How Encoders Work - A Practical Guide to Video and Audio Compression - page 191

I-frames can be placed into the bit stream where there is a major change in the pic-
ture content, for example, at a scene change or editing cut point. Nonlinear editors such as
Final Cut Pro introduce some hints when an edit or transition is placed into the finished
product. The compression software may be able to make use of this help in choosing when
to create a new I-frame.

9.5.2 Difference Frames (P-Frames)

A predicted frame ( P-frame ) is constructed using differences that are added to a key frame
or the most recent P-frame and are not necessarily delivered immediately after the I-frame
it is reconstructed from. The differences are assumed to be in the forward temporal direc-
tion as described in the earlier conceptual overview. Because P-frames cascade (one is built
on another), any coding errors will propagate and eventually the accumulation causes
visible artifacts.
Now take that movie film and compare two adjacent frames. Let's call them Frames
A and B. To view Frame A, it is projected into the viewer. Frame B, however, does not exist
as a viewable frame. It is a set of changes that must be applied to Frame A in order to
reconstruct Frame B. You cannot simply view Frame B. If there are any visible pixels in the
description of it, they will only cover a portion of the view screen. They must be drawn on
top of the contents of Frame A to be meaningful.
Frame B is a difference or delta frame. Add two more frames, C and D. They can also
be difference frames. The description of C is a set of visible differences between pictures
B and C, rather like those “spot the difference” puzzles in the newspapers. The descrip-
tion of Frame D is the visible differences that must be applied to make Frame D from a
fully reconstructed Frame C.
Figure 9-2 shows how this stacks up.

1 Mbyte 1 Mbyte 1 Mbyte 1 Mbyte 1 Mbyte

Sourceframe
1

Sourceframe
2

Sourceframe
3

Sourceframe
4

Sourceframe
5

Diff Diff

Diff

Diff

I - Frame P - Frame P - Frame P - Frame P - Frame

40 Kbyte 5 Kbyte 5 Kbyte 5 Kbyte 5 Kbyte

Figure 9-2 Difference frames.