Re: POTC Victory Jig video

There's also another common effect in dealing with a digital camera
recording a raster display (you'll often see this on the evening news since
they use video cameras)-- if the refresh rates are similar, one will often
draw part or all of the picture while the other is scanning back to the top
of the screen, resulting in a rolling or partially blacked-out display.

Ignus

"TheKorn" <TheKorn@xxxxxxxxxxx> wrote in message
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"Andy" <me@xxxxxxxxxxxxxxxxxxxx> wrote in
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Something to do with the video camera not able to sync to the DMD.
Same thing happens when you try to take a picture of a tv or arcade
monitor. I am sure Korn can explain it more eloquently. :)

I don't know if I can do it more eloquently, but I can certainly attempt
to use more words! :)

I'm not entirely sure how if it works exactly the same way with a DMD,
but it behaves almost identically to a TV, so let's pretend the DVD is a
(tube) TV for a moment...

What's really going on is that the entire screen isn't actually being lit
at any one point in time. In the case of a TV, there's exactly ONE line
being lit, and that line is moving down the screen really really quickly.
Human eyes aren't the fastest at responding to differences in light
color, so the eye itself has a natural 'carry over' effect.**

In addition (at least on a TV; not sure if this applies to a DMD or not),
the display itself also has a non-instant response time. I.E. if you
light a dot and then immediately extinguish it, it will keep glowing for
X hundredths of a second afterwards.

SOOooo, if you combine the displays natural carry-over with the human
eye's natural slowness to react to changes in light, you get a nice and
stable looking display.



Of course, a camera (and your family dog) sees things quite differently.
Both have a much faster response time to changes in light intensity. So
they both see (more or less) a really flickering half-picture with a
constantly moving bright spot.

In the case of the camera, if you increase the exposure time you can
usually get good enough results to show to friends, but it will not be
broadcast quality by any means. (That's what I did on the original POTC
video.) Too slow and all the images will blur together; not slow enough
and you'll get a hot stripe.

In the case of your dog, stop asking it to watch TV. :)





** This effect is not constant across the entire viewable area of the
eye. It's tied to the number of light/dark sensing receptors (I forget;
are those rods or cones?) in a particular area. In humans, that means
that the immediate center of vision is actually the *slowest* to respond
to light/dark changes. The peripheral vision is far faster at detecting
light changes. I've never tried it with a DMD, but with a 60 hz scanning
monitor if I look at it out of my peripheral vision I can see it
flickering.


--
Have a home video that's trapped on your camera? Want to share it on the
web or on DVD?

http://www.webwidevideo.com/


.

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