[PD] Spectrum graphing amplitude problem

[Ed Kelly]

My theory is that the brain automatically 're-frames' the image when you move your eyes. Here's a point - the eyes never move smoothly unless tracking a moving object to keep it stationary within the field of vision. They always dart about.
Cinema would not work if our eyes were digital, since the frame rate of our eyes would go out of sync with the pictures. Persistence of vision is why cinema works. Projectors (35mm) show each frame three times, so that there is no flicker to the image, so if there is a 'sampling rate' of the eye then it is certainly higher than the 72 flickers-per-second.
Flourescent gas discharge lamps flicker, albeit at a very high speed, so this explains the aliasing effect. This is a great way of explaining aliasing (like cartwheels in a 'Western' movie) because the light is 'sampling' the bicycle wheel.
My bicycle wheels are so buckled that this becomes very confusing to look at ;-)

Going back to the spectrum graphing amplitude problem - there should be a way to use peakit~ and build a graphical data structure-based array around its findings. I think data structures are the way forward for this sort of thing since they represent information in an abstract graphical way.
examples/ekext/peakit-listmoses.pd (but there's no graphical array yet)
Chun did a cool thing - it would be interesting to know how the video was made. Look up 'chun lee glass cloud' on youtube. Gem?

Best,
Ed


On 10/22/07, Roman Haefeli <reduzierer at yahoo.de> wrote:
> On Mon, 2007-10-22 at 17:33 -0500, Charles Henry wrote:
> > On 10/22/07, Martin Peach <martin.peach at sympatico.ca> wrote:
> > > Mathieu Bouchard wrote:
> > > >A very simple way to explain aliased frequencies would be: spin
 a bicycle
> > > >wheel. When you accelerate it beyond a certain point, it will
 begin to look
> > > >like it's going backwards instead. This is because the wheel
 speed,
> > > >together with the repetitiveness of the wheel's appearance, have
 crossed
> > > >the Nyquist frequency of your eye.
> > >
> > > That won't work in sunlight for example.
> >
> > Haven't you ever seen it?  (in sunlight that is)

As implied, I'm 99% positive I've seen it before.  You might still be
able to convince me that I haven't ;)

> i think, that is the point that martin is trying to make.
>
> i am not an expert either, but when i encountered this effect
 (watching
> the frontwheel, while cycling at night, for instance), there was
 always
> artificial light involved. to be more precise, there was always kind
 of
> a fluorescence or any other kind of 'gas discharge lamp' (is that the
> correct expression?) involved. it doesn't seem to work with common
 light
> bulbs, since their 'afterglow time' is too long.
>
> for me, it seems, that it is rather related to the light source and
 its
> frequency and not to a property of the eye.

hey, what do you know, there's even a wiki page on it:
http://en.wikipedia.org/wiki/Wagon-wheel_effect

Vision is in fact not purely analog.  Although I don't necessarily
think the "discrete frame" theory mentioned in the page is true, there
is a counterexample.  When your eyes make a saccade (fast movement),
there is no blurring.  There is an inhibition of the movement in
between the two visual scenes.
There is also nystagmus.  Even when images appear perfectly clear, the
eye moves in random oscillations.  When researchers (I forget the
citation/can't find it) used an eye camera to track random
oscillations and corrected for the movement (creating a static
retinotopic image), the perceived image disappears.  There may be
something involved in correcting for nystagmus that renders an image
in a series of apparently still frames.  I don't know for sure.

Chuck

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