Virtual Presentations with Perl

This summer, at yapc in Pittsburgh and again at the third Perl Conference, I was very fortunate to meet with a lot of friends from other regional Perl Monger groups. A lot of our groups have similar problems and frustrations. While larger groups like,, and others do have many active members who meet frequently, other groups have fewer than 5 or 10 members and don’t get together for a variety of reasons - geography, time constraints, projects at work, and so on.

Talking about this problem with Sarah Burcham of, we hit upon the idea of starting “virtual presentations”. If an active group like can have regular technical meetings, why can’t we host them live on the web, so a small, far-flung group in Missouri or Nebraska can join in?

So Sarah and I started to talk about this idea some more, and we started to understand how these “virtual presentations” needed to work. Cumbersome Java and proprietary clients were right out - we wanted to be promoting Perl, after all, and we needed this to be easy to use, easy to setup and easy to promote. We needed some way to synchronize the speaker’s slides to his presentation, and we needed some way for the remote group to ask questions and give feedback. So we immediately reduced the problem to the bare essentials. This got rid of all of the needless complexity that is the source of both great demos and countless headaches.

Since had a technical meeting scheduled a month after the Perl Conference, we started talking about how we could broadcast that presentation to the mongers in St. Louis.

We thought about RealAudio/RealVideo broadcasting to bring a presentation to a remote location. Neither RealAudio nor RealVideo require a large investment in hardware or software to capture and broadcast a presentation in real time. This approach, while perfectly valid, didn’t work for us, since the Philly side of the presentation was taking place behind a firewall. Assuming that we had the resources to transmit in real time, we couldn’t get the packets out of the building. There was also the problem of bandwidth utilization - ISI, The Institute for Scientific Information, was kind enough to host the meeting, and we didn’t want to overload the network by providing dozens of RealVideo feeds.

So, rather than trying to solve this problem with too many layers of technology, we just routed around it and used speaker phones. After all, if we are targeting small, far-flung groups, it should be easier to find a location with a speaker phone and a slow net connection than it is to find a location with sufficient unused bandwidth on a T1 line for realtime audio/video. A good host site can also use three-way calling to bring in two remote sites instead of one. While this technique may be much more limiting than using RealAudio or other internet audio broadcast, it is also much more accessible.

Now that we had the two-way communication problem solved, all that was left was synchronizing the audio presentation to the slides. Luckily, Kevin Lenzo had already solved this exact problem already with his two SlideShow scripts.

Kevin’s scripts are actually quite crafty. ‘SlideShow’ is actually a pair of scripts: one used in the presenter’s browser, and one used in each participant’s browser. The idea is very simple – whatever the presenter sees in his browser, the participants see in their browsers. When the presenter follows a link to a new web page, the presenter and all particiapnts should see the new page. (This includes pages that are part of the slide show as well as any other web page, such as

The first of these two scripts, master.cgi is used by the person giving the presentation. This is the script which fetches web pages and keeps everyone synchronized. After fetching a URL, master.cgi rewrites all HREF attributes in the page it fetches so that all links appear as HTTP GET requests to master.cgi. The rewritten links contain the original target URL as a parameter. The new version of the page is returned to the presenter’s browser, so that once he starts using master.cgi, all links followed are part of the presentation.

The master.cgi script also saves a version of each page to a shared file for the second script, nph-view.cgi to use. The nph-view.cgi is a server-push script which can send a multiple distinct web pages back in a single, long-lived response. Every time master.cgi updates the shared file, each nph-view.cgi instance sends another part in it’s multipart response to one participant’s browser. There is no limit to the number of users of nph-view.cgi, so long as the web server has enough resources to process one instance of this script for every participant. To avoid complications, master.cgi removes all hyperlinks before saving a page to the shared file, so that participants don’t inadvertantly stop nph-view.cgi from doing it’s job.

At this point, we have all the tools we need for a virtual presentation. We have a host site, with conference phone (or RealAudio/RealVideo server) a web server configured with the SlideShow scripts, and a network connection for the presenter to display his slides. All the remote users need is a speaker phone (or a high bandwidth connection for realtime audio) and a simple network connection. We can continue down this path and decorate this setup with an IRC channel, a perl/tk whiteboard and so forth. Here, we’ve used Perl where appropriate to make the simple things simple, and other techniques to make the hard things possible.

Since August, when Kevin, Sarah and I started discussing this idea, has hosted two of these virtual presentations. In September, we hosted Mark-Jason Dominus’ presentation on Strong Typing and Perl, with St. In October, we hosted Abigail’s presentation on Damian Conway’s Parse::RecDescent with both St. and

Both of these talks worked reasonably well. Many of the problems we encountered were not technical in nature. For example, when presenting dry material in front of a live audience, a good presenter can see the audience start to lose interest. When making the same presentation over a long-distance connection, it is difficult to discern whether the silence on the other end of the line means that the remote audience is rivited to their seats or asleep.

When presenting to both live and remote audiences simultaneously, it is more important for a speaker to project his voice in a manner so that everyone can hear him clearly. This is different from presenting unamplified to a small group, or using a microphone in front of a live audience.

Here are some other lessons we learned from our presentations:

  1. Test the audio connection early and often.

    This means getting the remote phone numbers as soon as possible. If using a conference phone, make sure that there are no long distance blocking issues to overcome. Test three way calling, if necessary.

  2. Test the SlideShow CGI scripts on your webserver.

    Make sure that you have the CGI scripts configured so that any change made in the ‘master’ window comes up in the ‘remote’ window. This testing can be done with two browser windows on one computer.

  3. Have a backup webserver configured and ready to use.

    Nothing is worse than starting a presentation and finding ‘no route to host’ to your shared server. Having a backup server on another host (on another network, if possible) can overcome some of these problems.

  4. Make many small HTML pages. Avoid scrolling.

    While a speaker standing in the front of a room can scroll down the page to show an example, remote sites do not have the same cues and cannot see the presenter scroll down the page. Try and rework the content so that you can click to the next point or example instead of scrolling down to it.

  5. Be clear on what you are highlighting.

    When making a point in view of your audience, moving the mouse around to highlight or underscore a point works nicely. Since remote viewers cannot see what you are doing with your mouse, be clear on what points you are highlighting. If possible, rework your slides to make the important points more obvious.

A lot of Perl Mongers are interested in sharing what they know, and many groups are active enough to hold regular technical meetings. If you are interested in spreading your knowledge around, consider inviting a remote group of mongers to your technical meeting. It’s not too difficult, and it’s actually easier if you use Perl!

Happy Mongering!



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