Aliasing - A new perspective out of Box

"Dirk Bell" <dirkman@erols.com> wrote in message news:<bf455r$jb3$1@bob.news.rcn.net>...
> "Craig" <crrea2@umkc.edu> wrote in message
> news:82396605.0307160454.51e76aab@posting.google.com...
> 
> <snipped>
> 
> > 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.
> >
> <snipped>
> 
> Craig et al,
> 
> I think this raises a really interesting topic. The eyes act as a lowpass
> filter.  IIRC blinking lights faster than about 30 Hz rep rate appear
> constant if the light is stationary with respect to the eye. I think that
> there are phenomena other than the eye "sampling" to explain what you are
> seeing.
I had a discussion about this with a professor when I was doing my
undergrad, the problem with testing the theory, is that the phenomena
happens at different times for different ppl, and it only seems to
occur at various angles, being off center of a ceiling fan by around
30 degrees, I cannot get it to occur (this is just with normal
lighting in the room, not the light present with the lamp) However,
the closer my eyes get from looking straight up at it, then the
"aliasing" occurs. Since I am not an expert on how the eye
specifically works, I would be making speculations based on my
observations.  However, from my personal observations, I don't think
the eye has so much to do with it.  I would argue that the possibility
that the fan is refracting light and that it is actualling doing the
"sampling" since it is rotating and my eye is just a off station
observer.  I did notice that if I look at the fan at night, while
laying in bed with only the light from a TV in an different room, or
from a single source, the effect is not as great, but as I increase
the light source and adjust my angle relavent to the fan I begin to
see more and more of the effects.

> With things that are rotating some of the explanations have to do with
> periodic speed variations and lighting conditions. A fan for example may be
> illuminated by fluorescent lights which do not provide constant
> illumination, but effectively pulse with twice the powerline frequency, so
> there is a sampling of sorts provided by the light.
Now, I also noticed that flickering lights dramatically increases the
effects seen from looking up, like several candles in the room, or the
TV as it flashes different light intensities during a program.
> 
> I have noticed that helicopter blades in sunlight do not appear as a
> constant blur but appear as a combination of a blur and a imprecisely timed
> strobed effect.  I have an explanation but I would be curious to hear others
> comments on this and the other phenomena mentioned.
> 
> I think that the effects when compared to the wheel turning in a movie are
> similar, but do not have the clean "sampling" result.  This probably needs
> to be considered in discussions.

It would be interesting to work out mathematically what we are
observing, and determine whether or we are seeing an "aliased signal"
or whether we are seeing a some form of optical illusion caused by
light defraction.
Who knows, I haven't really thought about this in years, except during
my first DSP class and first discussions about aliasing... who knows,
I think I may look at this a little more, see if we can come up with a
sound theory on this post.  Great topic.

-Craig
> 
> Dirk
> 
> Dirk A. Bell
> DSP Consultant

Bhanu Prakash Reddy wrote:
> 
> 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......"

Aliasing. You have just witnessed an effect of aliasing. When a wheel
spoke moves almost the distance between spokes in the time of one frame,
the wheel appears to move backwards. But you already knew that, didn't
you? The title shows it and you asked in a DSP group.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

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
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

[...]
> Also note that the aliased motion may appear to be reversed.
> 
which is a great illustration on the usefulness of negative frequency
concepts by the way... :o)

Robert

I swear that someone must be making hubs that free rotate from the
wheel.  Driving down I-95 a couple of months ago we passed a BMW or
Mercedes at about 70 MPH and both of us in the car swore that the hubs
were completely still or slightly moving backwards, so much so that we
slowed down, let the car come alongside again and stared for minutes. 
We were convinced that the hubs were free rotating from the wheel.

It could have been just the best example of this illusion I've ever
seen, but anytime before if I changed viewing location a bit, the effect
changed.  In this case it stayed the same as we changed viewing angle.

Anyone know if someone makes wheels with free rotating hubs or was this
just a REALLY good optical illusion??

We conjenctured that someone could make them with a little controllable
motor that could control the spin rate and direction.


Bhanu Prakash Reddy wrote:
> 
> 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......"

> I swear that someone must be making hubs that free rotate from the
> wheel.

Hi Ron,

I've seen this too; it's a new fancy-schmancy kind of free-spinning hubcap
accessory thingy. It's really weird to see a car pull up to a stop light,
come to a halt, then to see the "wheels" keep spinning.

Rick Armstrong
(reply address is bogus)

Ron Huizen wrote:
> I swear that someone must be making hubs that free rotate from the
> wheel.  Driving down I-95 a couple of months ago we passed a BMW or
> Mercedes at about 70 MPH and both of us in the car swore that the hubs
> were completely still or slightly moving backwards, so much so that we
> slowed down, let the car come alongside again and stared for minutes. 
> We were convinced that the hubs were free rotating from the wheel.
> 
> It could have been just the best example of this illusion I've ever
> seen, but anytime before if I changed viewing location a bit, the effect
> changed.  In this case it stayed the same as we changed viewing angle.
> 
> Anyone know if someone makes wheels with free rotating hubs or was this
> just a REALLY good optical illusion??
> 
> We conjenctured that someone could make them with a little controllable
> motor that could control the spin rate and direction.
> 
> 
snip

you can get free rotating hubs, as fas as I can tell it's basically an
extra hub plate with a bearing so, when you driving the look stationary
and when you stop they keep rotating for while.

I think they are quite expensive, but the buyers are probably the type
that spends more money on spoilers, stereo, blue neon light and such
than they do on the car .....

another explanation would be that there was flourecent lights on the 
street, but I'm not sure the effect is as good

-Lasse

Robert Malek wrote:
> 
> [...]
> > Also note that the aliased motion may appear to be reversed.
> >
> which is a great illustration on the usefulness of negative frequency
> concepts by the way... :o)
> 
> Robert

Oh? Convince me.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Paavo Jumppanen wrote:
> 
  ...

> As for why ti appears to be moving backwards, that depends upon the
> proximity of the rotational speed to the frames per second. If
> identical or harmonically related the tyre will appear stationary. If
> the rotational speed is slower it will appear to be rotation in the
> right direction but at a speed that is the difference between the
> frame rate and the rotational speed. If the rotational speed is
> slightly faster it will appear to be rotating backwards with speed
> that is the difference between the rotational speed and the frame
> rate.
> 
Off hand, I think that's the opposite of what happens.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������

Ron Huizen wrote:
> 
> I swear that someone must be making hubs that free rotate from the
> wheel.  Driving down I-95 a couple of months ago we passed a BMW or
> Mercedes at about 70 MPH and both of us in the car swore that the hubs
> were completely still or slightly moving backwards, so much so that we
> slowed down, let the car come alongside again and stared for minutes.
> We were convinced that the hubs were free rotating from the wheel.
> 
> It could have been just the best example of this illusion I've ever
> seen, but anytime before if I changed viewing location a bit, the effect
> changed.  In this case it stayed the same as we changed viewing angle.
> 
> Anyone know if someone makes wheels with free rotating hubs or was this
> just a REALLY good optical illusion??
> 
> We conjenctured that someone could make them with a little controllable
> motor that could control the spin rate and direction.
> 
Wonderful! Next time I get a chance, I'll make some. All it takes is a
way to mount and center a good bearing and apendulous weighted dummy hub
cap. Heve you looked at the wheel-mounted odometers on big
tractor-trailer rigs? They're less than 4" in diameter, yet maintain
oriention.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������