Interactive Optical Illusions - Explore & Train Your Vision System

In another thread I had mentioned how I often seek out optical illusions of
all types, in order to better train my perception and color-memory. If you
know what can trick your vision then you know how to override it from
memory. In my own experience this has become invaluable when having to
discern what white-balance might be best when instead the camera's own auto
white-balance or a recommended default just won't suffice. Or when some
optical distortion is occurring in some architectural photography or
fence-lines and I need to change my perspective so the shot is more
visually appealing rather than disturbing. (Though in one instance, I
exploited a subliminally visually-disturbing aspect to lines and shapes to
make a rather lack-luster image more interesting to the viewer. They never
could quite figure out what made them look at it so long or what made them
feel dizzy or off-balance by doing so. Yet it was only one edge in the
image that created this effect.)

In any case, I ran across an interesting web-site that covers most every
type of optical illusion that's been discovered or exploited for various
means and purposes. The best part is that nearly every one of them is
interactive in flash format. You can, in most all cases, adjust the
parameters to see when and if the illusion disappears or when it becomes
the primary force in your perception. In some instances I have discovered
some illusions from the demonstrations that weren't even mentioned (** see
below). If any of the flash applets include a large red button in the upper
right corner, you can use that show/hide a fuller battery of adjustment
tools.

There's also some good demonstrations listed in the Lightness library to
show everyone how unsharp-masking and contrast-optimizer/clarifier filter
parameters do their thing. In the Cornsweet and Mach-Band demos.

An Atlas of Visual Phenomena

http://lite.bu.edu/vision-flash10/applets/lite/lite/lite.html


(** An unmentioned illusion. When viewing the Gelatinous Ellipse in the
Forms library, at certain rotation speeds and ellipse shapes, you will find
a brighter white region, brighter than the white background, being induced
off the ends of and chasing the ends of the rotating black ellipse.)

An aside note to self: Interestingly, I haven't been able to find a
demonstration online anywhere of "Bidwell's Ghost" illusion that works for
my vision. The briefly-flashed red object never changes color for me.
Though this could be due to a hardware limitation.

On Mon, 28 Mar 2011 01:48:06 -0500, Better Info wrote:

Long ago when living on a South-Pacific island it was the first time I had
learned of this and everyone was telling me to watch for the green-flash to
see if it exists, one evening I saw it myself. But what surprised me even
more is that it wasn't green. It was an intense electric-blue, almost
purplish. The conditions were just right I guess. When I asked others about
it days later, my wondering why they called this blue-flash a green-flash,
they didn't believe me.

Here's the color(s) I remember:

http://www.eso.org/public/images/eso0812b/

image search: sunset "blue flash"

On Sun, 27 Mar 2011 20:12:43 -0700, ASCII wrote:

Better Info wrote:

An aside note to self: Interestingly, I haven't been able to find a
demonstration online anywhere of "Bidwell's Ghost" illusion that works for
my vision. The briefly-flashed red object never changes color for me.
Though this could be due to a hardware limitation.

Did you try this one? You can vary the blink rates
http://lite.bu.edu/new-lite/bidwell.swf
I get yellows and greens with the default parameters on a
HP 23" LCD monitor within XP Using Flash v10.2.153.1

BTW: Do you think it's might be the Bidwell's Ghost effect
that could explain the legendary 'green flash' seen at sunset
along the Pacific Ocean? At least it's a rumor here in San Diego

What am I supposed to see apart from alternating black and red
blinking?

Regards,

Eric Stevens

On Mon, 28 Mar 2011 02:39:12 -0700, ASCII wrote:

Better Info wrote:
On Mon, 28 Mar 2011 01:48:06 -0500, Better Info wrote:

Long ago when living on a South-Pacific island it was the first time I had
learned of this and everyone was telling me to watch for the green-flash to
see if it exists, one evening I saw it myself. But what surprised me even
more is that it wasn't green. It was an intense electric-blue, almost
purplish. The conditions were just right I guess. When I asked others about
it days later, my wondering why they called this blue-flash a green-flash,
they didn't believe me.

Here's the color(s) I remember:

http://www.eso.org/public/images/eso0812b/

image search: sunset "blue flash"

Blue and green are so close
spectrally that it suggests more of a refractory
than psychological phenomenon.

The point being, is that it actually is a real phenomenon and can appear in
any color of the spectrum, and it's not just an illusion due to afterimage
effects in the human eye. Though I don't discount that some people
reporting this have seen one of the refracted edges of sun in red or
orange, which briefly flashed, and then induced a greenish or bluish
afterimage. Much depends on the brightness of the sun as it's setting too.
In some instances the sun is very easy to look at when on the horizon.
Other times its still painfully bright.

(Mild afterimages always appear in their complimentary colors, though if
very bright, will slowly fade back and forth between first the
complimentary and then the original colors and back again and again, each
time becoming less and less apparent. This is not psychological by the way,
it's caused by the chemical changes in the nerves in your retina that
detect light and color. The chemicals attempting to reset themselves to a
neutral condition, to put it in simple terms.)

That reminds me, in my search for other applets to demonstrate
Bidwell's-Ghost, I ran across this interesting article on creating
positive-afterimages. You might find this interesting reading:
http://www.science-project.com/_members/science-projects/1985/02/1985-02-fs.html


When I watched for it that time it was very easy to look at so there wasn't
really any "flash" per se, just that the very last bit of it ducking below
the ocean horizon turned intensely blue for a moment, then it was gone.
There were no clouds on the horizon at the time, just sky and ocean. I
recall, now that I've been reminiscing about it, that at the time I got the
impression that it was even being colored by the water itself, or the long
distance the light took while skimming just inches above the ocean water
causing the color I had seen. Because everyone had told me it only appears
green to those who have seen it. I figured there must be another
explanation for the color I saw. Only later did I find out that the
atmosphere alone could refract it into an intense blue.

As for my belief that an afterimage effect might be the cause of some
reports, use any camera flash and cover the flash with a strong red or
orange filter. Or just flash it at something strongly red or orange
colored. See what color of afterimage remains after firing it. I suspect
that when the edges of the sun are rippling in the atmosphere and are still
very bright, this (a colored afterimage) could cause some of the
green-flash reports.

On Mon, 28 Mar 2011 02:53:11 -0700, ASCII wrote:

Eric Stevens wrote:
On Sun, 27 Mar 2011 20:12:43 -0700, ASCII wrote:

Better Info wrote:

An aside note to self: Interestingly, I haven't been able to find a
demonstration online anywhere of "Bidwell's Ghost" illusion that works for
my vision. The briefly-flashed red object never changes color for me.
Though this could be due to a hardware limitation.

Did you try this one? You can vary the blink rates
http://lite.bu.edu/new-lite/bidwell.swf
I get yellows and greens with the default parameters on a
HP 23" LCD monitor within XP Using Flash v10.2.153.1

BTW: Do you think it's might be the Bidwell's Ghost effect
that could explain the legendary 'green flash' seen at sunset
along the Pacific Ocean? At least it's a rumor here in San Diego

What am I supposed to see apart from alternating black and red
blinking?

Regards,

Eric Stevens

If you relax and let your focus drift toward infinity like when you look at
'magic eye' images there's some secondary color effects that appear.

I'm afraid not. :-(

Regards,

Eric Stevens