Editing MPEG-1 (section)

===Motion vectors===
To decrease the amount of temporal redundancy in a video, only blocks that change are updated, (up to the maximum GOP size). This is known as conditional replenishment. However, this is not very effective by itself. Movement of the objects, and/or the camera may result in large portions of the frame needing to be updated, even though only the position of the previously encoded objects has changed. Through motion estimation, the encoder can compensate for this movement and remove a large amount of redundant information.

The encoder compares the current frame with adjacent parts of the video from the anchor frame (previous I- or P- frame) in a diamond pattern, up to a (encoder-specific) predefined [[radius]] limit from the area of the current macroblock. If a match is found, only the direction and distance (i.e. the [[wikt:vector|''vector'']] of the ''motion'') from the previous video area to the current macroblock need to be encoded into the inter-frame (P- or B- frame). The reverse of this process, performed by the decoder to reconstruct the picture, is called [[motion compensation]].

A predicted macroblock rarely matches the current picture perfectly, however. The differences between the estimated matching area, and the real frame/macroblock is called the prediction error. The larger the amount of prediction error, the more data must be additionally encoded in the frame. For efficient video compression, it is very important that the encoder is capable of effectively and precisely performing motion estimation.

Motion vectors record the ''distance'' between two areas on screen based on the number of pixels (also called pels). MPEG-1 video uses a motion vector (MV) precision of one half of one pixel, or half-pel. The finer the precision of the MVs, the more accurate the match is likely to be, and the more efficient the compression. There are trade-offs to higher precision, however. Finer MV precision results in using a larger amount of data to represent the MV, as larger numbers must be stored in the frame for every single MV, increased coding complexity as increasing levels of interpolation on the macroblock are required for both the encoder and decoder, and [[wikt:law of diminishing returns|diminishing returns]] (minimal gains) with higher precision MVs. Half-pel precision was chosen as the ideal trade-off for that point in time. (See: [[qpel]])

Because neighboring macroblocks are likely to have very similar motion vectors, this redundant information can be compressed quite effectively by being stored [[Pulse-code modulation|DPCM]]-encoded. Only the (smaller) amount of difference between the MVs for each macroblock needs to be stored in the final bitstream.

P-frames have one motion vector per macroblock, relative to the previous anchor frame. B-frames, however, can use two motion vectors; one from the previous anchor frame, and one from the future anchor frame.<ref name=hp_transcoding>{{Citation |first1=Susie J. |last1=Wee |first2=Bhaskaran |last2=Vasudev |first3=Sam |last3=Liu |title=Transcoding MPEG Video Streams in the Compressed Domain |date=March 13, 1997 |publisher=[[Hewlett-Packard]] |url=http://www.hpl.hp.com/personal/Susie_Wee/PAPERS/hpidc97/hpidc97.html |access-date=2016-11-11 |archive-url=https://web.archive.org/web/20070817191927/http://www.hpl.hp.com/personal/Susie_Wee/PAPERS/hpidc97/hpidc97.html |archive-date=2007-08-17|citeseerx=10.1.1.24.633 }}</ref>

Partial macroblocks, and black borders/bars encoded into the video that do not fall exactly on a macroblock boundary, cause havoc with motion prediction. The block padding/border information prevents the macroblock from closely matching with any other area of the video, and so, significantly larger prediction error information must be encoded for every one of the several dozen partial macroblocks along the screen border. DCT encoding and quantization (see below) also isn't nearly as effective when there is large/sharp picture contrast in a block.

An even more serious problem exists with macroblocks that contain significant, random, ''edge noise'', where the picture transitions to (typically) black. All the above problems also apply to edge noise. In addition, the added randomness is simply impossible to compress significantly. All of these effects will lower the quality (or increase the bitrate) of the video substantially.