Re: CCTV/PC's

[James Derrick <james@xxxxxxx>]

On Wed, 03 May 2000 09:06:02 +0100, you wrote:

>At 22:58 02/05/00 +0100, you wrote:
>Inside, I've had a reliable system working by simply subtracting
>one frame from the next, and setting a threshold for the sum of the
>absolute pixel differences (you can then also display the 'difference'
>image, to see what has caused the triggering).
Sounds good- what res & colour depth are you using?

The Linux video4linux drivers and ImageMagic libraries seem to give
you a lot for free to write this kind of thing, although this
afternoon is already booked!

>It's trickier outside, with different areas continuously brightening
and
>darkening at different rates due to shadows and reflections.
Very true- with a typical day's images and xamin to aminate them one
after the other the major change is indeed shadows tracking the moving
sun and clouds.

>You really
>need to do proper movement detection then, which is where
mathematicians
>wake up and get to work and us engineers doze off at the sight of large
>matrices and funny symbols <g> .
How about doing the same as PIRs do- set up a mask image to define
'trigger zones' and loose the rest of the image. Then you could only
trigger if two adjacent zones detect movement. This software 'double
knock' can then be tweaked to remove tracking sun shadows and trigger
more on, say a path facing the camera.

Taking pairs of images seems to remove most of the shadow difference,
but is of course no use if the burgular is stationary.

I did some of the squiggly symbols at Uni, but happily have managed to
forget the full horror over the last few years. Image processing text
books were quite good as they had a lot of pictures in them, although
most were of tanks taken from above with different feature extraction
methods applied to the image.

Can't think why tanks were so photogenic... ;-)

>The PIRs with built in cameras aren't
>really the answer, as the PIR is most sensitive to movement across the
>sensor, so you tend to get pictures of the sides of people's faces...
What an excellent point! That sounds like bitter experience.

James
---
James Derrick    james@xxxxxxx, Cramlington, Near Newcastle, England
Forwarding Service: jderrick@xxxxxxx
Beyond the Horizon of the place we lived when we were young,
In a World of Magnets and Miracles. Pink Floyd.

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