According to Apple, a method and apparatus for performing motion estimation in a digital video system is disclosed. Specifically, the present invention discloses a system that quickly calculates estimated motion vectors in a very efficient manner. In one embodiment, a first multiplicand is determined by multiplying a first display time difference between a first video picture and a second video picture by a power of two scale value. This step scales up a numerator for a ratio. Next, the system determines a scaled ratio by dividing that scaled numerator by a second first display time difference between said second video picture and a third video picture. The scaled ratio is then stored calculating motion vector estimations. By storing the scaled ratio, all the estimated motion vectors can be calculated quickly with good precision since the scaled ratio saves significant bits and reducing the scale is performed by simple shifts.

Here's Apple's background on the invention: "Digital based electronic media formats are finally on the cusp of largely replacing analog electronic media formats. Digital compact discs (CDs) replaced analog vinyl records long ago. Analog magnetic cassette tapes are becoming increasingly rare. Second and third generation digital audio systems such as Mini-discs and MP3 (MPEG Audio--layer 3) are now taking market share from the first generation digital audio format of compact discs.

"The video media formats have been slower to move to digital storage and digital transmission formats than audio media. The reason for this slower digital adoption has been largely due to the massive amounts of digital information required to accurately represent acceptable quality video in digital form and the fast processing capabilities needed to encode compressed video. The massive amounts of digital information needed to accurately represent video require very high-capacity digital storage systems and high-bandwidth transmission systems.

"However, video is now rapidly moving to digital storage and transmission formats. Faster computer processors, high-density storage systems, and new efficient compression and encoding algorithms have finally made digital video transmission and storage practical at consumer price points. The DVD (Digital Versatile Disc), a digital video system, has been one of the fastest selling consumer electronic products in years. DVDs have been rapidly supplanting Video-Cassette Recorders (VCRs) as the pre-recorded video playback system of choice due to their high video quality, very high audio quality, convenience, and extra features. The antiquated analog NTSC (National Television Standards Committee) video transmission system is currently in the process of being replaced with the digital ATSC (Advanced Television Standards Committee) video transmission system.

"Computer systems have been using various different digital video encoding formats for a number of years. Specifically, computer systems have employed different video coder/decoder methods for compressing and encoding or decompressing and decoding digital video, respectively. A video coder/decoder method, in hardware or software implementation, is commonly referred to as a 'CODEC.'

"The best digital video compression and encoding systems used by computer systems have been the digital video systems backed by the Motion Pictures Expert Group commonly known by the acronym MPEG. The three most well known and highly used digital video formats from MPEG are known simply as MPEG-1, MPEG-2, and MPEG-4. VideoCDs (VCDs) and early consumer-grade digital video editing systems use the early MPEG-1 digital video encoding format. Digital Versatile Discs (DVDs) and the Dish Network brand Direct Broadcast Satellite (DBS) television broadcast system use the higher quality MPEG-2 digital video compression and encoding system. The MPEG-4 encoding system is rapidly being adapted by the latest computer based digital video encoders and associated digital video players.

"The MPEG-2 and MPEG-4 standards compress a series of video frames or video fields and then encode the compressed frames or fields into a digital bitstream. When encoding a video frame or field with the MPEG-2 and MPEG-4 systems, the video frame or field is divided into a rectangular grid of pixelblocks. Each pixelblock is independently compressed and encoded.

"When compressing a video frame or field, the MPEG-4 standard may compress the frame or field into one of three types of compressed frames or fields: Intra-frames (I-frames), Unidirectional Predicted frames (P-frames), or Bi-Directional Predicted frames (B-frames). Intra-frames completely independently encode an independent video frame with no reference to other video frames. P-frames define a video frame with reference to a single previously displayed video frame. B-frames define a video frame with reference to both a video frame displayed before the current frame and a video frame to be displayed after the current frame. Due to their efficient usage of redundant video information, P-frames and B-frames generally provide the best compression."

And here's Apple's summary of the invention: "A method and apparatus for performing motion estimation in a video codec is disclosed. Specifically, the present invention discloses a system that quickly calculates estimated motion vectors in a very efficient manner without requiring an excessive number of division operations..

"In one embodiment, a first multiplicand is determined by multiplying a first display time difference between a first video picture and a second video picture by a power of two scale value. This step scales up a numerator for a ratio. Next, the system determines a scaled ratio by dividing that scaled numerator by a second first display time difference between said second video picture and a third video picture. The scaled ratio is then stored to be used later for calculating motion vector estimations. By storing the scaled ratio, all the estimated motion vectors can be calculated quickly with good precision since the scaled ratio saves significant bits and reducing the scale is performed by simple shifts thus eliminating the need for time consuming division operations."

The inventors are Barin Geoffry Haskell, David William Singer, Adriana Dumitras and Atul Puri. The graphic below illustrates a high-level block diagram of one possible digital video encoder system.