Editing MPEG-1 (section)

===Frame/picture/block types===
MPEG-1 has several frame/picture types that serve different purposes. The most important, yet simplest, is '''I-frame'''.

====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).

====P-frames====
"P-frame" is an abbreviation for "Predicted-frame". They may also be called forward-predicted frames or [[wikt:inter-|inter-]]frames (B-frames are also inter-frames).

P-frames exist to improve compression by exploiting the [[wikt:temporal|temporal]] (over time) [[wikt:redundancy|redundancy]] in a video. P-frames store only the ''difference'' in image from the frame (either an I-frame or P-frame) immediately preceding it (this reference frame is also called the ''[[wikt:anchor|anchor]] frame'').

The difference between a P-frame and its anchor frame is calculated using ''motion vectors'' on each ''macroblock'' of the frame (see below). Such motion vector data will be embedded in the P-frame for use by the decoder.

A P-frame can contain any number of intra-coded blocks (DCT and Quantized), in addition to any forward-predicted blocks (Motion Vectors).<ref name=hp_transcoding/>

If a video drastically changes from one frame to the next (such as a [[cut (transition)|cut]]), it is more efficient to encode it as an I-frame.

====B-frames====
"B-frame" stands for "bidirectional-frame" or "bipredictive frame". They may also be known as backwards-predicted frames or B-pictures. B-frames are quite similar to P-frames, except they can make predictions using both the previous and future frames (i.e. two anchor frames).

It is therefore necessary for the player to first decode the next I- or P- anchor frame sequentially after the B-frame, before the B-frame can be decoded and displayed. This means decoding B-frames requires larger [[data buffer]]s and causes an increased delay on both decoding and during encoding. This also necessitates the decoding time stamps (DTS) feature in the container/system stream (see above). As such, B-frames have long been subject of much controversy, they are often avoided in videos, and are sometimes not fully supported by hardware decoders.

No other frames are predicted from a B-frame. Because of this, a very low bitrate B-frame can be inserted, where needed, to help control the bitrate. If this was done with a P-frame, future P-frames would be predicted from it and would lower the quality of the entire sequence. However, similarly, the future P-frame must still encode all the changes between it and the previous I- or P- anchor frame. B-frames can also be beneficial in videos where the background behind an object is being revealed over several frames, or in fading transitions, such as scene changes.<ref name=Didier_MPEG/><ref name=bmrc_mpeg2_faq/>

A B-frame can contain any number of intra-coded blocks and forward-predicted blocks, in addition to backwards-predicted, or bidirectionally predicted blocks.<ref name=bmrc_mpeg2_faq/><ref name=hp_transcoding/>

====D-frames====
MPEG-1 has a unique frame type not found in later video standards. "D-frames" or DC-pictures are independently coded images (intra-frames) that have been encoded using DC transform coefficients only (AC coefficients are removed when encoding D-frames&mdash;see DCT below) and hence are very low quality. D-frames are never referenced by I-, P- or B- frames. D-frames are only used for fast previews of video, for instance when seeking through a video at high speed.<ref name=Didier_MPEG/>

Given moderately higher-performance decoding equipment, fast preview can be accomplished by decoding I-frames instead of D-frames. This provides higher quality previews, since I-frames contain AC coefficients as well as DC coefficients. If the encoder can assume that rapid I-frame decoding capability is available in decoders, it can save bits by not sending D-frames (thus improving compression of the video content). For this reason, D-frames are seldom actually used in MPEG-1 video encoding, and the D-frame feature has not been included in any later video coding standards.