hook_ffmpeg_html() uses FFmpeg to convert images
  to a video; hook_scianimator() uses the JavaScript
  library SciAnimator to create animations;
  hook_r2swf() uses the hook_ffmpeg_html(x, options)hook_scianimator(x, options)
hook_r2swf(x, options)
x[1] is
  the plot base filename, and x[2] is the file
  extensionanimation.fun, e.g. you can set
  opts_knit$set(animation.fun = hook_scianimator).