Follow-up to issues posted about generating image(cache) derivatives manually, i.e. to pre-populate all derivatives so they're not being generated on demand.
http://drupal.org/node/331222#comment-4863926
http://drupal.org/node/1122896#comment-4863924
I was able to effect a cron-triggered regeneration of image derivatives as follows:
1. Create a view with a page display which displays all desired image derivatives as fields in a list of nodes. I.e. so that loading this view's URL in a browser triggers image derivative generation. You don't have to use a pager; we'll be making wget wait a delay before each request.
2. For sake of simplicity, disable all blocks for this view's URL. I.e. try to only have the view display the image derivatives and as few links as possible.
3. Launch wget in a cron job on some period, e.g. every 2hrs, like so:
5 */2 * * * wget -P /tmp/wget-drupal -nH -w 5 -m -r -q -I sites/default/files/styles http://mysite.com/view-url > /dev/null 2>&1