Here’s rionhunter's response, I've added my take on this below:
I made a response to this, but unfortunately, tumblr has a way of eating up anything more than 10 lines long, and it got a little lost. So, even though I’m not Hank, I thought I would make a full post explaining the science.
To understand why it’s happening, though, I’m going to have to quickly explain to you what is happening first.
Hopefully we all know that animation (and film) is just a collection of images, flashed in quick succession. The motion that we see, however, is pieced together in our brains, thanks to a thing called ‘persistence of vision’.
Persistence of Vision is caused by the lag in your brain. Seriously.
That brief instant it takes for your brain to understand what it’s seeing is the reason you’re able to watch movies. And we should be thankful for that brief instant.
Light comes into your eyeballs, and it’s crazy hectic data. There’s so much stuff happening all the time everywhere. And while our brains are good, they can’t process everything they’re seeing at light speed. Everything we perceive through our retinas is just light, bouncing off other things. We all know that, but it’s something we often forget.
The brain processes one instant of reality, then a snapshot of the next, and then the next, and so on, and pieces them together to create motion.
This is everything. This is your entire reality. The perception of instances blended together to form a delicious smoothy of senses.
For motion to be consistent, however, what it’s seeing needs to resemble what it was seeing the moment before. For example, for objectX to look like it’s moving, it needs to mostly be where it was the microsecond before, but slightly not.
Basically, you need to think about those ol’ claymations kids make, where the lego slowly edges fowards. You need to take that concept, and apply it to everything you’ve ever known and loved.
If objectX doesn’t overlap where it was before, it’ll look liked it appeared there out of nowhere or a whole new objectX. This is when the illusion of movement is broken. It doesn’t occur in live-action movies or reality as much, because it’s hard to break the illusion of reality when you’re in reality, whereas to create a realistic perception of reality, from nothing, on a screen?
Yeah, a little trickier.
In an industry setting, animators have to create at least 25 frames for every second of footage (FPS). And sometimes, in that 25 frames, animators need to have something move so fast on a frame, that it doesn’t overlap its previous self.
Their solution, as you probably know, is to stretch and contort their object in a way that’s not dissimilar from motion blur with cameras. Especially when you acknowledge that motion blur is everything that’s happening for that 1/25th of a second.
Again, a lot of this is common knowledge, but it’s a matter of how it all pieces together to work.
As you can see here, in figure A, the hotdogs are smoothly sliding out at a consistent speed, which means, if you were to mark each spot they were in every frame, the marks would make a straight line.
The intervals between each marking isn’t very much, because they’re moving quite slowly. The hotdogs are mostly overlapping themselves between each frame.
Now remember that the illusion of movement is all in your brain, where it looks for something that resembled the instant before, and projects trajectory into your concious.
The only reason you’re able to reverse the flow of hotdogs is because they look so similar, and because it’s literally all in your head.
When you make yourself think the flow of hotdogs is going into this fine gentleman’s pants, you’re making yourself believe that, in one frame, hotdogX moves almost a whole hotdog length down, instead of only a little bit of a hotdog length up.
And because it’s almost a whole hotdog length down, in just one frame, the distance of the intervals along the hotdog’s trajectory increases, which means it travels more distance in the same amount of time.
In that one instance of perceived reality (IPR)(Don’t use that anywhere serious, I just made that up), the hotdog moves 9 pixels, instead of 2 (approx.)(I’m not going to count them)
So, to summarize the answer to your question (aka TL:DR);
The reason why the ‘dogs fly into his pants faster is because your brain lag enables you to perceive motion through light (it likes things that look the same). And when things look the same, you can screw with your brain something hardcore.
When you force your brain to see things at different intervals, it can change how you perceive them.
I don’t totally agree with rionhunter's explanation. It's true that persistence of vision and related phenomena of visual perception are responsible for the fact that films and TV don’t look like the series of still frames that they are. But to me, none of that explains the directional perception of the hotdogs or their (apparent) speed.
Today’s films, TV shows, and digitally animated features aim for 24-30 frames per second. Hand-drawn animation like the Disney films of yore used to get away with as little as 12 images per second (each doubled to create a total of 24 frames per second). And yes, they would distort images in between to create a motion-blur type effect.
But the GIF illusion above reminds me more of the spinning dancer illusion than anything else:
Almost instantaneously, that dancer will appear to spin in one direction. A majority of you will see it in a certain direction over the other, but I don’t want to lead the witness, so to speak, by telling you which. Most of you, given enough brain cramping, will be able to reverse the direction of the dancer, just like you reversed the direction of the happy hot dogs (try using your peripheral vision to make it switch).
This initial reaction/reversal trickery is due to the lack of a depth reference in both images. It’s likely that our perceived position below Hot Dog Man tricks our brain into assuming the franks are flying away to the right. And without a reference point that makes the opposite impossible, you can readily make the opposite possible and perceive the hot dogs falling into his pants. Incidentally, you may have heard that your directional preference determines whether you are “right-brained” or “left-brained”, but that’s BS, because brain-sidedness is a BS concept to begin with.
As for their apparent speed? In each of the frames of this GIF, a sausage moves vertically by 12 pixels and horizontally by 4 pixels (I measured). It doesn’t matter which direction you perceive them moving, that’s how far they go in either direction. It’s likely that they appear to move faster when entering his pants (appearing to move down and to the left) because the background is moving to the right. You know how sometimes when you’re stopped at a stoplight and the car next to you will move and you suddenly feel like you’re moving backwards? It’s like that. It’s an illusion of self-motion. On the other hand, when the hotdogs move in the same direction as the drumsticks, the illusion of motion is reduced because the background reference is interpreted differently by your brain. What’s especially cool about this is that the hotdogs move the same distance no matter what, your brain is just doing that thing it always does where it lies to you.
So there ya go John and Hank and everyone else. That’s my take on the hot dog man illusion GIF. Science side … out.
(Hot dog GIF by Lacey Micallef)