We include a 1x1 pixel image on the event homepage (it's not visible). When that image is requested from the server we record it as a pageview hit for that particular URL.
The total visitors number is calculated based on the number of unique IP addresses who visit the page.
The code for this can be found here if you'd like to see it: https://github.com/hypertiny/notches/blob/master/app/controllers/notches/hits_controller.rb#L5-L11
We include a 1x1 image in the email. When that image is requested from the server we increment the open count.
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