There are a few of these (or similar) on display at the San Francisco Exploratorium; well worth a visit if you've never been.
I made a simple RMagick script to generate these from animation frames. Turns out it even works well with full-color images. See: http://doormouse.org/illusion/
(interactivity only works in browsers that support the HTML5 range element)
Agreed; it's exactly the same principle as a flipbook or movie projector. Though it seems arbitrary that those things have ceased to be seen as optical illusions.
The reason IMO it's more an optical illusion is your brain extrapolates it as "_ behind a fence" even though you know it's just some funny lines behind the screen. 90% of the background image is blocked but you easily block out all the vertical lines and just see the object behind the fence.
This technique, used with space between the grating and the image, is what they are using for glasses-free digital 3D screens. You could think of this as a completely flat parallax barrier screen, only it's physically moved to show different views of the image. Autostereoscopy works because the angle of your two eyes does the moving for you (there are only two frames — one for the left eye view and one for the right eye view).
It's a simple algorithm, and you could carry it out by hand in an image editor without much difficulty.
Just overlay N layers (the frames of your animation) on top of one another. For every Nth column of pixels, delete the next N-1 columns following it (in all layers). Then, for each layer at index X in the stack, translate the entire layer horizontally by X pixels.
Your overlay for viewing the animation is just a black opaque rectangle with every Nth column being deleted to reveal the image beneath.
Looking at it, yes, that's pretty much it. Lenticular images (aka, sports flips, those odd things that move when you tilt them, etc) are based on the same principle except with a lens to show each "band" depending on viewing angle rather then moving a striped sheet over it to obscure a sequence of frames.
Looks like an automated algorithm to me, probably based on some vision research. Lack of a link with more details bothered me as it was a pretty cool hack.
Your response is pointless. Just flag it and move on. Your fist sandwiches should be reserved for people who waste comments saying things don't belong on HN. No tagging or categories does not mean that type of comment is a helpful alternative. It's worse.
I don't know why you think my comment is supposed to be an alternative to something. What is your comment supposed to be an alternative to? My comment? Huh?
I'm specifically trying to discourage the original submitter from putting off-topic material on HN. There are plenty of sites that cater to whatever stuff. I realize there is a gray area and this argument has been rehashed countless times, but come on, this is irrelevant in every single way.
Feel free to ding my karma again, I have plenty. Might as well put it to use griping about the same people who will vote these comments down. If nobody is afraid to break the HN taboo of complaining about the quality of submissions ("oh he's complaining, he must be a new guy, haha I'll link him to the guidelines and put him in his place, yeah!") then people will keep submitting crap.
I didn't want to reply to throw fuel in this flame, but felt that I should put an end to it. I merely pointed out the link (which you already are familiar with) to end the flames that were going on. Fwiw, I didn't even ding your karma down.
Yes, any community will inevitably be filled with a few neighbors with lawn-mowers going off in the middle of the night, but it's worth a gentle reminder. Heck, yes this may very much be off topic (to you and some others), but you have to also realize that the community shapes up the discussions. In this very thread, there are discussions about how this could be done interactively and even some examples at that too.
And before I upvote or downvote, I look at the username, their recent actions and their karma level. I saw the post user's profile and learned that the user is new to this community and let it slide, for hey we were all noobs once here and have probably done some off-topic things.
"people will keep submitting crap." Oh yes, they will! But, don't worry, HN is a pretty good community as a whole at smelling out B.S. :-)
Sorry, I didn't mean to imply "you" specifically as the one dinging my karma. I kind of meant for everything after that first sentence that referred to "you" to mean anyone reading it. You can't vote down all of my comments 5 times on your own.
For what it's worth, I'm mostly disappointed in stuff like this because, while many people were noobs at some point: 1) anyone is free to lurk until they feel un-noob-like 2) you can show some restraint when posting links.
I've only made a few submissions, and I think they were pretty ok. Out of the 10, 1 was off topic, and only partially.
Here is what bugs me the most: I used to be able to read the 'new' page and see cool stuff. Some of it wouldn't get to the top/front page. Most of it, even the stuff in 'new', was usually pretty good. As it is now, only the top/front page of HN is good to read. Going through 'new' is like becoming a human crap filter.
Please don't just submit any crap you feel like because maybe it will get voted to the front. Only submit good stuff.
"HN is not something that you just dump something on. It's not a big truck. It's a series of people complaining."
I made a simple RMagick script to generate these from animation frames. Turns out it even works well with full-color images. See: http://doormouse.org/illusion/ (interactivity only works in browsers that support the HTML5 range element)