Eyz-On
CVR
User Guide
Eyz-On
CVR
User Guide
44
21214
Video Compression Limitations and Artifacts
Introduction
The
Eyz-On
supports several types of video compression including JPEG,
MPEG4, and H.264. While the JPEG option stores video information as
individual images the MPEG4 and H.264 algorithms store several video frames
to make a complete video clip.
JPEG Mode
When the
Eyz-On
is in the JPEG mode images are stored as a single frame. In
this mode each file contains one and only one frame and each frame is
independent of all the other frames that are contained in other files. Since each
file is an individual frame, the video image in the frame can be saved,
transmitted or displayed as a single image.
When the
Eyz-On
detects that an event has happened and the
Eyz-On
is configured to record images it records the currently captured image
and a number of additional images. The number of images saved depends on
the setting for the event, but generally each image saved is an individual file and
the number of files saved is fairly consistent between events as long as the
setting for the event length is not altered.
MPEG4 & H.264 Mode
MPEG4 and H.264 algorithms contain a number of features that allow them to
compress video much better than JPEG. If a particular video sequence contains
30 JPEG images (and therefore 30 files) each compressed JPEG image
contains all the information necessary to reconstruct the image. But if the same
video sequence is recorded in MPEG4 or H.264 a very different result emerges.
The MPEG4 and H.264 file contains a series of frames labeled “I”, “B” and “P”.
The “I” frames are very similar to JPEG images in that they contain all the
information necessary to reconstruct the image.
The “B” and “P” frames contain
only differences in the image from the preceding “I” frame.
(Note: In this
discussion we
will only
refer to “B” frames.)
Therefore, an MPEG4 or H.264
video clip consisting of 30 frames would be a single file containing one
“I” frame
and 29 “B” frames. This file size would result in a tremendous savings over the
30 individual JPEG images, when a series of video images is involved.
Effects of I, B & P Frames
Although there is tremendous memory saving using MPEG4 or H.264 over
normal JPEG there are some unique differences when video clips are recorded.
This limitation is due to the fact that in order to reconstruct an image frame you
need to have at least one “I” frame in the sequence and the “I” frame must be
the first frame in the sequence. If the MPEG4 or H.264 algorithm only consisted
of “I” frames then the algorithm would in essence be Motion JPEG and the
resulting savings in file size would be lost. This limitation that the first field in the
MPEG4 or H.264 video clip be an “I” frame has some direct consequences in
the way that
Eyz-On
records video.
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