Inside Macintosh: QuickTime
QuickDraw provides a means of displaying images with high color resolution in pixel maps or on screens with lower color resolution. By dithering the destination image, QuickDraw fools your eyes into seeing colors that are not actually available on the display screen. Unfortunately, the error-diffusion technique used by QuickDraw takes longer than just drawing pixels by directly looking them up in a color table. The drawing delays imposed by standard dithering are unacceptable when working with movies.
To alleviate this problem, Apple has developed a technique that allows faster dithering to destinations that use 8 bits per pixel. Fast dithering uses lookup tables created by the Image Compression Manager. All the decompressors supplied by Apple can use fast dithering.
Apple decompressors use fast dithering when copying from image band buffers to 8-bit destinations. If the accuracy for decompression is above normal, then the decompressors use true error diffusion rather than fast dithering. Note that video sequences are normally displayed at normal or low accuracy so that you can obtain maximum display speed during decompression.