I want to understand the technical background why it is not possible to track an HTTPS website with Piwik, when Piwik itself is installed on an HTTP server?
Somebody said, that if you would do that the browser would come up with an error message, but why?
I mean you do an HTTPS request, and on the bottom of the site is the tracking code to the HTTP Piwik site, that gets requested immediately. What's wrong about that?
All resources (such as the requests to Piwik) of the site requested through HTTPS have to go through the very same protocol since, otherwise, you will receive varying warning messages from different browsers (along the lines of "Your connection to XYZ is encrypted, however it contains resources that are not secure [...]").
So, to alleviate the problem, also Piwik has to be available via SSL. The Piwik Javascript snipped already checks the protocol and redirects the user to the respective protocol. Now all you need to ensure is that your Piwik installation resides somewhere that has a valid SSL certificate.