X6

MPEG glossary 373
In a closed GOP, B frames of the last subgroup may contain only backward
predictions or references to the preceding P frame, but no references to the following I
frame, since it belongs to the next GOP.
I frames
Intra-frames: In these pictures, the entire image information of a frame is saved and
only information from this frame is used ("intra-frame encoded"). In contrast to the I
frame, P and B frames save only the differences between the current frame, and
preceding and/or following frame are also found in MPEG video (P frame = "predicted
frame", B frame = "bidirectional frame", see Prediction (view page 374)).
Interlace
For historical reasons, pictures in a movie are always recorded and transmitted in the
form of two fields, first the lines with even numbers and then those with odd
numbers. These fields are alternatively displayed with a double-frame rate. The (lazy)
eye of the viewer or the processing of the TV tube puts the two frames together to
form one.
The output image First field Second field
You normally don’t have to worry about field processing. The video material goes
through the entire processing chain as fields and is exported again as fields or burned
onto DVD or shown on TV when played back on a DVD as a full picture. Only in
certain rare conditions is it necessary to delve deeper into this process. Two problems
can occur:
Interlace artifacts
To be displayed on a computer monitor the two fields must be combined to form a
full screen.
These two fields are not the same, since two fields are created during the recording
(between which a 1/50 of a second gap is evident). Moving objects can therefore
produce artifacts at the vertical edges.