Scaling HTML slides for different projectors with full-page zoom (Shallow Thoughts)

Akkana's Musings on Open Source, Science, and Nature.

Sun, 20 Jun 2010

Scaling HTML slides for different projectors with full-page zoom

Regular readers probably know that I use HTML for the slides in my talks, and I present them either with Firefox in fullscreen mode, or with my own Python preso tool based on webkit.

Most of the time it works great. But there's one situation that's always been hard to deal with: low-resolution projectors. Most modern projectors are 1024x768, and have been for quite a few years, so that's how I set up my slides. And then I get asked to give a talk at a school, or local astronomy club, or some other group that has a 10-year-old projector that can only handle 800x600. Of course, you never find out about this ahead of time, only when you plug in right before the talk. Disaster!

Wait -- before you object that HTML pages shouldn't use pixel values and should work regardless of the user's browser window size: I completely agree with you. I don't specify absolute font sizes or absolute positioning on web pages -- no one should. But presentation slides are different: they're designed for a controlled environment where everyone sees the same thing using the same software and hardware.

I can maintain a separate stylesheet -- that works for making the font size smaller but it doesn't address the problem of pictures too large to fit (and we all like to use lots of pictures in presentations, right?) I can maintain two separate copies of the slides for the two sizes, but that's a lot of extra work and they're bound to get out of sync.

Here's a solution I should have thought of years ago: full-page zoom. Most major browsers have offered that capability for years, so the only trick is figuring out how to specify it in the slides.

IE and the Webkit browsers (Safari, Konqueror, etc.) offer a wonderful CSS property called zoom. It works like this:

body {
  zoom: 78.125%;
}

78.125% is the ratio between an 800-pixel projector and a 1024-pixel one. Just add this line, and your whole page will be scaled down to the right size. Lovely!

Lovely, except it doesn't work on Firefox (bug 390936). Fortunately, Firefox has another solution: the more general and not yet standardized CSS transform, which Mozilla has implemented as the Mozilla-specific property -moz-transform. So add these lines:

body {
  position: absolute; left: 0px; top: 0px;
  -moz-transform: scale(.78125, .78125);
}

The position: absolute is needed because when Firefox scales with -moz-transform, it also centers whatever it scaled, so the slide ends up in the top center of the screen. On my laptop, at least, it's the upper left part of the screen that gets sent to the projector, so slides must start in the upper left corner.

The good news is that these directives don't conflict; you can put both zoom and -moz-transform in the same rule and things will work fine. So I've added this to the body rule in my slides.css:

  /* If you get stuck on an 800x600 projector, use these:
  zoom: 78.125%;
  position: absolute; left: 0px; top: 0px;
  -moz-transform: scale(.78125, .78125);
   */

Uncomment in case of emergency and all will be well. (Unless you use Opera, which doesn't seem to understand either version.)

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[ 11:14 Jun 20, 2010    More tech/web | permalink to this entry ]