The Art of Giving Two-Projector Talks
(for example at Siggraph)

Marc Levoy, October, 1996

In the short space of four years, this howto has become completely obsolete. Nobody uses 35mm slides any more. One day, PCs (and Powerpoint) might support two-screen talks (which will require two video outputs). If and when they do, the advice given here will become relevant again. Otherwise not.

-Marc Levoy, August, 2000

Talks at Siggraph generally use two side-by-side slide projectors. This is an unusual presentation format, and there is an art to making effective use of it. There are also some important rules of thumb that if ignored will render your talk confusing and your audience annoyed.

We begin by observing that a talk is inherently a linear presentation of information. Unlike a paper or a hypertext document, a talk conveys information in a strictly sequential order. The audience can neither look forward nor back, and it cannot compare two aspects of the talk except mentally. A sequence of slides is also a linear presentation. The audience sees only one slide at a time and, assuming that the slide contains text, they read it from top to bottom. Comparisons are possible only within one slide or, to a limited extent, between two successive slides.

By contrast, side-by-side slide projection presents two concurrent streams of information. There are several ways that you as a speaker and that your audience can interpret these two visual streams. Your goals as a speaker are (a) to design a coherent strategy for using the two streams, and (b) to make sure that your audience understands your strategy. Here is a non-exhaustive list of possible strategies:

  1. An A/B comparison. Examples are two images, two equations, or two algorithms. The ability to present two images at once is enormously powerful; it is the main reason that Siggraph offers side-by-side slide projection. To use this power effectively, make sure you are comparing apples with apples. The only difference between the two images should be the single feature you are comparing; all other aspects of the two images should be identical, e.g. same viewpoint, same lighting, same sampling rate, etc. For comparisons between text slides, use the slide titles to cue your audience into what you are doing, e.g. by labeling the left slide "Previous Method" and the right slide "New Method". Do not use the same title on both slides; half your audience will think you are displaying two copies of the same slide. If the text is similar on the two slides, highlight the differences in a different color and draw your audience's attention to the differences, e.g. by saying, "These two equations are identicial except for this term in the denominator..." Don't force your audience to perform pattern matching between the two slides in order to uncover the differences. You may also use the optical pointer, but first read the advice in item 4 below.

  2. An image and an elaboration. Examples of elaborations are detail images, a cartoon drawing, a performance table or graph, or a text explanation. This scheme works especially well if you have several pairs of images and accompanying elaborations. Once the audience catches onto your scheme, make sure that each elaboration refers only to the image it accompanies. Don't mix data from successive pairs.

  3. A sequence of slides shown two at a time. If your talk is naturally linear, you don't need to force it into a nonlinear format. Assuming that the two slides have unrelated titles and that you narrate them in a consistent pattern (usually left, then right as seen by the audience - see item 3 below), the audience will catch on quickly, and they will enjoy the opportunity to look ahead.

Whichever strategy you employ, make sure that your audience understands it. Don't be afraid to add phrases to your talk like, "Here is a comparison between..." If you switch strategies in mid-talk, which may be appropriate depending on the material to be presented, alert your audience to the switch with an appropriate phrase like, "These next five pairs of slides show typical scenes with and without..."

To conclude, here are some rules of thumb about giving two-projector presentations. Break them only with care!

  1. Avoid changing only one of your slides. The audience will spend their time (and attention) staring at the unchanged slide looking for subtle differences. Once they decide it hasn't changed, they will be annoyed with you. If you have nothing new to say on one side, insert a black slide. These are blank pieces of plastic equal in size and thickness to a slide mount, and they are available at most photo supply stores. Do not use black images. They do not translate to completely black slides.

  2. If you must change only one slide, don't perform the change by triggering only one slide projector using the remote controls at podium. Given the natural confusion between audience left/right versus speaker left/right, and under the pressure of the moment, you will undoubtedly make a mistake. In every Siggraph session, there is one speaker who makes this mistake, then spends precious minutes shuttling back and forth trying to get resynchronized. The right solution is to always advance both projectors at once and use duplicate slides where necessary to hold an image over several advances.

  3. Refer to the left or right slide AS SEEN BY THE AUDIENCE, not as seen by you, who are facing the other way. The use of "near" and "far" is similarly speaker-centered and should be avoided.

  4. Use the optical pointer, but don't abuse it. If you use it to underscore whether you are referring to the left or right slide, wave the pointer briefly across the slide, then turn it off. Audiences are easily distracted by a wandering laser spot. Note that an optical pointer does not replace good graphics. You cannot draw with it, although many speakers try. In a similar vein, the best way to highlight a feature on a drawing is to draw it in a different color. The pointer works, but it is fleeting. Remember that you know your material cold, but your audience doesn't. That's why you're up there.

© 1996 Marc Levoy
Last update: October 13, 1996