User Guide

310 Appendix E, Digital Compression
• P-pictures (Predicted pictures), coded with respect to the nearest previous I- or P-
pictures. This technique is called forward prediction and is illustrated in Figure 7.
Predicted pictures provide more compression and serve as a reference for B-pictures and
future P-pictures.
• B-pictures (Bidirectional pictures), use both past and future pictures as a reference. This
technique is called bidirectional prediction and is illustrated in Figure 7.
Group of Pictures A group of pictures (or GOP) typically has 15 pictures in the following
order: I-B-B-P-B-B-P-B-B-P-B-B-P-B-B-I. 15 represents the interval at which I-pictures repeat.
When encoding video, you can usually adjust the size of GOPs depending on storage
requirements, thereby affecting the placement of I-pictures and the location of scene cuts.
Figure 7: Forward and bidirectional prediction
1234567
1342675
FORWARD
PREDICTION
BIDIRECTIONAL
PREDICTION
INTRA PICTURE
PREDICTED PICTURE
BIDIRECTIONAL PICTURE
PICTURE DISPLAY
ORDER
PICTURE TRANSMIT
ORDER
I BBPBBPB PBBPPBBBPBBPBBIBBPBBBB
1 30292827262524232221201918171615141312111098765432
2 B-pictures between
reference pictures
I-picture every 15th
frame (1/2 second)
1 Second
Type of picture:
Frame number: