the one with minimum cost function and the macro block at this location is the best match. There is a reduction in computation by a factor of 9 in this algorithm. For p=7, while ES evaluates cost for 225 macro-blocks, TSS evaluates only for 25 macro blocks. Two Dimensional Logarithmic Search TDLS is closely related to TSS however it is more accurate for estimating motion vectors for a large search window size. The algorithm can be described as follows, Start with search location at the center Select an initial step size say, S = 8 Search for 4 locations at a distance of S from center on the X and Y axes Find the location of point with least cost function If a point other than center is the best matching point, Select this point as the new center Repeat steps 2 to 3 If the best matching point is at the center, set S = S/2 If S = 1, all 8 locations around the center at a distance S are searched Set the motion vector as the point with least cost function New Three Step Search TSS uses a uniformly allocated checking pattern and is prone to miss small motions. NTSS is an improvement over TSS as it provides a center biased search scheme and has provisions to stop halfway to reduce the computational cost. It was one of the first widely accepted fast algorithms and frequently used for implementing earlier standards like MPEG 1 and H.261. The algorithm runs as follows: Start with search location at center Search 8 locations +/- S pixels with S = 4 and 8 locations +/- S pixels with S = 1 around location (0,0) Pick among the 16 locations searched, the one with minimum cost function If the minimum cost function occurs at origin, stop the search and set motion vector to (0,0) If the minimum cost function occurs at one of the 8 locations at S = 1, set the new search origin to this location Check adjacent weights for this location, depending on location it may check either 3 or 5 points The one that gives lowest weight is the closest match, set the motion vector to that location If the lowest weight after the first step was one of the 8 locations at S = 4, the normal TSS procedure follows Pick among the 9 locations searched, the