Other frames from the group are coded with use of the reference to the previous frame. Such frames may include the elements coded independently from all other frames similar to JPEG, and the elements that are coded better if they have been motion compensated. Compression jargon typically calls the elements of the image 'macroblocks'. Speaking briefly, MPEG- 2, H.261 and H.263 standards differ from each other by frame resolution, color modes, frame rate, number of frames involved in motion compensation, the accuracy and maximum length of motion vectors and some other specific parameters. It should be noted that implementing all modes of the standard in software-only based codec is a very difficult task.
Motion compensation requires a huge number of arithmetic operations.
For instance, for a video with QSIF resolution and frame rate of 5 frames per second the standard algorithm of motion vector search in the area pixels needs about arithmetic operations per second, which overcomes the performance of Pentium-II 400 MHz processor. For this reason simplest video codecs either do not use motion compensation at all, or use it in a very restricted pixel area, or implement an algorithm which does not guarantee the best minimization of difference signal. For a given bit rate this results in the loss of image quality, the appearance of typical image distortions like trails after moving objects, strong blocking and smoothing effects. The present codec use the complete motion compensation capacity of the standard. Next, the video quality depends substantially on many other coding parameters such as quantizer scales, macroblock types decision, structure of pictures group etc.