Method and apparatus for variable accuracy inter-picture timing specification for digital video encoding with reduced requirements for division operations
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.
1. For a stream comprising first, second, and third video pictures, a method comprising:
computing a scaling value that is based on (i) a particular power of two value, (ii) a first order difference value between an order value for the third video picture and an order value for the first video picture, and (iii) a second order difference value between an order value for the second video picture and the order value for the first video picture;
computing a motion vector associated with the second video picture by bit-shifting a product of the scaling value and a motion vector associated with the third video picture, wherein a number of bits shifted by said bit-shifting is based on said particular power of two value; and
decoding the second video picture by using the computed motion vector.
2. The method of claim 1 , wherein an order value for a video picture is representative of a temporal relationship for the video picture with respect to another video picture.
3. The method of claim 1 , wherein an order value for a video picture is representative of a position for the video picture in a sequence of video pictures.
4. The method of claim 3 , wherein the sequence is a sequence for outputting video pictures.
5. The method of claim 1 , wherein the scaling value is directly proportional to the second order difference value.
6. The method of claim 1 , wherein the scaling value is inversely proportional to the first order difference value.
7. The method of claim 1 , wherein the order value for the second video picture is derived from a value stored in a slice header that is associated with the second video picture.
8. The method of claim 1 , wherein computing the motion vector associated with the second video picture comprises performing a mathematical operation that is equivalent to performing a multiplication and a division by said particular power of two value.