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:
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
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.
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!
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
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.
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.
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