 A video coding format does not dictate all algorithms used by a codec implementing the format.For example, a large part of how video compression typically works is by finding similarities between video frames (block-matching), and then achieving compression by copying previously-coded similar subimages (e.g., macroblocks) and adding small differences when necessary.Finding optimal combinations of such predictors and differences is an NP-hard problem, meaning that it is practically impossible to find an optimal solution.