WP22 STREAMING MEDIA SPOTLIGHT SERIES JUNE 2017 SPONSORED CONTENT
How Does Video Compression Work?
To reduce video size, encoders implement different
types of compression schemes. Intra-frame compression
algorithms reduce the file size of keyframes (also called
Intra-coded frames, or I-frames) using techniques similar to
standard digital image compression. Other codec algorithms
apply inter-frame compression schemes to intelligently
process the frames between keyframes. Looking at the
example in Figure 2, you’ll see three frames of video showing
a bicyclist riding past a tree. The first frame on the left is a
keyframe encoded using intra-frame compression. You’ll
notice that the tree and background don’t change in the next
t wo frames, so the video encoder doesn’t reprocess that part
of the image; it just processes and stores data representing
only the changed parts of the image—the bicyclist, in
this case—in those successive frames of the shot. By not
processing the entire shot in every frame, the encoder is able
to reduce the file size, and therefore the amount of data that
has to be decoded on the playback device. AWS Elemental
has its own algorithms that it uses to analyze I-frames and
the differences in subsequent frames, storing information on
the changes between these frames in Predicted frames (or
P-frames), processing only the parts of the image that change.
There are also Bi-directional predicted frames (or B-frames),
which can look backward and for ward at other frames and
store the changes (see Figure 3).
Figure 2. In this image, the frame on the left is a keyframe (also called an I-frame). Encoders like those from AWS Elemental analyze what will change in subsequent
frames (called delta frames, Predicted frames or P-frames), and only store the data for the parts of the image that change from frame to frame.
Figure 3. I-frames serve as the reference point for the beginning of a new shot, P-frames process the changes in the shot, and B-frames look forward and backward
for what’s changed between frames.