It's an interlaced frame and the motion

estimation compensation is done at the frame level.

In the field prediction mode we have an example here

where this field could be a top field or a

bottom field, and can be predicted from either the top

field or the bottom field of the previous frame in time.

While in this example, we're looking at the bottom field of

this particular frame at time t, let's say, which can be predicted

from the first field of the same frame, the same time instance,

or the bottom field of the previous field, times t minus one.

So in this case, the distance here is one 30th of a

second, while the distance between these two fields is one60th of a second.

Another motion compensation mode provided in MPEG-2 is the dual prime mode.

This mode is applicable only to P pictures and to GOPs that

have no B pictures between these P pictures and the reference frames.

So for frame pictures, two motion vectors are associated with each field.

So for this top field here, the motion vector v1 and

pv1 are associated with this, this dotted line.

If we, if we denote by pv1 X, the horizontal component

of this vector, then pv1 X is encoded in terms of v1x.

And, as you can see here, V1x connects this

top fields which are one 30th of a second apart.

While pv1 corrects the bottom field of this frame and the top

field of this one, which are one 60th of a frame apart.