How does the persistence of vision work

The persitence of vision phenomenom is caused by a residual image wich is printed in the retina of the eye and last for a 1/25e sec duration.

A residual image is an optical phenomenon that causes the brain to continue to perceive an image. Even when its exposure to the eye is over.

When you scroll through several static frames at a fairly high rate, you can reproduce the illusion of movement in one fluid motion.

This is why the persistence of vision is very important to understand in the film industry. But also in the animation industry!

Observe how we already can fool the human eyes with only 2 frames :


Thaumatrope d’un oiseau en cage
A thaumatrope of a bird in a cage

It is in fact 2 separated pictures printed on each side of a same support. Then we make it rotate quickly in order to just see one final picture.

This technique is used for Thaumatropes. They are small optical toys that are rotated to see the 2 pictures on each side of disc, to be like one.


Its use in the film industry

Now we know that an image will stay on the retina for 1/25 sec. So, scrolling quickly several frames, can fool the visual perception. Or even recreate the illusion of movement.

However, the final visual rendering is still jerky and lack of fluidity. It’s important to know that under 20 frames/sec, the human eye is still able to perceive the flashing pictures. It is because the visual stimulus is still insufficient.

Beyond 20 frames, the succession of pictures will not be noticeable and the scrolling of pictures will become fluid.

The invention of the film made it possible to capture video sequences at a 24 frames/sec rate and thus to have a good visual fluidity.

The first cinematographic systems offered reduced frame rates near 10 or 16 frames/sec. However, this was still interprtetable by the human eye in term of illusion of movement.

That said, a movie projector remains visible under a 16 frames/sec rate and can become problematic. Hence the appearance of the film and its frames rate at 24/sec.

With the digital age, depending to the differents video formats and the projection systems, the rates can vary. They can go up to 30, 60 frames/sec, offering the viewer exceptional quality and fluidity!

For a better understanding of the different frame rates and their use, watch the video below :



Persistence of vision in animation

In the animation field, the beauty of the thing is that we do not necessarily need a 24/sec frame rate to have a good fluidity of movement.

Let’s start with the traditional 2D animation. For this animation technique, we will usually animate at a 12/sec frame rate and the pictures will not have the natural “motion blur” (unless you add some manually by drawing it yourself).

This is good news for us because even at a 12/sec frame rate, just imagine the time it will take and the production cost for a 1:30 movie! The work is reduced by half! It is a huge time and money saving for animation studios.

For those who have less budget, a limited animation can be done at a 10 or 8/sec frame rate. It is still jerky, but it lends itself well to the cartoony universe in which the characters move quickly between 2 poses and then hold on one pose for a few seconds, before moving on to the next (snappy animation).

In 2D digital animation, we usually animate on 2. It means that we do a drawing that will be exposed on 2 frames. And when comes an action phase, we will animate on 1 (1 drawing per frame) because of the fast movement.

When an element is animated by interpolation, such as a ball or a camera movement, we will go to 24 frames/sec in order to obtain the best possible fluidity.

In 3D, we usually animate at a 24 frames/sec rate or more, depending on the video format to which the final animation will be exported.


In te IT field

The cathode ray tubes (old TV or computer screens) of the past used interlacing for image projection.

But scanning the image line by line left place to the refresh rate. This means that the image flashes and disappears / reappears at a certain rate expressed in Hertz (Hz).

With a frequency of 85 Hz on modern RTC screens, one can minimize the perception of flickering glow.

Liquid crystal displays (LCD) can have a refresh rate between 60 and 144 Hz!

Technology has evolved to offer us the best visual quality and fluidity of image. Whether in the cinema as in our living room platforms!


Some optical illusion examples that plays with persistence of vision





If you have other examples of optical illusions that use the persistence of vision, feel free to share them in the comments section below.

And now, play on!




Nugget
Nugget

I am a professional 2D and 3D animator who worked for years on Animated TV Series and Video Games. (see complete profil description on the sidebar of the Blog). Have fun!

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