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From: whb@castle.ed.ac.uk (H Bruce)
Newsgroups: comp.graphics
Subject: Inter-frame differencing under vary lighting conditions.
Message-ID: <9444@castle.ed.ac.uk>
Date: 3 Apr 91 13:43:37 GMT
Distribution: comp
Organization: Dept. Electrical Engineering, Edinburgh University, Scotland, UK.
Lines: 25

I am compressing a 4 frame sequence of grey scale images 1 second apart by
sending the first frame then subsequent frame differences.

This works fine but if the lighting changes then the grey scale values
change enough to signal a pixel difference when one has not occured.
This can be overcome by calculating the mean (m0) and standard deviation (sd0) 
of the first image and applying them to the "normalised" pixel values of the
subsequent images.

ie    p = (p-m)/sd * sd0 + m0

However if on object moves and changes size between frames the mean and
standard deviation can change substantially so that two subsequent 
"normalised" images still have different illumination levels.

To overcome this I can only guess that a frame difference is necessary
before the normalisation so that parts of the image that vary are not 
included in the statistical calculations.

Is this a sensible approach or can anyone recommend and alternative algorithm
to overcome this problem ?

Thanks,

Henry Bruce.