Apple MA434Z/A User Manual - Page 410

At 3-bit resolution, you begin to see a gradient from black to white, although the graph is still choppy. 8 bits, 256 values total Graph of 8-bit image Finally, at 8 bits, you can see a smooth transition, and the graph line is almost straight. These graphs demonstrate that more bits used to represent an image results in finer color transitions. Digital film compositing refers to bit depth on a per-channel basis, so 8 bits refers to 8 bits per channel, or 32 bits total for an RGBA (Red, Green, Blue, and Alpha) image. Shake can calculate up to 32 bits per channel, or 128 bits total in an RGBA image. To complicate things, because 8 bits equals 1 byte, images in Shake are set with a byte value of 1 (8 bits), 2 (16 bits), or 4 (32 bits, or float). Ultimately, the color depth you decide to work in depends on the destination of the end result. For example, because of the responsiveness of film and the size of the screen, an image that looks fine at 8 bits on video can look terrible on film. On the other hand, higher bit depths are more processor-intensive. You need to strike a balance between quality and speed. Avoiding Color Banding Most non-film composites work fine at 8 bits, which is typically the standard output of most 3D renderers and paint packages. However, there are times when you need to use a higher bit depth to process your images-requiring you to increase an image's bit depth to 16 bits, or a whopping 65,000 (more or less) values per channel. A typical example of when higher bit depths are better is whenever you process a ramp of similar color values (for example, the light-to-medium blue found within an image of the sky) across a wide screen space. An 8-bit image, though sometimes indiscernible from 16 bits on a computer monitor, will probably exhibit color banding when printed to film. If your sky image is generated in 8 bits in a different software package, there is no immediate improvement if you bump it up to 16 bits in Shake. In this example, the ramp needs to be generated in 16 bits to take advantage of the extra precision of 16 bits (for example, using the Shake Ramp or RGrad node). 410 Chapter 15 Image Processing Basics