In-loop Deblocking is a technique used in encoding and decoding MPEG-4 AVC video. Since video compression within a frame is done in blocks of pixels, there are often artifacts created due to adjacent blocks using different estimations of the values of pixels on the border of each. The difference between adjacent blocks is sometimes visible, resulting in this block structure becoming visible.
While older encoding technologies like MPEG-2 allow deblocking to occur during playback, AVC goes a step further and allows the encoder to make deblocking decisions, called in-loop deblocking, and pass information on these decisions to the decoder by saving them as part of the video. Not only does this make the deblocking itself more accurate, but it also improves with encoding quality. It also slows down video encoding and decoding speed, and like certain other advanced features of AVC, like CABAC, may cause problems playing AVC video on older computers.