Random patterns inside envelop

https://www.google.com/search?q=inside+of+envelope+pattern&client=firefox-b&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjOw-H09o7OAhWEOxoKHejAAj0Q_AUICCgB&biw=1440&bih=757#imgrc=4kXI06hLXbHebM%3A