Editing MPEG-1 (section)

====I-frames====
"I-frame" is an abbreviation for "[[wikt:I-frame|Intra-frame]]", so-called because they can be decoded independently of any other frames. They may also be known as I-pictures, or keyframes due to their somewhat similar function to the [[key frame]]s used in animation. I-frames can be considered effectively identical to baseline [[JPEG]] images.<ref name=bmrc_mpeg2_faq/>

High-speed seeking through an MPEG-1 video is only possible to the nearest I-frame. When cutting a video it is not possible to start playback of a segment of video before the first I-frame in the segment (at least not without computationally intensive re-encoding). For this reason, I-frame-only MPEG videos are used in editing applications.

I-frame only compression is very fast, but produces very large file sizes: a factor of 3&times; (or more) larger than normally encoded MPEG-1 video, depending on how temporally complex a specific video is.<ref name=Didier_MPEG/> I-frame only MPEG-1 video is very similar to [[MJPEG]] video. So much so that very high-speed and theoretically lossless (in reality, there are rounding errors) conversion can be made from one format to the other, provided a couple of restrictions (color space and quantization matrix) are followed in the creation of the bitstream.<ref name=smith_transcoding>{{Citation |first1=Soam |last1=Acharya |first2=Brian |last2=Smith |title=Compressed Domain Transcoding of MPEG |pages=3 |year=1998 |publisher=[[Cornell University]], [[IEEE Computer Society]], [[IEEE]] International Conference on Multimedia Computing and Systems |url=http://citeseer.ist.psu.edu/acharya98compressed.html |access-date=2016-11-11 |url-status=live |archive-url=http://archive.wikiwix.com/cache/20110223164151/http://citeseer.ist.psu.edu/acharya98compressed.html |archive-date=2011-02-23 }} – (Requires clever reading: says quantization matrices differ, but those are just defaults, and selectable){{registration required|s}}</ref>

The length between I-frames is known as the [[group of pictures]] (GOP) size. MPEG-1 most commonly uses a GOP size of 15–18. i.e. 1 I-frame for every 14-17 non-I-frames (some combination of P- and B- frames). With more intelligent encoders, GOP size is dynamically chosen, up to some pre-selected maximum limit.<ref name=bmrc_mpeg2_faq/>

Limits are placed on the maximum number of frames between I-frames due to decoding complexing, decoder buffer size, recovery time after data errors, seeking ability, and accumulation of IDCT errors in low-precision implementations most common in hardware decoders (See: [[IEEE]]-1180).