Aliasing - A new perspective out of Box

Started by Bhanu Prakash Reddy July 16, 2003
"Craig" <crrea2@umkc.edu> schrieb im Newsbeitrag
news:82396605.0307160454.51e76aab@posting.google.com...
> "Glen Herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message
news:<Tr6Ra.74831$ye4.51585@sccrnsc01>...
> > "Bhanu Prakash Reddy" <itsbhanu@yahoo.com> wrote in
message
> > news:28192a4d.0307152153.355783d1@posting.google.com... > > > HI all, > > > > > > Thank you very much for the participants in " Negative
Frequencies"
> > > Thread. Here is another question from me. > > > > > > Yesterday I was watching a movie. In that, hero was chasing villain
in
> > > a BMW car at a speed of 100 Miles/Hr.I was observing the car keenly > > > and suddenly it appeared that the tyres of the car are rotating at > > > only 8 miles/hr(I didnt > > > measure..felt that its RPM is very less).The reason i feel is
because
> > > of Aliasing. My eyes are sampling the scene at less than the Nyquist > > > rate. Our CPU (Brain) samples through the sensor called Eye at a > > > particular rate..because it has to spare some time for other 4
sensors
> > > also( In some cases 5 sensors ;) > > > > It isn't because the brain is sampling, but because the movie or TV
camera
> > is sampling. > However, you can also observe the same thing when you stare at a > ceiling fan, or a car driving down the road with spokes wheels, etc. > So in that instance, it can be argued that your brain is "sampling".
I guess our Visual System is not a perfect low-pass without aliasing. As i think most of our nerves are operating in some kind of "pulse frequency modulation". Meaning: Information is not processed continuously "analog" but somehow "quantized". This includes the first nerve cells in the eye detecting light and continues with the nerve cells feed by them and the whole brain. Maybe the details of this are already worked out ... But something else is dangerous here: Our Consciousness ! As already discovered our Consciousness is only a very small fraction of our "Brain Power". Only a fraction of our "sensoric stimulation" gets into our consciousness already "symbolized" by our "cognitive filters" which are inherited/trained since we live. And also other things may play important roles here: - Mechanical resonances of our eyes and the related body. - Periodical Muscle movement of our eyes It's a very complex and fed back signal processing system ... Raymund Hofmann
On Fri, 18 Jul 2003 18:54:46 -0400, Jerry Avins <jya@ieee.org> wrote:

>Eric Jacobsen wrote: >> >> On Wed, 16 Jul 2003 21:32:28 -0400, Jerry Avins <jya@ieee.org>
wrote:
>> >> >Bhanu Prakash Reddy wrote: >> >> >> >> > >> >> > It isn't because the brain is sampling, but because the movie
or TV camera
>> >> > is sampling. >> >> > >> >> > Yes, it is aliasing just like is often described here. >> >> > >> >> > Also note that the aliased motion may appear to be reversed. >> >> > >> >> > -- glen >> >> >> >> Even if u see really (not in a movie) a fast moving vehicle, u
observe
>> >> the same situation with its tyres.In that case who is
sampling...brain
>> >> only right??? >> >> >> >> -bp$ >> > >> >No. You won't see the effect in uniform light. At night, reflections >> >from parts of the wheel when they assume specific ankles can act like
a
>> >synchronized stroboscope, making the wheel appear stationary. >> > >> >Jerry >> >> At night artificial lighting creates a shutter effect since the lights >> turn off 120 times a second with 60Hz power and 100 times a second >> with 50 Hz power. This creates a visual sampling system because you >> only get to sample the object when the lights are actually on. >> Although one doesn't perceive the flicker, one does perceive the >> aliasing effects that can happen. >> >> I've personally witnessed this visual aliasing many times, understood >> it, and therefore never gave it much more thought beyond initial >> interest. Once a few years ago while driving home in the daylight I >> observed the same visual aliasing in the wheel of the car in my side >> rear-view mirror. It took me a while to wrap my brain around that >> one since it was natural daylight illumination. I was beginning to >> worry that I was on "The Truman Show" for a while until I
realized
>> that the mechanical vibration in the rear-view mirror was producing >> the same shuttering effect achieved with AC electric artificial >> lighting at night. >> >> Eric Jacobsen >> Minister of Algorithms, Intel Corp. >> My opinions may not be Intel's opinions. >> http://www.ericjacobsen.org > >I see that too, but people claim to see the effect with illumination by >headlights! Then it's reflection at specific orientations. The >explanation I rejected was that the eye itself sampled. Tain't so. If it >were, we would observe beats. > >Jerry
Agreed, and the phenomena would be as repeatable in natural daylight as it is at night, which it isn't. Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
On Sat, 19 Jul 2003 12:30:56 +0200, "raymund hofmann"
<filter001@desinformation.de> wrote:

>> > It isn't because the brain is sampling, but because the movie or TV >camera >> > is sampling. >> However, you can also observe the same thing when you stare at a >> ceiling fan, or a car driving down the road with spokes wheels, etc. >> So in that instance, it can be argued that your brain is
"sampling". I'd join in disagreeing with this notion. I've yet to see an instance where this effect can't be otherwise explained. I've seen no evidence from this effect or others to indicate that the brain samples as revealed by any aliasing effects. ...
>And also other things may play important roles here: > >- Mechanical resonances of our eyes and the related body.
Not only resonance but the effects of external vibration sources. I think IEEE Spectrum or some related publication had a lot of discussion a few years ago about the well-known "Frito effect" which is experienced by examining an oscilloscope trace while munching on Fritos. The mechanical vibration transmitted from your teeth to your eye sockets will do all kinds of wacky things to your perception of the oscilloscope trace. Note: The young whipper-snappers in labs equipped with only the latest equipment may miss out on this since it won't work unless the scope trace is swept. Some of the new all-digital machines don't do this. An old boss of mine (the founder of the company, actually) used to tease new technicians when they had difficulty with lab measurements. He would very carefully explain to them with the usual air of authority and knowledge that they could sometimes improve their measurements by rapping the top of their head with their knuckles while observing oscilloscope traces (this works with quickly swept spectrum analyser traces as well). We'd always know if he'd been around if we saw a confused tech sitting at a bench bonking himself on the head while staring at a scope. The effect is the same as Frito chewing, it jitters your eyeballs and screws up your perception. I've always been amazed at how much deviation in the scope trace you can observe but the rest of your field of view is stable. Human vision must have some kickass image stabilization to do that, but the fact that you can see a continuous (but very wiggly and sometimes highly deviated) scope trace is further evidence to me that the brain doesn't sample. If it does it's at some incredibly unbelievably high rate, which I don't think is the case.
>- Periodical Muscle movement of our eyes > >It's a very complex and fed back signal processing system ... > >Raymund Hofmann > >
Eric Jacobsen Minister of Algorithms, Intel Corp. My opinions may not be Intel's opinions. http://www.ericjacobsen.org
Hello,
a bit late to comment on this thread, but still...

The aliasing phenomena observer with the human vision
is not only depending on the light sources.

It is well known that the brain operates at a few diffrent
frequencies, where (approx) 16 Hz is one of the major
fundamentals. 

Also as mentioned in an earlier post, the central area of the
visual field is optimised for detail but less for movements
while the outer part is optimised to catch movements.

so, the aliasing effect, with wheels spinning backwards
or standing still can be observed in fine sunlight.

-j
"Glen Herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message
news:<Tr6Ra.74831$ye4.51585@sccrnsc01>...
> "Bhanu Prakash Reddy" <itsbhanu@yahoo.com> wrote in message > news:28192a4d.0307152153.355783d1@posting.google.com... > > HI all, > > > > Thank you very much for the participants in " Negative
Frequencies"
> > Thread. Here is another question from me. > > > > Yesterday I was watching a movie. In that, hero was chasing villain in > > a BMW car at a speed of 100 Miles/Hr.I was observing the car keenly > > and suddenly it appeared that the tyres of the car are rotating at > > only 8 miles/hr(I didnt > > measure..felt that its RPM is very less).The reason i feel is because > > of Aliasing. My eyes are sampling the scene at less than the Nyquist > > rate. Our CPU (Brain) samples through the sensor called Eye at a > > particular rate..because it has to spare some time for other 4 sensors > > also( In some cases 5 sensors ;) > > It isn't because the brain is sampling, but because the movie or TV camera > is sampling.
However, you can also observe the same thing when you stare at a ceiling fan, or a car driving down the road with spokes wheels, etc. So in that instance, it can be argued that your brain is "sampling". Its always need to get a real world example of the problem you experience and can duplicate in Matlab, or on a DSP board, gives you a bit better understanding I think.
> > Yes, it is aliasing just like is often described here. > > Also note that the aliased motion may appear to be reversed. > > -- glen
> > It isn't because the brain is sampling, but because the movie or TV camera > is sampling. > > Yes, it is aliasing just like is often described here. > > Also note that the aliased motion may appear to be reversed. > > -- glen
Even if u see really (not in a movie) a fast moving vehicle, u observe the same situation with its tyres.In that case who is sampling...brain only right??? -bp$
"Bhanu Prakash Reddy" <itsbhanu@yahoo.com> wrote in message
news:28192a4d.0307152153.355783d1@posting.google.com...
> HI all, > > Thank you very much for the participants in " Negative Frequencies" > Thread. Here is another question from me. > > Yesterday I was watching a movie. In that, hero was chasing villain in > a BMW car at a speed of 100 Miles/Hr.I was observing the car keenly > and suddenly it appeared that the tyres of the car are rotating at > only 8 miles/hr(I didnt > measure..felt that its RPM is very less).The reason i feel is because > of Aliasing. My eyes are sampling the scene at less than the Nyquist > rate. Our CPU (Brain) samples through the sensor called Eye at a > particular rate..because it has to spare some time for other 4 sensors > also( In some cases 5 sensors ;)
It isn't because the brain is sampling, but because the movie or TV camera is sampling. Yes, it is aliasing just like is often described here. Also note that the aliased motion may appear to be reversed. -- glen
HI all,

Thank you very much for the  participants in " Negative Frequencies"
Thread. Here is another question from me.

Yesterday I was watching a movie. In that, hero was chasing villain in
a BMW car at a speed of 100 Miles/Hr.I was observing the car keenly
and suddenly it appeared that the tyres of the car are rotating at
only 8 miles/hr(I didnt
measure..felt that its RPM is very less).The reason i feel is because
of Aliasing. My eyes are sampling the scene at less than the Nyquist
rate. Our CPU (Brain) samples through the sensor called Eye at a
particular rate..because it has to spare some time for other 4 sensors
also( In some cases 5 sensors ;)

Does any one has got any other reason to say.....?

Now comes my doubt...some times it appeared to me that the tyres are
rotating in the opposite direction of
the car....Why is it so???


Best Regrads,
BP$
" Working with DIGITall Passion......"