knitr mp4 movie embedding does not work on Windows XP - html

Question

I knit a Rmd file (to html) with a chunk producing a mp4 movie :
```{r clock, fig.width=7, fig.height=6, fig.show='animate'}
par(mar = rep(3, 4))
for (i in seq(pi/2, -4/3 * pi, length = 12)) {
plot(0, 0, pch = 20, ann = FALSE, axes = FALSE)
arrows(0, 0, cos(i), sin(i))
axis(1, 0, "VI"); axis(2, 0, "IX")
axis(3, 0, "XII"); axis(4, 0, "III"); box()
}
```
knitrgenerates the following html code for embedding the mp4 movie :
<p><video controls="controls" loop="loop"><source src="figure/clock.mp4" type="video/mp4" />video of chunk clock</video></p>
The mp4 movie is well created in the figure subfolder but it does not appear in the html output when I open it with a Windows XP machine using either Chrome, Firefox or Explorer.
Here is a (temporary) example : http://stla.overblog.com/ellipse-chart-test - this is not the "clock" example but this is exactly the same rendering problem. I see the movie with Chrome on a Windows Vista machine, but not with my Windows XP machine.
What is the explanation ? Is it really a problem with the OS or with browser versions ?

Answer 1

tl;dr Browsers really use the OS to perform some media decoding tasks. Work around it by a) providing alternate media streams b) using the most compatible media format for your audience c) using a plugin (i.e. Flash), or d) recommend installing an MP4 plugin.
This is in fact a 'problem' with the OS. Many browsers, just as some other programs on a particular platform, use the operating system resources to acomplish a given task. This is particularly true when it comes to procedures protected by intellectual property rights.
Your codec (h.264 aka "MP4") happens to be a particulary fiercely fought over piece of IP. Thus, browsers don't go to lengths to licence the IP at hand, but rather use the licenced codecs of the host system.
In your case, Windows XP happens not to be able to decode the media format of your video, and the browser doesn't seem to be able to do that by itself.
Your alternatives now:
Give additional media streams with your video tag (see Wikipedia for an example)
Trying to find out what browser the majority of users is using on XP, then choose a natively supported format (either webm for Chrome or ogg for Firefox)
Just use Flash to play the MP4 (as in the pre-HTML5 days)
Telling users to install an OS-level plugin to play h.264; you could even do that in the fallback text. I won't recommend a specific product, but there are many.