b-log – betriebsraum weblog

Software Development, Human-Computer Interaction, Projects…

Efficient Gesture Recognition and Corner Finding in AS3

July 21, 2009

Gesture recognition and corner finding are two techniques that can be useful for all sorts of things. For example, gesture recognition can be used to recognise shapes in graphics applications, to control the user interface on a mobile device or to build creative games. In this context, I’m referring to shapes drawn with an input device that may or may not leave an ink trail on the screen, and not to physical gestures, such as waving a hand (however, most of the time a drawn shape also implies a physical gesture). Corner finding is a technique that finds all corner points of an arbitrary stroke and can be used for shape simplification, node finding or another exciting field, which I’m going to reveal later this year… ;)

While gesture recognition and corner finding are not necessarily related to each other, they are both concerned with the processing and recognition of lines and can be utilised to create interesting forms of human-computer interfaces. Furthermore, both approaches have in common that extensive research has been carried out and new algorithms have been continuously developed. Some of those algorithms are quite complex to implement and require advanced mathematics…

The »$1 Gesture Recognizer« (Wobbrock et al. 2007) and the »ShortStraw« Corner Finder (Wolin et al. 2008) are two very efficient algorithms that are straightforward to implement, i.e. using only basic mathematics and trigonometry, and (despite of or because of their simplicity) outperform most other approaches. The »$1 Gesture Recognizer« is a template-based recognition algorithm that matches a gesture against one or more stored templates, which is the preferred method for many use cases since it requires no training period by the user (such as in Rubine’s algorithm, for example). The gestures are also rotation, position and scale invariant, meaning that a gesture will be recognised even if it’s drawn with a different rotation, position or scale than that of the stored template. However, the algorithm can be easily modified to disable those features if, for instance, a recogniser is needed that also takes the rotation of drawn shapes into account.
»ShortStraw« is a corner finding algorithm for polylines that returns all corner points for the input points of any free-form stroke by calculating and comparing »straw distances«.

If you want to know more about how exactly they work, you can read the papers about the »$1 Gesture Recognizer« and »ShortStraw« (btw, this also fits nicely into my new series of blog posts about relevant academic papers).

I’ve implemented both algorithms in ActionScript 3 and put together a simple Flex based demo application for testing (as for the »$1 Gesture Recognizer«, there are some versions for different languages available but I’ve implemented it from scratch as I wanted to understand the approach. For ShortStraw I couldn’t find any source code). You can view the demo and download all source code at the end of the article. The main classes for $1 can be found in de.betriebsraum.gestures.recognizer.* and for ShortStraw in de.betriebsraum.geom.cornerfinder.ShortStraw.
The DollarRecognizer class has some convenience methods to add and remove DollarTemplates and the main recognize() method, which accepts an array of points and returns an instance of DollarResult with the id and score of the best match.
ShortStraw has two public static methods: getCornerIndices() and a convenience method getCornerPoints(), which both accept an array of input points. getCornerIndices() returns the array indices of all found corners in the source array and getCornerPoints() returns an array of new points. It’s also noteworthy that there are a lot of other handy methods in the classes, for example to resample points or calculate the median, so have a look at them too…

Please note that I haven’t explicitly tuned the implementations to squeeze out the last bit of performance with various AS3 optimisation tricks and hacks as they perform quite well for me.
Here’s the demo (you can view and download the full source code by right-clicking and selecting “View Source” in the demo).

$1 Gesture Recognizer & ShortStraw Demo

$1 Gesture Recognizer & ShortStraw Demo (click on the image)

Filed under: Flex/AS3, HCI, RIA

8 Responses to “Efficient Gesture Recognition and Corner Finding in AS3”

  1. […] petit article sur la reconnaissance des gestes en As3 et une petite […]

  2. alex says:

    great app! works really good.

  3. Og2t says:

    Good to see you blogging again Felix! Amazing post, I will try the corner finder with my face tracking experiment. Keep Flashin’!

  4. jrg says:

    fascinating subject, thanks for this!

  5. Flo says:

    Hi, thanks for sharing!
    Really efficient… You rock!

  6. Simon says:

    Great little app. I love it!

  7. Joshua Koo says:

    Hi Felix,

    Thank you for the recommendation of these 2 papers, (it sparked some interest of reading papers in me) and the great AS3 implementation.

    I’ve ported the shortstraw implementation for Javascript, and it works pretty well too, and I’ll try to post the js implementation on my site shortly.

    Glad to have found your blog!


  8. […] first package I looked at was offered up by a Mr. Felix at bertiesbraum.de. Felix’s gesture recognizer consists of AS3 implementations of the “$1 Gesture […]

  9. RobertMeevy says:
    Your comment is awaiting moderation. This is a preview, your comment will be visible after it has been approved.

    Hi Crypto Fans,

    I won $30,000 in !!! 100% Deposit Match up to $300 !!!
    won the #1 tournament prize worth $50 on January 10, playing Tropical Treat and will sure to be back for more fun packed action on the slots. https://rebrand.ly/0nygm slot makina oyunlar?± indir slots era free coins link slot v casino online casino slot machines free play online casino slots tips and tricks detailed description Horseshoe Bend Slot Canyon Tours, U.S. more details Should I Play a Progressive Jackpot Slots Game? Progressive jackpot slots online offer the chance of some massive real money wins.
    Enjoy our free, instant play Las Vegas slot machine games – there is no need to sign-up and no need to download software – just click and enjoy! About Our Free Slots from Las Vegas There are over games to choose from and we add more all the time. more info here Be that as it may, YouTube casino streaming has grown to be quite big thesedays. of these Namun situs judi uang asli sultanplay menyediakan slot online terbaru dan slot online terbaik. https://bit.ly/3gwYTF4 May 12, ?  According to officials, “an isolated thunderstorm crossed the San Rafael Swell” on Monday afternoon, causing flash flooding and “nickel-sized hail” in area slot canyons. click for more info Latest News ???? iOS Emoji Changelog ???· Mask Wearing Emoji Now Smiles First Look: New Emojis in iOS ??†• New Emojis In Final List For ??“? Samsung One UI Emoji Changelog ??—“ When are the Emojis Coming to iPhone? ??¤– Android Emoji Changelog ???? Now Anyone Can Feed a Baby.
    8/24/?  GTA Online casino chips are acquired by exchanging GTA$ with the cashier at a rate of 1 chip = GTA$1, and this was initially capped at a maximum of 20, chips in . https://rebrand.ly/uzo3a Download ( MB) Description of Slots Casino – Jackpot Mania is modded game in this mod unlimited money, coins and everything for android Slots Casino – Jackpot Mania this is best android apk . https://cutt.us/Bjzku slots of vegas mod apk slot attendant job duties slot machines for real money slot games in vegas free slot machines quick hits https://gamesforrealmoney.blogspot.com

Add a comment

Follow comments to this post by subscribing to the comment feed.