WHITEPAPERS

Primatte chromakey software offers powerful, ultra-precision matte-control functions for superb quality image compositing. Primatte, which runs on a wide variety of SGI workstations, was originally developed by IMAGICA Corp. (Tokyo) in 1992, with the purpose of realizing the highest quality chromakey processing for digital imagery. Primatte is resolution-independent software, and supports video, HDTV and even film images. The basic algorithm utilized in Primatte was originally presented at the 8th NICOGRAPH Conference and the 23rd Imaging Technology Conference (both in 1992) and a U.S. patent was granted in 1994. Primatte is a standardized SGI software product.

Continued development of Primatte was transferred from PHOTRON LTD. (Tokyo), and is now being handled by IMAGICA, the original developers of Primatte.

You may have already heard of the "blue-screen process," it is one of the most popular special effects used in the motion picture and video industries today. To create this effect, the cinematographer takes a shot of the foreground object in front of a blue screen (this is often called "blue background") and then uses this shot to create an intermediate matte that discriminates based on the blue screen hue and density. This then allows compositing the foreground object onto the background scene.

Figure: 1 Shooting with bluescreen and chromakey processing

In the video industry, this process is called a "chromakey"(Figure 1) and the matte signals are created by converting the hue difference of the R, G, and B signals to an amplitude differential. Matte signals are then applied to the gate input of the switcher to create the compositing effect.

This same process can be used on computer generated imagery by processing the digitized picture.

The quality of a composite image depends on whether a precise and identical matte image can be achieved for the foreground object. For example, let's consider a magenta and a red foreground object shot in front of blue screen. Magenta contains a bluish color component, so the difference of hue achieved is smaller than that from the red object. Since the key value also varies, it makes it difficult to get a clean matte image for each of the red and magenta objects using the same conditions. Blue-spill removal from the edges of the foreground object and creating mattes from translucent objects, like smoke or steam, are among the most difficult conditions that must be resolved for clear composited images.

To resolve the above-mentioned difficulties, the following Features have been incorporated into the Primatte product:

• A unique method of calculating key values.
• Superb color processing on the soft-key portions of the image.
• Clean and precise blue-spill removal functions.
• Easy set-up and operation.

The following sections explain these Primatte Features in more detail.