Surveillance Suitable Compression

Without deploying a tape-type recorder, a hard drive or a DVD recorder on helicopters or in other demanding environments and missions, then video compression onto a solid-state medium is unavoidable. But what compression scheme is most suitable?

Laserdyne has chosen the MJPEG2000 scheme because it applies the same encoding to every frame.

By encoding every frame, MJPEG2000 compression treats video as a sequence of still frames. It also encodes each frame as a whole, not as a series of tiles or blocks, as is the case with MPEG 1, 2 & 4 encoding schemes.  In these schemes each block is separately compressed, so that detail at block boundaries may be masked.

These two attributes of MJPEG2000 are ideally suited to surveillance applications, where individual frames or stills, may need to be viewed to recover information like licence plates or faces.

Although higher compression ratios are achievable with MPEG based schemes, these schemes are not well suited to surveillance applications. They have been optimized for ‘motion’ video where certain types of spatial and temporal distortion are tolerable, even imperceptible, to human vision – ideal for watching movies.

In the case of a moving camera in mobile surveillance, the fidelity of each frame, when viewed as a still, can be compromised. This does not happen with the MJPEG2000 scheme.

Sample video still frame, with MPEG2 encoding	Zoomed detail from sample with MPEG2 encoding: facial detail compromised to the extent that even gender is difficult to determine
Sample video still frame, with MJPEG2000 encoding	Zoomed detail from sample with MJPEG2000 encoding: facial detail better preserved

These examples contrast the potential differences between MPEG2-I and MJPEG2000 encoding.

In these examples, both codecs were configured to operate at a rate of 0.8MB/s for PAL video. One one side they were compressed using the MJPEG2000 codec, on the other using an i-frame only MPEG2 codec.

By using MJPEG2000 to encode each frame as a still, without any block-based compression artifacts, the fidelity of each image is not affected by camera motion nor by the location of the frame within the video sequence – both of which are potential problems with MPEG based schemes for this application.

In summary: MPEG 1, 2 & 4 compression schemes are good for motion pictures, while the MJPEG2000 compression scheme is good for all-frame fidelity.