A SAD comparison is the following: For all pixels A in a block 1 and B in a block 2, sum up abs(A - B). The total is the SAD of the two blocks. It compares e

A SAD comparison is the following: For all pixels A in a block 1 and B in a block 2, sum up abs(A - B). The total is the SAD of the two blocks. It compares each of the pixels of Block A with the corresponding pixel in Block B. Its the simplest possible block comparison metric that still takes into account all individual pixels. Its popular not only because of its inherent simplicity, but also because of the psadbw MMX/SSE instruction that makes it extremely fast on x86 CPUs. A motion search generally involves a whole lot of SADs, and since the motion search is the core of any video encoder, making the SADs faster has a huge impact on performance.