Animation basics: The optical illusion of motion - TED-Ed

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How do animators make still images come to life? Are the images really moving, or are they merely an optical illusion? TED-Ed takes you behind the scenes to reveal the secret of motion in movies.

Lesson and animation by TED-Ed.

Take a series of still, sequential images.

Let's look at them one by one.

Faster.

Now, let's remove the gaps,

go faster still.

Wait for it...

...bam!

Motion!

Why is that?

Intellectually, we know we're just looking

at a series of still images,

but when we see them change fast enough,

they produce the optical illusion

of appearing as a single, persistent image

that's gradually changing
form and position.

This effect is the basis
for all motion picture technology,

from our LED screens of today

to their 20th century
cathode ray forebearers,

from cinematic film projection

to the novelty toy,

even, it's been suggested,

all the way back to the Stone Age

when humans began painting on cave walls.

This phenomenon of perceiving
apparent motion

in successive images

is due to a characteristic
of human perception

historically referred
to as "persistence of vision."

The term is attributed

to the English-Swiss
physicist Peter Mark Roget,

who, in the early 19th century,

used it to describe
a particular defect of the eye

that resulted in a moving object

appearing to be still when
it reached a certain speed.

Not long after,

the term was applied
to describe the opposite,

the apparent motion of still images,

by Belgian physicist Joseph Plateau,

inventor of the phenakistoscope.

He defined persistence of vision

as the result of successive afterimages,

which were retained and then
combined in the retina,

making us believe that what we were seeing

is a single object in motion.

This explanation was widely accepted

in the decades to follow

and up through the turn
of the 20th century,

when some began to question

what was physiologically going on.

In 1912, German
psychologist Max Wertheimer

outlined the basic primary
stages of apparent motion

using simple optical illusions.

These experiments led him to conclude

the phenomenon was due to processes

which lie behind the retina.

In 1915, Hugo Munsterberg,

a German-American pioneer
in applied psychology,

also suggested that the apparent motion

of successive images

is not due to their being
retained in the eye,

but is superadded
by the action of the mind.

In the century to follow,

experiments by physiologists

have pretty much confirmed
their conclusions.

As it relates to the illusion
of motion pictures,

persistence of vision has
less to do with vision itself

than how it's interpreted in the brain.

Research has shown that different aspects

of what the eye sees,

like form,

color,

depth,

and motion,

are transmitted to different
areas of the visual cortex

via different pathways from the retina.

It's the continuous interaction

of various computations
in the visual cortex

that stitch those different
aspects together

and culminate in the perception.

Our brains are constantly working,

synchronizing what we see,

hear,

smell,

and touch

into meaningful experience

in the moment-to-moment
flow of the present.

So, in order to create the illusion

of motion in successive images,

we need to get the timing of our intervals

close to the speed at which our brains
process the present.

So, how fast is the present
happening according to our brains?

Well, we can get an idea

by measuring how fast
the images need to be changing

for the illusion to work.

Let's see if we can figure it out

by repeating our experiment.

Here's the sequence presented

at a rate of one frame per two seconds

with one second of black in-between.

At this rate of change

with the blank space
separating the images,

there's no real motion perceptible.

As we lessen the duration of blank space,

a slight change in position
becomes more apparent,

and you start to get an inkling
of a sense of motion

between the disparate frames.

One frame per second,

two frames per second,

four frames per second.

Now we're starting to get
a feeling of motion,

but it's really not very smooth.

We're still aware of the fact

that we're looking at separate images.

Let's speed up,

eight frames per second,

twelve frames per second.

It looks like we're about there.

At twenty-four frames per second,

the motion looks even smoother.

This is standard full speed.

So, the point at which we lose
awareness of the intervals

and begin to see apparent motion

seems to kick
in at around eight to twelve frames per second.

This is in the neighborhood

of what science has determined

to be the general threshold
of our awareness

of seeing separate images.

Generally speaking, we being
to lose that awareness

at intervals of around 100
milliseconds per image,

which is equal to a frame rate of

around ten frames per second.

As the frame rate increases,

we lose awareness
of the intervals completely

and are all the more convinced

of the reality of the illusion.