Video watermarking

The software is being thought as a shopify plugin, run from my computer. I sell videos produced by myself in a shopify store. For selling digital stuff, shopify request the files to be uploaded to their site and then they deliver the file once the customer buys it.

The proposal of the software if fighting piracy through a very elementary video watermarking strategy. The idea is to print somehow in a non visible way a date of upload in all videos that have had more than 3 sales in any window time from current day to a month ago. The software, installed in my computer, should carry out the following actions:
Check daily which videos have sold more than 3 units in my shopify store
Looking for those video files in my computer and watermarking them with today’s date
Reuploading them to shopify with this watermark
Keeping a table per video of the reuploading dates.
Then, if at any time I find a video of myself in a file sharing site, I will need to manually download it and the software should allow me to turn visible the watermark. In this way, I will know when the video was uploaded. Then I can see which were the customers who bought that specific copy, from the date of upload to the following date of upload.

This would give me a number of “suspicious” customers, any number between 3 and whatever I sold of one single video in one day, let’s say 20. Let’s suppose I have 15 suspects. Then the next time I find another video in a file sharing site I do the same and this second list will be crossed with the first one and  this will narrow the list.
Given my estimated sales volume, in no more than 2 or 3 found files I should be able to “catch” the pirat.