Re: CCTV/PC's

[Nigel Orr <Nigel.Orr@xxxxxxx>]

At 10:07 07/05/00 +0100, you wrote:
>Sounds good- what res & colour depth are you using?

Just 8-bit, monochrome.  I suspect mono makes it much more straightforward,
there's probably no significant advantage in using colour for detection.

>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!

I used the drivers that came with an Imagenation CT100 card- the cards are
a bit pricey (it was purchased for another use, the motion detection and
recording system got chucked together in a couple of lunchtimes to meet a
pressing need...)

>How about doing the same as PIRs do- set up a mask image to define
>'trigger zones' and loose the rest of the image.

I've seen a system on the web that does just that.  I can't remember where
I saw it, but the screenshot had a picture of someone climbing over a wall,
with various rectangles drawn, showing the sensitive areas.  Obviously, you
can choose those to be areas where the light changes are consistent.

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

If the burglar stays stationary all the time, that's fine by me :-)  But
you're right, any motion detection system will miss sufficiently small
movements.  I suspect that none but the most professional burglars could
move slowly enough to avoid triggering most systems inside, but when you
have to cope with outside changes, it might be easier to defeat.

>>sensor, so you tend to get pictures of the sides of people's
faces...
>What an excellent point! That sounds like bitter experience.

Happily not bitter- I just thought about it when I saw some PIRs with
built-in cameras in a catalogue- seems like a good idea at first... same
for the smoke detector ones, I guess, the detector should be on the
ceiling, either so it works properly, or so it doesn't look out of place,
so you get piccies of the tops of people's heads as they walk under, and
only see their distorted faces at the very edges of the picture, if it's a
fish-eye lens...

Happily, most PIRs do detect movement towards them _reasonably_ well,
though not as well as movement across.

Nigel


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