optical illusions are both fun and they
help us understand how human visual
system works they also give us hints
about hot computer vision systems can be
hacked and how we can hack the human
visual system to build image and video
processing algorithms and realistic
virtual reality systems let's first talk
about what some optical illusions are
and how they work we call it an illusion
when things appear to be different from
how they really are take a look at this
very famous checker shadow illusion for
example the area labeled as a looks
darker than the area labeled as B
however they're both the identical shade
of grey grey appears white if it's in
shadow and it appears black if it's on
your bright light or look at this
Rubik's Cube take a look at the blue
tiles on the Left cube and the yellow
tiles on the right cube what colors do
you see most people see blue and yellow
but they're actually the exact same
shade of grey no tricks in the video you
can pause the video and mask out the
cube using a piece of paper if you don't
believe so why does this happen your
brain assumes the illuminant to infer
this true color of an object a blue
shirt looks blue whether it's indoors
illuminated by LED lights or outdoors on
a sunny day this is called color
constancy or in the case of the
checkerboard example brightness
constancy and the first illusion your
brain assumes that one of these tiles
gets less light due to the perceived
shadow and concludes that it must be a
brighter color in the second illusion
the image on the left appears to have a
yellow of illuminant whereas the one on
the right is under blue light therefore
the colors are interpreted relative to
the type of illuminant cameras also
mimic this behavior to produce
natural-looking colors if you're
interested in how it's done you can
check out my earlier video on how
cameras process images this was an
example of the color illusion now let's
take a look at an example of a motion
illusion you've probably seen this
rotating snakes illusion before the
image you are looking at is entirely
stationary nothing here actually moves
but they appear to be moving when you're
not looking and it doesn't matter if
it's on a display or not if you print it
on a paper it would still appear moving
some claim that even cats can
see the solution we don't know for sure
why this happens but it's probably due
to some processing delay in our visual
system different parts of the scene can
be processed at different speeds
depending on the brightness contrast and
saccadic eye movements
the reason the wheels stopped moving
when we look at them might be because
the part of the image that we look at is
processed at a relatively constant speed
the motion at the periphery might be
amplified further because of our
peripheral vision is more sensitive to
movement it also makes sense from an
evolutionary perspective to have higher
motion sensitivity at the periphery so
that we can move our gaze quickly if
something moves to see it better
speaking of moving things let's take a
look at this illusion Sara at the centre
cross for a while and you'll start
seeing a green disc running around there
is no actual green in the video it's
just the missing disc running in a
circle it gets more interesting if you
keep looking if you stare at the center
for about another 15 to 20 seconds and
the magenta disks disappear entirely and
you see only the green disc which
doesn't really exist
so why does this happen the solution is
actually a combination of three
different illusions first of all nothing
is really rotating the apparent motion
is a result of our brain filling in the
gaps between the frames if you think
about it nothing you watch on a display
has a truly continuous movement it's a
series of discrete frames displayed one
after another
okay this was probably something you
already knew so why does the green come
from the green disc is an opposite color
after image that we see as a result of
the original adaptation finally if you
fixate at the centre for long enough the
magenta disappear because it seems that
our brain does some sort of background
subtraction if you look at a point for a
while the non moving parts in the
background start disappearing this
phenomenon is called truck slurs fading
this is one of the reasons we
unconsciously make small micro cyclic
eye movements those movements are
believed to prevent the retinal image
from fading up next we have the
silencing illusion when objects move we
start ignoring other types of changes
for example now we see that the colors
of the disks are constantly changing
now look at the center of the circle
once the discs start moving we barely
see any color change size and shape
changes disappear as well
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the solution shows how motion affects
the appearance of objects it's very
interesting that simply moving an object
can silence where is a visual change
this phenomenon can be factored in video
quality assessment algorithms to make
more perceptually accurate quality
judgments there are many other types of
delusions that we don't even realize
that are illusions illusory contours can
be an example of that for example there
are no triangles in this image our
visual system is evolved to perceive the
world under normal conditions for
example everyday objects we see have
continuous borders if some portions of
the edges are missing or have some
occlusion that our brain just fills them
in look at these two rectangles for
example why do we even see two
rectangles here but not three because we
see objects as a whole rather than in
parts we assume that those two parts of
the rectangle are connected this
phenomenon is linked to the laws of
closure and continuity in guessed salt
psychology here is another very famous
solution take a look at this grid but
don't stare at a single point do you see
the flickering grey dots what our eyes
do here is actually very similar to what
edge detection and image sharpening
algorithms ooh the perceived brightness
at a point depends not only on the
absolute brightness at that point but
also on its surroundings the surrounding
neurons reduce the activity of the ones
at the center and this is called lateral
inhibition in digital image processing
we use very similar filters that
subtract surrounding pixel values from
the center once we can easily simulate
this illusion by applying a laplacian of
gaussian or difference of gaussians
filter to the screen in image and video
processing this type of filter is used
to process and enhance images to solve a
variety of problems humans are not the
only ones who see optical illusions
animals see some of them even machines
can be fooled by using images called
adversarial patches those adversarial
patches cause image classifiers and
object detectors to ignore or miss
classify what they see in the input
lastly I want to share a very
interesting illusion that I recently ran
into keep your eyes on the cross as we
cycle through these phases they start to
look like creepy caricatures this
phenomenon is called the flash face
distortion effect it was discovered by
chance and is speculated to be linked to
the perceptual adaptation that helps us
recognize faces alright that's all for
today I hope you liked it subscribe for
more videos and as always thanks for
watching
stay safe and see you next time ba ba ba
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