Autodesk 62205-051408-9001 User Guide - Page 728

27 Adding and Removing Grain Adding Grain to Clips The Add Grain operator synthesizes noise patterns and applies them to clips. It modulates the intensity of the noise according to the luma (or density) of the image pixels using a granularity curve that specifies the gain for the luma values of each channel. Gain (intensity) Luma (brightness) With the Add Grain operator, you can: • Add grain to video or computer-generated images to blend them smoothly with other film elements in the composite (for example, film backgrounds). • Add grain to match the grain in various film shots that were shot on different types of film or under different shooting conditions. • Add grain (and degrade the quality of the images) to make a resulting clip more film-like or "realistic." Some people believe we are culturally conditioned to accept film-quality images as reality. The dynamic texture of grain can also be an artistic choice. It is like a noise or texture that can be subliminal at times. About Granularity and Graininess Granularity is the objective measurement of the size of grain clusters across an area of uniform tone. It is high in dark regions and midtones, and low in highlights. The granularity curve represents the granularity from the shadows up to the highlights. For an image with 8 bits per color channel, the X-axis of the curve ranges from 0 to 255. If you are working with 10 bits per channel, the range is from 0 to 1023. The film emulsions listed in the Film Stock list on the Add Grain Controls panel have specific granularity curves provided by the manufacturer (Kodak). Each type of film stock has a distinct grain "fingerprint" which is described by the granularity curve. Since the fingerprint is specific, all elements of a composite need to have been shot on the same stock. Instead, you can de-grain elements (with various fingerprints) before you create the mattes, and then re-grain the entire composite afterwards to match the grain. For example, apply a Remove Grain operator before the Keyer for the foreground layer, another on the background layer, and apply the Add Grain operator after the composite. Graininess is the subjective perception of the amount of grain. The eye is distracted by fine detail, bright highlights, or busy movement, paying less attention to grain in such cases. A sharp image appears less grainy than when poorly focused. Large areas of uniform tone reveal grain patterns most clearly. Using the Add Grain Operator The grain from the Add Grain operator is very subtle and controllable. There are two approaches to adding grain with this operator: • You can use the granularity curves for a film stock. • You can use the Custom option to exactly match the grain of another clip. When you choose a film stock from the Film Stock list, the built-in granularity curves for the corresponding Kodak emulsion are used, and the RGB Gain parameters that you set are translated as scaling factors for the curves. 712