing is off, the Grain is applied evenly to the entire image as shown here.