This setting specifies whether there will be two, one, or no B-frames between the reference
frames within a GOP. GOP structure, along with GOP size, determines the number of I-,
P-, and B-frames that will be used during transcoding.
The GOP structure you choose depends on how far apart P-frames should be spaced.
Since a P-frame is predicted from the previous reference frame (either an I-frame or a
P-frame), if there are one or two B-frames in between, the prediction must cover the
distance objects can move over the duration of two to three frames.
In principle, the less average motion there is from one frame to the next, the farther apart
P-frames can be spaced, and the greater the compression can be. For most video material,
the IBBP structure is a good choice. Material with unusually fast motion throughout the
entire sequence may benefit from an IBP or IP structure, but in such cases a relatively
high bit rate (6 to 8 Mbps for SD video) may be required for good quality.