Open or Closed GOPs
An open GOP allows the B-frames from one GOP to look at an I- or P-frame from an
adjacent GOP. Open GOPs are more efficient, but cannot be used with multi-angle or
mixed-angle titles. A closed GOP keeps all references within itself, standing alone with
respect to other GOPs. DVD Studio Pro works with either type in single-angle titles and
requires closed GOPs with multi-angle and mixed-angle titles.
It is important to know that the same pattern can provide different results when used
with an open or closed GOP. For example, the integrated MPEG encoder uses a closed
GOP type, and it starts its IBBP pattern with an I-frame. Other encoders using an open
GOP may start with a B-frame. In this case, starting with a B-frame is a little more efficient;
if you start with an I-frame, an extra P-frame must be added to the end (a GOP cannot
end with a B-frame).