Image diffing using CSS

Since the last post on CSS was popular here is another simple CSS trick that I found useful in the past.

Let’s say we want to find the difference between these two images:

You can do this by simply inverting the colors and then stacking them together at 50% opacity. This can be achieved with a single CSS filter:
-webkit-filter: invert(100%) opacity(50%);

What you should get back is a gray image except for places where the two images differ. This works because inverting will flip the color (255 - c) and opacity will blend the two colors equally (c1 + c2 / 2). So the resulting color is ((255 - c1) + c2) / 2 which is just rgb(127.5, 127.5, 127.5) whenever the overlapping pixel colors c1 and c2 are the same.

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Serverless WebRTC using QR codes

For those not familiar, WebRTC is a technology for peer-to-peer communication between browsers. But despite being peer to peer, WebRTC still requires setting up servers for exchanging session descriptions before the browsers can talk to each other (this is called “signaling”).

If you don’t want to manage your own servers, you can use some external service for signaling (and a public STUN server). But let’s say just for fun we don’t want to use any external services either. Then one interesting approach by Chris Ball is to manually exchange the session description by pasting them into IM or email.

I thought it would be cool to extend his demo by transmitting the data using QR codes instead. Then you will be able to establish a WebRTC connection between a phone and a desktop (or two phones with each other) by just pointing their cameras at each other.

After gluing some QR code libraries together, this was the result:

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6DOF Positional Tracking with the Wiimote

In this post I am going to talk about how to track the absolute position and orientation of a wiimote in 3D space.

This is certainly nothing new and was done over seven years ago. Oliver Kreylos, the creator of the video I just linked to, wrote a very good writeup for how he did the motion tracking along with source code. But we will see how to reimplement the same effect with just one function using a computer vision library, OpenCV!

To see what we can do with the motion tracking, here’s a simple demo that plots the position of the wiimote as I am using tracing letters in the air (also shown is a white square that represents 4 LEDs along with its projected image onto the wiimote’s camera):

And another that shows my hand:

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Computing CSS matrix3d transforms

I was cleaning out some old notes from my previous job and found some math scribbles for computing CSS transforms and thought I would share it. For some context, I was working on a page which had an image that looked like this:

I wanted to add an easter egg where I could use the screens of those devices to display arbitrary things. I thought it would just be a simple matter of pixel pushing with translate/rotate/scale/etc using CSS transforms but couldn’t really get things to line up perfectly.

Frustrated, I started trying to solve it analytically instead. That means for any given shape, I need to solve for the perspective transform that warps an element into that shape. Once that is solved, it is easy to write a WYSIWYG helper script for outputting the CSS. Here’s the final result:

See the Pen ifnqH by Franklin Ta (@fta) on CodePen.

See the coffeescript tab for the code. Or paste this gist into console to try it out on any page that has jQuery. You will need to change the selector to whatever element you want to add the dots to.

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Predicting the next Math.random() in Java

A while back a friend of mine hosted a programming competition where you were given 9 random numbers one at a time and you had to guess the rank (the sorted position amongst the 9) of each as they are coming in. The goal was to submit a strategy in Java that will maximize the number of these games won. The best solution can guess right about 3.3% of the time but I figured if he was going to let me execute arbitrary code I might as well cheat and guess correctly 100% of the time. So I set out to see if I can predict the rest of the numbers in the stream after seeing just one.

So I started digging through java.util.Random’s source and found that it is just a linear congruential generator which can be easily to cracked to obtain the original seed. The rest of this post will show how (if you just want to get the code, you can find it here).

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Solving CAPTCHAs on Project Euler

A couple weeks ago I started doing Project Euler problems. Unfortunately this was during their security breach so login was disabled so I couldn’t get credit for any of the problems I solved. Now that account functionality is restored, I want to resubmit my answers (about 60 of them) on my actual login. Since they have a 30 second rate limiting between answer submissions, it would’ve taken me half an hour to manually resubmit. This was a little too tedious and boring so I wanted to automate it. But they do have a CAPTCHA you need to fill out for each submission to prevent this…

Fortunately their CAPTCHAs are were really weak (EDIT: Project Euler made their CAPTCHAs significantly harder since this post): digits only, uniform font/font-size/font-color/background, well separated, unobstructed, etc. It seems like the only variation is that each digit is slightly rotated. Here are a few typical examples:

Writing a naive solver for these CAPTCHAs is pretty easy. The rest of this post will outline how (link to code is at the end).

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