1. Field of the Invention
The present invention relates to motion vector search apparatus and method that favorably search for motion vectors between frames of digital moving pictures.
2. Related Background Art
Conventionally, a block matching method has been frequently used as a motion vector search method in a motion compensation inter-frame prediction coding method.
The block matching method is a method in which a block in a reference frame in the same position as a motion search target block (N×N pixel area) is used as a center to calculate matching evaluation functions with blocks that shifted in units of pixels within a search range of ±M×±M pixels, the best matching block is searched, and the amount and direction of the shift to the matching block is used as the motion vector. As the matching evaluation function, a sum of absolute values of differences between pixel values calculated on a pixel to pixel basis for all pixels within the block (i.e., a difference evaluation value) may be used, for example.
The block matching method, however, has a problem in that searches must be conducted a great number of times, which requires an enormous amount of calculation when this system is realized. For this reason, various simplification techniques such as a method of searching with multiple steps, such as, for example, a 3-step search method have been suggested.
In the 3-step search method, a search is made at rough search-point intervals as the first step, and then the second search is made at search-point intervals each of which is a half of the interval for the first step around the best matching position as a center, instead of the all-pixel position search. In this manner, in the 3-step search method, the same process is repeated in a plurality of steps to narrow the coarse search-point intervals to close ones, thereby searching for a motion vector.
Accordingly, the amount of calculation can be significantly reduced with the 3-step search method than with the block matching method that performs the all-pixel position search described above. However, the 3-step search method entails a possibility of finding a motion vector completely different from the search direction sought if an erroneous search is made in the first step.