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From: mrd@ecs.soton.ac.uk (Mark Dobie)
Newsgroups: comp.graphics
Subject: Re: Inter-frame differencing under vary lighting conditions.
Message-ID: <7373@ecs.soton.ac.uk>
Date: 5 Apr 91 13:46:22 GMT
References: <9444@castle.ed.ac.uk>
Distribution: comp
Organization: University of Southampton, UK
Lines: 37

In <9444@castle.ed.ac.uk> whb@castle.ed.ac.uk (H Bruce) writes:

>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.

Try reading

"Illumination Independent Change Detection for Real World Image
Sequences" by Kurt Skifstad and Ramesh Jain

in Computer Vision, Graphics and Image Processing, vol 46, pp 387-399
(1989)

They discuss several methods with the particular aim of being robust
against changes in illumination of the scene. They describe the
normalisation technique you use applied to regions in the images. They
continue with a method derived from a graphics shading model, which
proves the overall best performer.

				Mark.

-- 

Mark Dobie                              M.Dobie@uk.ac.soton.ecs (JANET)
University of Southampton		M.Dobie@ecs.soton.ac.uk (Bitnet)